[PATCH v2 0/5] Fix the racy split index problem

[PATCH v2 0/5] Fix the racy split index problem

[SZEDER Gábor]

This is the second attempt to fix the racy split index problem, which
causes occasional failures in several random test scripts when run
with 'GIT_TEST_SPLIT_INDEX=yes'.  The important details are in patches
1 and 5 (corresponding to v1's 3 and 5).

Changes since v1:

  - Most importanly, patch 5 adds a second is_racy_timestamp()
    condition to prepare_to_write_split_index() to deal with those
    racily clean cache entries that were copied from the shared index
    by unpack_trees() while it created a new index.
    This should fix the remainig failures in 't3903-stash.sh' with
    split index enabled.

    Furthermore, together with patch 4 they add a bunch of comments to
    the most (alas, not all) conditions in
    prepare_to_write_split_index()'s first loop, trying to explain in
    plain English which cache entries are handled by each of those
    conditions.  I might have gone a bit overboard with the comments,
    but I think they would have helped me a lot while working on these
    fixes...

  - Patch 1 adds one more failing test to demonstrate the raciness
    after unpack_trees() created a new index.

    Updated some of the comments in those tests, and I also reordered
    some tests, because I found that it's more logical to explain them
    in this order.  (However, this made 'range-diff' think that it's a
    total rewrite, which is not really true... but even though I could
    convince it otherwise by adjusting the --creation-factor, the
    results were utterly unreadable.)

  - Patch 4 is new and...  interesting?  I'd like to hear Duy's
    thoughts on this one.

  - Patch 3 is new and...  I hesitate to call it a bugfix, because,
    luckily, there is no real and visible bug, but it's a fix
    nonetheless.

  - Commit message updates here and there.

  - Addressed Ævar's nits in patch 2.

  - The first two patches from v1 have already been merged to
    'master' (cff90bdc5c (Merge branch 'sg/split-index-test',
    2018-09-24)).


SZEDER Gábor (5):
  split-index: add tests to demonstrate the racy split index problem
  t1700-split-index: date back files to avoid racy situations
  split-index: count the number of deleted entries
  split-index: don't compare stat data of entries already marked for
    split index
  split-index: smudge and add racily clean cache entries to split index

 cache.h                     |   2 +
 read-cache.c                |   2 +-
 split-index.c               | 121 +++++++++++++++++---
 t/t1700-split-index.sh      |  49 +++++----
 t/t1701-racy-split-index.sh | 214 ++++++++++++++++++++++++++++++++++++
 5 files changed, 348 insertions(+), 40 deletions(-)
 create mode 100755 t/t1701-racy-split-index.sh

Range-diff:
1:  71d973e4c0 < -:  ---------- t1700-split-index: drop unnecessary 'grep'
2:  5d46f58a07 < -:  ---------- t0090: disable GIT_TEST_SPLIT_INDEX for the test checking split index
3:  e60fa2dd9e < -:  ---------- split index: add a test to demonstrate the racy split index problem
-:  ---------- > 1:  a5b46af0ff split-index: add tests to demonstrate the racy split index problem
4:  799c9c7739 ! 2:  ed26c9703e t1700-split-index: date back files to avoid racy situations
    @@ -43,7 +43,7 @@
      
     +# Create a file named as $1 with content read from stdin.
     +# Set the file's mtime to a few seconds in the past to avoid racy situations.
    -+create_file () {
    ++create_non_racy_file () {
     +	cat >"$1" &&
     +	test-tool chmtime =-5 "$1"
     +}
    @@ -56,7 +56,7 @@
      
      test_expect_success 'add one file' '
     -	: >one &&
    -+	create_file one &&
    ++	create_non_racy_file one &&
      	git update-index --add one &&
      	git ls-files --stage >ls-files.actual &&
      	cat >ls-files.expect <<-EOF &&
    @@ -65,7 +65,7 @@
      
      test_expect_success 'modify original file, base index untouched' '
     -	echo modified >one &&
    -+	echo modified |create_file one &&
    ++	echo modified | create_non_racy_file one &&
      	git update-index one &&
      	git ls-files --stage >ls-files.actual &&
      	cat >ls-files.expect <<-EOF &&
    @@ -74,7 +74,7 @@
      
      test_expect_success 'add another file, which stays index' '
     -	: >two &&
    -+	create_file two &&
    ++	create_non_racy_file two &&
      	git update-index --add two &&
      	git ls-files --stage >ls-files.actual &&
      	cat >ls-files.expect <<-EOF &&
    @@ -83,7 +83,7 @@
      
      test_expect_success 'add original file back' '
     -	: >one &&
    -+	create_file one &&
    ++	create_non_racy_file one &&
      	git update-index --add one &&
      	git ls-files --stage >ls-files.actual &&
      	cat >ls-files.expect <<-EOF &&
    @@ -92,7 +92,7 @@
      
      test_expect_success 'add new file' '
     -	: >two &&
    -+	create_file two &&
    ++	create_non_racy_file two &&
      	git update-index --add two &&
      	git ls-files --stage >actual &&
      	cat >expect <<-EOF &&
    @@ -101,7 +101,7 @@
      test_expect_success 'set core.splitIndex config variable to true' '
      	git config core.splitIndex true &&
     -	: >three &&
    -+	create_file three &&
    ++	create_non_racy_file three &&
      	git update-index --add three &&
      	git ls-files --stage >ls-files.actual &&
      	cat >ls-files.expect <<-EOF &&
    @@ -113,7 +113,7 @@
     +test_expect_success 'set core.splitIndex config variable back to true' '
      	git config core.splitIndex true &&
     -	: >three &&
    -+	create_file three &&
    ++	create_non_racy_file three &&
      	git update-index --add three &&
      	BASE=$(test-tool dump-split-index .git/index | grep "^base") &&
      	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
    @@ -122,7 +122,7 @@
      	EOF
      	test_cmp expect actual &&
     -	: >four &&
    -+	create_file four &&
    ++	create_non_racy_file four &&
      	git update-index --add four &&
      	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
      	cat >expect <<-EOF &&
    @@ -131,7 +131,7 @@
      test_expect_success 'check behavior with splitIndex.maxPercentChange unset' '
      	git config --unset splitIndex.maxPercentChange &&
     -	: >five &&
    -+	create_file five &&
    ++	create_non_racy_file five &&
      	git update-index --add five &&
      	BASE=$(test-tool dump-split-index .git/index | grep "^base") &&
      	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
    @@ -140,7 +140,7 @@
      	EOF
      	test_cmp expect actual &&
     -	: >six &&
    -+	create_file six &&
    ++	create_non_racy_file six &&
      	git update-index --add six &&
      	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
      	cat >expect <<-EOF &&
    @@ -149,7 +149,7 @@
      test_expect_success 'check splitIndex.maxPercentChange set to 0' '
      	git config splitIndex.maxPercentChange 0 &&
     -	: >seven &&
    -+	create_file seven &&
    ++	create_non_racy_file seven &&
      	git update-index --add seven &&
      	BASE=$(test-tool dump-split-index .git/index | grep "^base") &&
      	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
    @@ -158,7 +158,7 @@
      	EOF
      	test_cmp expect actual &&
     -	: >eight &&
    -+	create_file eight &&
    ++	create_non_racy_file eight &&
      	git update-index --add eight &&
      	BASE=$(test-tool dump-split-index .git/index | grep "^base") &&
      	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
    @@ -167,19 +167,19 @@
      
      test_expect_success 'shared index files expire after 2 weeks by default' '
     -	: >ten &&
    -+	create_file ten &&
    ++	create_non_racy_file ten &&
      	git update-index --add ten &&
      	test $(ls .git/sharedindex.* | wc -l) -gt 2 &&
      	just_under_2_weeks_ago=$((5-14*86400)) &&
      	test-tool chmtime =$just_under_2_weeks_ago .git/sharedindex.* &&
     -	: >eleven &&
    -+	create_file eleven &&
    ++	create_non_racy_file eleven &&
      	git update-index --add eleven &&
      	test $(ls .git/sharedindex.* | wc -l) -gt 2 &&
      	just_over_2_weeks_ago=$((-1-14*86400)) &&
      	test-tool chmtime =$just_over_2_weeks_ago .git/sharedindex.* &&
     -	: >twelve &&
    -+	create_file twelve &&
    ++	create_non_racy_file twelve &&
      	git update-index --add twelve &&
      	test $(ls .git/sharedindex.* | wc -l) -le 2
      '
    @@ -188,13 +188,13 @@
      	git config splitIndex.sharedIndexExpire "16.days.ago" &&
      	test-tool chmtime =$just_over_2_weeks_ago .git/sharedindex.* &&
     -	: >thirteen &&
    -+	create_file thirteen &&
    ++	create_non_racy_file thirteen &&
      	git update-index --add thirteen &&
      	test $(ls .git/sharedindex.* | wc -l) -gt 2 &&
      	just_over_16_days_ago=$((-1-16*86400)) &&
      	test-tool chmtime =$just_over_16_days_ago .git/sharedindex.* &&
     -	: >fourteen &&
    -+	create_file fourteen &&
    ++	create_non_racy_file fourteen &&
      	git update-index --add fourteen &&
      	test $(ls .git/sharedindex.* | wc -l) -le 2
      '
    @@ -203,14 +203,14 @@
      	just_10_years_ago=$((-365*10*86400)) &&
      	test-tool chmtime =$just_10_years_ago .git/sharedindex.* &&
     -	: >fifteen &&
    -+	create_file fifteen &&
    ++	create_non_racy_file fifteen &&
      	git update-index --add fifteen &&
      	test $(ls .git/sharedindex.* | wc -l) -gt 2 &&
      	git config splitIndex.sharedIndexExpire now &&
      	just_1_second_ago=-1 &&
      	test-tool chmtime =$just_1_second_ago .git/sharedindex.* &&
     -	: >sixteen &&
    -+	create_file sixteen &&
    ++	create_non_racy_file sixteen &&
      	git update-index --add sixteen &&
      	test $(ls .git/sharedindex.* | wc -l) -le 2
      '
    @@ -219,7 +219,7 @@
      		git config core.sharedrepository "$mode" &&
      		git config core.splitIndex true &&
     -		: >one &&
    -+		create_file one &&
    ++		create_non_racy_file one &&
      		git update-index --add one &&
      		echo "$modebits" >expect &&
      		test_modebits .git/index >actual &&
-:  ---------- > 3:  899e5090d3 split-index: count the number of deleted entries
-:  ---------- > 4:  02646f1a1b split-index: don't compare stat data of entries already marked for split index
5:  38b5f0fe72 ! 5:  290ac26055 split-index: smudge and add racily clean cache entries to split index
    @@ -22,25 +22,29 @@
         them with smudged stat data, i.e. with file size set to 0.  When
         subsequent git commands read the index, they will notice that the
         smudged stat data doesn't match with the file in the worktree, and
    -    then go on to check the file's content.
    +    then go on to check the file's content and notice its dirtiness.
     
         In the above example, however, in the second 'git update-index'
    -    prepare_to_write_split_index() gathers all cache entries that should
    -    be written to the new split index.  Alas, this function never looks
    -    out for racily clean cache entries, and since the file's stat data in
    -    the worktree hasn't changed since the shared index was written, it
    -    won't be replaced in the new split index.  Consequently,
    -    do_write_index() doesn't even get this racily clean cache entry, and
    -    can't smudge its stat data.  Subsequent git commands will then see
    -    that the index has more recent mtime than the file and that the (not
    -    smudged) cached stat data still matches with the file in the worktree,
    -    and, ultimately, will erroneously consider the file clean.
    +    prepare_to_write_split_index() decides which cache entries stored only
    +    in the shared index should be replaced in the new split index.  Alas,
    +    this function never looks out for racily clean cache entries, and
    +    since the file's stat data in the worktree hasn't changed since the
    +    shared index was written, it won't be replaced in the new split index.
    +    Consequently, do_write_index() doesn't even get this racily clean
    +    cache entry, and can't smudge its stat data.  Subsequent git commands
    +    will then see that the index has more recent mtime than the file and
    +    that the (not smudged) cached stat data still matches with the file in
    +    the worktree, and, ultimately, will erroneously consider the file
    +    clean.
     
         Modify prepare_to_write_split_index() to recognize racily clean cache
    -    entries, and mark them to be added to the split index.  This way
    -    do_write_index() will get these racily clean cache entries as well,
    -    and will then write them with smudged stat data to the new split
    -    index.
    +    entries, and mark them to be added to the split index.  Note that
    +    there are two places where it should check raciness: first those cache
    +    entries that are only stored in the shared index, and then those that
    +    have been copied by unpack_trees() from the shared index while it
    +    constructed a new index.  This way do_write_index() will get these
    +    racily clean cache entries as well, and will then write them with
    +    smudged stat data to the new split index.
     
         Note that after this change if the index is split when it contains a
         racily clean cache entry, then a smudged cache entry will be written
    @@ -49,19 +53,19 @@
         an entry in the split index replacing an outdated entry in the shared
         index.  It did affect a few tests in 't1700-split-index.sh', though,
         because they actually check which entries are stored in the split
    -    index; the previous patch made the necessary adjustments.  And racily
    -    clean cache entries and index splitting are rare enough to not worry
    -    about the resulting duplicated smudged cache entries, and the
    -    additional complexity required to prevent them is not worth it.
    +    index; a previous patch in this series has already made the necessary
    +    adjustments in 't1700'.  And racily clean cache entries and index
    +    splitting are rare enough to not worry about the resulting duplicated
    +    smudged cache entries, and the additional complexity required to
    +    prevent them is not worth it.
     
         Several tests failed occasionally when the test suite was run with
         'GIT_TEST_SPLIT_INDEX=yes'.  Here are those that I managed to trace
         back to this racy split index problem, starting with those failing
         more frequently, with a link to a failing Travis CI build job for
    -    each.  The highlighted line shows when the racy file was written,
    -    which is not always in the failing test (but note that those lines are
    -    in the 'after failure' fold, and your browser might unhelpfully fold
    -    it up before you could take a good look).
    +    each.  The highlighted line [2] shows when the racy file was written,
    +    which is not always in the failing test but in a preceeding setup
    +    test.
     
           t3903-stash.sh:
             https://travis-ci.org/git/git/jobs/385542084#L5858
    @@ -75,7 +79,7 @@
           t2200-add-update.sh:
             https://travis-ci.org/git/git/jobs/382543426#L3051
     
    -      t0090-cache-tree:
    +      t0090-cache-tree.sh:
             https://travis-ci.org/git/git/jobs/416583010#L3679
     
         There might be others, e.g. perhaps 't1000-read-tree-m-3way.sh' and
    @@ -84,6 +88,10 @@
         [1] In the branch leading to the merge commit v2.1.0-rc0~45 (Merge
             branch 'nd/split-index', 2014-07-16).
     
    +    [2] Note that those highlighted lines are in the 'after failure' fold,
    +        and your browser might unhelpfully fold it up before you could
    +        take a good look.
    +
         Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
     
      diff --git a/cache.h b/cache.h
    @@ -121,39 +129,94 @@
      			base = si->base->cache[ce->index - 1];
     -			if (ce == base)
     +			if (ce == base) {
    -+				/*
    -+				 * Racily clean cache entries must be
    -+				 * written to the split index, so the
    -+				 * subsequent do_write_index() can smudge
    -+				 * their stat data.
    -+				 */
    -+				if (!ce_uptodate(ce) &&
    -+				    is_racy_timestamp(istate, ce))
    ++				/* The entry is present in the shared index. */
    ++				if (ce->ce_flags & CE_UPDATE_IN_BASE) {
    ++					/*
    ++					 * Already marked for inclusion in
    ++					 * the split index, either because
    ++					 * the corresponding file was
    ++					 * modified and the cached stat data
    ++					 * was refreshed, or because there
    ++					 * is already a replacement entry in
    ++					 * the split index.
    ++					 * Nothing more to do here.
    ++					 */
    ++				} else if (!ce_uptodate(ce) &&
    ++					   is_racy_timestamp(istate, ce)) {
    ++					/*
    ++					 * A racily clean cache entry stored
    ++					 * only in the shared index: it must
    ++					 * be added to the split index, so
    ++					 * the subsequent do_write_index()
    ++					 * can smudge its stat data.
    ++					 */
     +					ce->ce_flags |= CE_UPDATE_IN_BASE;
    ++				} else {
    ++					/*
    ++					 * The entry is only present in the
    ++					 * shared index and it was not
    ++					 * refreshed.
    ++					 * Just leave it there.
    ++					 */
    ++				}
      				continue;
     +			}
      			if (ce->ce_namelen != base->ce_namelen ||
      			    strcmp(ce->name, base->name)) {
      				ce->index = 0;
    +@@
    + 				 * the split index.
    + 				 * Nothing to do.
    + 				 */
    ++			} else if (!ce_uptodate(ce) &&
    ++				   is_racy_timestamp(istate, ce)) {
    ++				/*
    ++				 * A copy of a racily clean cache entry from
    ++				 * the shared index.  It must be added to
    ++				 * the split index, so the subsequent
    ++				 * do_write_index() can smudge its stat data.
    ++				 */
    ++				ce->ce_flags |= CE_UPDATE_IN_BASE;
    + 			} else {
    + 				/*
    + 				 * Thoroughly compare the cached data to see
     
      diff --git a/t/t1701-racy-split-index.sh b/t/t1701-racy-split-index.sh
      --- a/t/t1701-racy-split-index.sh
      +++ b/t/t1701-racy-split-index.sh
     @@
      
    - for trial in 0 1 2 3 4
    + for trial in $trials
      do
    --	test_expect_failure "update the split index when the shared index contains a racily clean cache entry #$trial" '
    -+	test_expect_success "update the split index when the shared index contains a racily clean cache entry #$trial" '
    - 		test_when_finished "rm -f .git/index .git/sharedindex.*" &&
    +-	test_expect_failure "update the split index when a racily clean cache entry is stored only in the shared index $trial" '
    ++	test_expect_success "update the split index when a racily clean cache entry is stored only in the shared index #$trial" '
    + 		rm -f .git/index .git/sharedindex.* &&
      
      		# The next three commands must be run within the same
     @@
    - 		# corresponding replacement cache entry with smudged
    - 		# stat data should be added to the new split index, so
    - 		# the file wont appear clean for subsequent git commands.
    + 		# entry of racy-file is only stored in the shared index.
    + 		# A corresponding replacement cache entry with smudged
    + 		# stat data should be added to the new split index.
     -		#
     -		# Alas, such a smudged replacement entry is not added!
      		git update-index --add other-file &&
      
    - 		check_cached_diff
    + 		# Subsequent git commands should notice the smudged
    +@@
    + 
    + for trial in $trials
    + do
    +-	test_expect_failure "update the split index after unpack trees() copied a racily clean cache entry from the shared index $trial" '
    ++	test_expect_success "update the split index after unpack trees() copied a racily clean cache entry from the shared index #$trial" '
    + 		rm -f .git/index .git/sharedindex.* &&
    + 
    + 		# The next three commands must be run within the same
    +@@
    + 		# index.  A corresponding replacement cache entry
    + 		# with smudged stat data should be added to the new
    + 		# split index.
    +-		#
    +-		# Alas, such a smudged replacement entry is not added!
    + 		git read-tree -m HEAD &&
    + 
    + 		# Subsequent git commands should notice the smudged
-- 
2.19.0.361.gafc87ffe72

[PATCH v2 1/5] split-index: add tests to demonstrate the racy split index problem

[SZEDER Gábor]

Ever since the split index feature was introduced [1], refreshing a
split index is prone to a variant of the classic racy git problem.
There are a couple of unrelated tests in the test suite that
occasionally fail when run with 'GIT_TEST_SPLIT_INDEX=yes', but
't1700-split-index.sh', the only test script focusing solely on split
index, has never noticed this issue, because it only cares about how
the index is split under various circumstances and all the different
ways to turn the split index feature on and off.

Add a dedicated test script 't1701-racy-split-index.sh' to exercise
the split index feature in racy situations as well; kind of a
"t0010-racy-git.sh for split index" but with modern style (the tests
do everything in &&-chained list of commands in 'test_expect_...'
blocks, and use 'test_cmp' for more informative output on failure).

The tests cover the following sequences of index splitting, updating,
and racy file modifications, with the last two cases demonstrating the
racy split index problem:

  1. Split the index while adding a racily clean file:

       echo "cached content" >file
       git update-index --split-index --add file
       echo "dirty worktree" >file    # size stays the same

     This case already works properly.  Even though the cache entry's
     stat data matches with the modifid file in the worktree,
     subsequent git commands will notice that the (split) index and
     the file have the same mtime, and then will go on to check the
     file's content and notice its dirtiness.

  2. Add a racily clean file to an already split index:

       git update-index --split-index
       echo "cached content" >file
       git update-index --add file
       echo "dirty worktree" >file

     This case already works properly.  After the second 'git
     update-index' writes the newly added file's cache entry to the
     new split index, it basically works in the same way as case #1.

  3. Split the index when it (i.e. the not yet splitted index)
     contains a racily clean cache entry, i.e. an entry whose cached
     stat data matches with the corresponding file in the worktree and
     the cached mtime matches that of the index:

       echo "cached content" >file
       git update-index --add file
       echo "dirty worktree" >file
       # ... wait ...
       git update-index --split-index --add other-file

     This case already works properly.  The shared index is written by
     do_write_index(), i.e. the same function that is responsible for
     writing "regular" and split indexes as well.  This function
     cleverly notices the racily clean cache entry, and writes the
     entry to the new shared index with smudged stat data, i.e. file
     size set to 0.  When subsequent git commands read the index, they
     will notice that the smudged stat data doesn't match with the
     file in the worktree, and then go on to check the file's content
     and notice its dirtiness.

  4. Update the split index when it contains a racily clean cache
     entry:

       git update-index --split-index
       echo "cached content" >file
       git update-index --add file
       echo "dirty worktree" >file
       # ... wait ...
       git update-index --add other-file

     This case already works properly.  After the second 'git
     update-index' the newly added file's cache entry is only stored
     in the split index.  If a cache entry is present in the split
     index (even if it is a replacement of an outdated entry in the
     shared index), then it will always be included in the new split
     index on subsequent split index updates (until the file is
     removed or a new shared index is written), independently from
     whether the entry is racily clean or not.  When do_write_index()
     writes the new split index, it notices the racily clean cache
     entry, and smudges its stat date.  Subsequent git commands
     reading the index will notice the smudged stat data and then go
     on to check the file's content and notice its dirtiness.

  5. Update the split index when a racily clean cache entry is stored
     only in the shared index:

       echo "cached content" >file
       git update-index --split-index --add file
       echo "dirty worktree" >file
       # ... wait ...
       git update-index --add other-file

     This case fails due to the racy split index problem.  In the
     second 'git update-index' prepare_to_write_split_index() decides,
     among other things, which cache entries stored only in the shared
     index should be replaced in the new split index.  Alas, this
     function never looks out for racily clean cache entries, and
     since the file's stat data in the worktree hasn't changed since
     the shared index was written, the entry won't be replaced in the
     new split index.  Consequently, do_write_index() doesn't even get
     this racily clean cache entry, and can't smudge its stat data.
     Subsequent git commands will then see that the index has more
     recent mtime than the file and that the (not smudged) cached stat
     data still matches with the file in the worktree, and,
     ultimately, will erroneously consider the file clean.

  6. Update the split index after unpack_trees() copied a racily clean
     cache entry from the shared index:

       echo "cached content" >file
       git update-index --split-index --add file
       echo "dirty worktree" >file
       # ... wait ...
       git read-tree -m HEAD

     This case fails due to the racy split index problem.  This
     basically fails for the same reason as case #5 above, but there
     is one important difference, which warrants the dedicated test.
     While that second 'git update-index' in case #5 updates
     index_state in place, in this case 'git read-tree -m' calls
     unpack_trees(), which throws out the entire index, and constructs
     a new one from the (potentially updated) copies of the original's
     cache entries.  Consequently, when prepare_to_write_split_index()
     gets to work on this reconstructed index, it takes a different
     code path than in case #5 when deciding which cache entries in
     the shared index should be replaced.  The result is the same,
     though: the racily clean cache entry goes unnoticed, it isn't
     added to the split index with smudged stat data, and subsequent
     git commands will then erroneously consider the file clean.

Note that in the last two 'test_expect_failure' cases I omitted the
'#' (as in nr. of trial) from the tests' name on purpose for now, as
it confuses 'prove' into thinking that those tests failed
unexpectedly.

[1] In the branch leading to the merge commit v2.1.0-rc0~45 (Merge
    branch 'nd/split-index', 2014-07-16).

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 t/t1701-racy-split-index.sh | 218 ++++++++++++++++++++++++++++++++++++
 1 file changed, 218 insertions(+)
 create mode 100755 t/t1701-racy-split-index.sh

diff --git a/t/t1701-racy-split-index.sh b/t/t1701-racy-split-index.sh
new file mode 100755
index 0000000000..ebde418d7e
--- /dev/null
+++ b/t/t1701-racy-split-index.sh
@@ -0,0 +1,218 @@
+#!/bin/sh
+
+# This test can give false success if your machine is sufficiently
+# slow or all trials happened to happen on second boundaries.
+
+test_description='racy split index'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	# Only split the index when the test explicitly says so.
+	sane_unset GIT_TEST_SPLIT_INDEX GIT_FSMONITOR_TEST &&
+	git config splitIndex.maxPercentChange 100 &&
+
+	echo "cached content" >racy-file &&
+	git add racy-file &&
+	git commit -m initial &&
+
+	echo something >other-file &&
+	# No raciness with this file.
+	test-tool chmtime =-20 other-file &&
+
+	echo "+cached content" >expect
+'
+
+check_cached_diff () {
+	git diff-index --patch --cached $EMPTY_TREE racy-file >diff &&
+	tail -1 diff >actual &&
+	test_cmp expect actual
+}
+
+trials="0 1 2 3 4"
+for trial in $trials
+do
+	test_expect_success "split the index while adding a racily clean file #$trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		# The next three commands must be run within the same
+		# second (so both writes to racy-file result in the same
+		# mtime) to create the interesting racy situation.
+		echo "cached content" >racy-file &&
+
+		# Update and split the index.  The cache entry of
+		# racy-file will be stored only in the shared index.
+		git update-index --split-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Subsequent git commands should notice that racy-file
+		# and the split index have the same mtime, and check
+		# the content of the file to see if it is actually
+		# clean.
+		check_cached_diff
+	'
+done
+
+for trial in $trials
+do
+	test_expect_success "add a racily clean file to an already split index #$trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		git update-index --split-index &&
+
+		# The next three commands must be run within the same
+		# second.
+		echo "cached content" >racy-file &&
+
+		# Update the split index.  The cache entry of racy-file
+		# will be stored only in the split index.
+		git update-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Subsequent git commands should notice that racy-file
+		# and the split index have the same mtime, and check
+		# the content of the file to see if it is actually
+		# clean.
+		check_cached_diff
+	'
+done
+
+for trial in $trials
+do
+	test_expect_success "split the index when the index contains a racily clean cache entry #$trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		# The next three commands must be run within the same
+		# second.
+		echo "cached content" >racy-file &&
+
+		git update-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Now wait a bit to ensure that the split index written
+		# below will get a more recent mtime than racy-file.
+		sleep 1 &&
+
+		# Update and split the index when the index contains
+		# the racily clean cache entry of racy-file.
+		# A corresponding replacement cache entry with smudged
+		# stat data should be added to the new split index.
+		git update-index --split-index --add other-file &&
+
+		# Subsequent git commands should notice the smudged
+		# stat data in the replacement cache entry and that it
+		# doesnt match with the file the worktree.
+		check_cached_diff
+	'
+done
+
+for trial in $trials
+do
+	test_expect_success "update the split index when it contains a new racily clean cache entry #$trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		git update-index --split-index &&
+
+		# The next three commands must be run within the same
+		# second.
+		echo "cached content" >racy-file &&
+
+		# Update the split index.  The cache entry of racy-file
+		# will be stored only in the split index.
+		git update-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Now wait a bit to ensure that the split index written
+		# below will get a more recent mtime than racy-file.
+		sleep 1 &&
+
+		# Update the split index when the racily clean cache
+		# entry of racy-file is only stored in the split index.
+		# An updated cache entry with smudged stat data should
+		# be added to the new split index.
+		git update-index --add other-file &&
+
+		# Subsequent git commands should notice the smudged
+		# stat data.
+		check_cached_diff
+	'
+done
+
+for trial in $trials
+do
+	test_expect_failure "update the split index when a racily clean cache entry is stored only in the shared index $trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		# The next three commands must be run within the same
+		# second.
+		echo "cached content" >racy-file &&
+
+		# Update and split the index.  The cache entry of
+		# racy-file will be stored only in the shared index.
+		git update-index --split-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Now wait a bit to ensure that the split index written
+		# below will get a more recent mtime than racy-file.
+		sleep 1 &&
+
+		# Update the split index when the racily clean cache
+		# entry of racy-file is only stored in the shared index.
+		# A corresponding replacement cache entry with smudged
+		# stat data should be added to the new split index.
+		#
+		# Alas, such a smudged replacement entry is not added!
+		git update-index --add other-file &&
+
+		# Subsequent git commands should notice the smudged
+		# stat data.
+		check_cached_diff
+	'
+done
+
+for trial in $trials
+do
+	test_expect_failure "update the split index after unpack trees() copied a racily clean cache entry from the shared index $trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		# The next three commands must be run within the same
+		# second.
+		echo "cached content" >racy-file &&
+
+		# Update and split the index.  The cache entry of
+		# racy-file will be stored only in the shared index.
+		git update-index --split-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Now wait a bit to ensure that the split index written
+		# below will get a more recent mtime than racy-file.
+		sleep 1 &&
+
+		# Update the split index after unpack_trees() copied the
+		# racily clean cache entry of racy-file from the shared
+		# index.  A corresponding replacement cache entry
+		# with smudged stat data should be added to the new
+		# split index.
+		#
+		# Alas, such a smudged replacement entry is not added!
+		git read-tree -m HEAD &&
+
+		# Subsequent git commands should notice the smudged
+		# stat data.
+		check_cached_diff
+	'
+done
+
+test_done
-- 
2.19.0.361.gafc87ffe72

[PATCH v2 2/5] t1700-split-index: date back files to avoid racy situations

[SZEDER Gábor]

't1700-split-index.sh' checks that the index was split correctly under
various circumstances and that all the different ways to turn the
split index feature on and off work correctly.  To do so, most of its
tests use 'test-tool dump-split-index' to see which files have their
cache entries in the split index.  All these tests assume that all
cache entries are written to the shared index (called "base"
throughout these tests) when a new shared index is created.  This is
an implementation detail: most git commands (basically all except 'git
update-index') don't care or know at all about split index or whether
a cache entry is stored in the split or shared index.

As demonstrated in the previous patch, refreshing a split index is
prone to a variant of the classic racy git issue.  The next patch will
fix this issue, but while doing so it will also slightly change this
behaviour: only cache entries with mtime in the past will be written
only to the newly created shared index, but racily clean cache entries
will be written to the new split index (with smudged stat data).

While this upcoming change won't at all affect any git commands, it
will violate the above mentioned assumption of 't1700's tests.  Since
these tests create or modify files and create or refresh the split
index in rapid succession, there are plenty of racily clean cache
entries to be dealt with, which will then be written to the new split
indexes, and, ultimately, will cause several tests in 't1700' to fail.

Let's prepare 't1700-split-index.sh' for this upcoming change and
modify its tests to avoid racily clean files by backdating the mtime
of any file modifications (and since a lot of tests create or modify
files, encapsulate it into a helper function).

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 t/t1700-split-index.sh | 49 ++++++++++++++++++++++++------------------
 1 file changed, 28 insertions(+), 21 deletions(-)

diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
index be22398a85..c8acbc50d0 100755
--- a/t/t1700-split-index.sh
+++ b/t/t1700-split-index.sh
@@ -8,6 +8,13 @@ test_description='split index mode tests'
 sane_unset GIT_TEST_SPLIT_INDEX
 sane_unset GIT_FSMONITOR_TEST
 
+# Create a file named as $1 with content read from stdin.
+# Set the file's mtime to a few seconds in the past to avoid racy situations.
+create_non_racy_file () {
+	cat >"$1" &&
+	test-tool chmtime =-5 "$1"
+}
+
 test_expect_success 'enable split index' '
 	git config splitIndex.maxPercentChange 100 &&
 	git update-index --split-index &&
@@ -31,7 +38,7 @@ test_expect_success 'enable split index' '
 '
 
 test_expect_success 'add one file' '
-	: >one &&
+	create_non_racy_file one &&
 	git update-index --add one &&
 	git ls-files --stage >ls-files.actual &&
 	cat >ls-files.expect <<-EOF &&
@@ -83,7 +90,7 @@ test_expect_success 'enable split index again, "one" now belongs to base index"'
 '
 
 test_expect_success 'modify original file, base index untouched' '
-	echo modified >one &&
+	echo modified | create_non_racy_file one &&
 	git update-index one &&
 	git ls-files --stage >ls-files.actual &&
 	cat >ls-files.expect <<-EOF &&
@@ -102,7 +109,7 @@ test_expect_success 'modify original file, base index untouched' '
 '
 
 test_expect_success 'add another file, which stays index' '
-	: >two &&
+	create_non_racy_file two &&
 	git update-index --add two &&
 	git ls-files --stage >ls-files.actual &&
 	cat >ls-files.expect <<-EOF &&
@@ -155,7 +162,7 @@ test_expect_success 'remove file in base index' '
 '
 
 test_expect_success 'add original file back' '
-	: >one &&
+	create_non_racy_file one &&
 	git update-index --add one &&
 	git ls-files --stage >ls-files.actual &&
 	cat >ls-files.expect <<-EOF &&
@@ -174,7 +181,7 @@ test_expect_success 'add original file back' '
 '
 
 test_expect_success 'add new file' '
-	: >two &&
+	create_non_racy_file two &&
 	git update-index --add two &&
 	git ls-files --stage >actual &&
 	cat >expect <<-EOF &&
@@ -218,7 +225,7 @@ test_expect_success 'rev-parse --shared-index-path' '
 
 test_expect_success 'set core.splitIndex config variable to true' '
 	git config core.splitIndex true &&
-	: >three &&
+	create_non_racy_file three &&
 	git update-index --add three &&
 	git ls-files --stage >ls-files.actual &&
 	cat >ls-files.expect <<-EOF &&
@@ -253,9 +260,9 @@ test_expect_success 'set core.splitIndex config variable to false' '
 	test_cmp expect actual
 '
 
-test_expect_success 'set core.splitIndex config variable to true' '
+test_expect_success 'set core.splitIndex config variable back to true' '
 	git config core.splitIndex true &&
-	: >three &&
+	create_non_racy_file three &&
 	git update-index --add three &&
 	BASE=$(test-tool dump-split-index .git/index | grep "^base") &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
@@ -265,7 +272,7 @@ test_expect_success 'set core.splitIndex config variable to true' '
 	deletions:
 	EOF
 	test_cmp expect actual &&
-	: >four &&
+	create_non_racy_file four &&
 	git update-index --add four &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
 	cat >expect <<-EOF &&
@@ -279,7 +286,7 @@ test_expect_success 'set core.splitIndex config variable to true' '
 
 test_expect_success 'check behavior with splitIndex.maxPercentChange unset' '
 	git config --unset splitIndex.maxPercentChange &&
-	: >five &&
+	create_non_racy_file five &&
 	git update-index --add five &&
 	BASE=$(test-tool dump-split-index .git/index | grep "^base") &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
@@ -289,7 +296,7 @@ test_expect_success 'check behavior with splitIndex.maxPercentChange unset' '
 	deletions:
 	EOF
 	test_cmp expect actual &&
-	: >six &&
+	create_non_racy_file six &&
 	git update-index --add six &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
 	cat >expect <<-EOF &&
@@ -303,7 +310,7 @@ test_expect_success 'check behavior with splitIndex.maxPercentChange unset' '
 
 test_expect_success 'check splitIndex.maxPercentChange set to 0' '
 	git config splitIndex.maxPercentChange 0 &&
-	: >seven &&
+	create_non_racy_file seven &&
 	git update-index --add seven &&
 	BASE=$(test-tool dump-split-index .git/index | grep "^base") &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
@@ -313,7 +320,7 @@ test_expect_success 'check splitIndex.maxPercentChange set to 0' '
 	deletions:
 	EOF
 	test_cmp expect actual &&
-	: >eight &&
+	create_non_racy_file eight &&
 	git update-index --add eight &&
 	BASE=$(test-tool dump-split-index .git/index | grep "^base") &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
@@ -326,17 +333,17 @@ test_expect_success 'check splitIndex.maxPercentChange set to 0' '
 '
 
 test_expect_success 'shared index files expire after 2 weeks by default' '
-	: >ten &&
+	create_non_racy_file ten &&
 	git update-index --add ten &&
 	test $(ls .git/sharedindex.* | wc -l) -gt 2 &&
 	just_under_2_weeks_ago=$((5-14*86400)) &&
 	test-tool chmtime =$just_under_2_weeks_ago .git/sharedindex.* &&
-	: >eleven &&
+	create_non_racy_file eleven &&
 	git update-index --add eleven &&
 	test $(ls .git/sharedindex.* | wc -l) -gt 2 &&
 	just_over_2_weeks_ago=$((-1-14*86400)) &&
 	test-tool chmtime =$just_over_2_weeks_ago .git/sharedindex.* &&
-	: >twelve &&
+	create_non_racy_file twelve &&
 	git update-index --add twelve &&
 	test $(ls .git/sharedindex.* | wc -l) -le 2
 '
@@ -344,12 +351,12 @@ test_expect_success 'shared index files expire after 2 weeks by default' '
 test_expect_success 'check splitIndex.sharedIndexExpire set to 16 days' '
 	git config splitIndex.sharedIndexExpire "16.days.ago" &&
 	test-tool chmtime =$just_over_2_weeks_ago .git/sharedindex.* &&
-	: >thirteen &&
+	create_non_racy_file thirteen &&
 	git update-index --add thirteen &&
 	test $(ls .git/sharedindex.* | wc -l) -gt 2 &&
 	just_over_16_days_ago=$((-1-16*86400)) &&
 	test-tool chmtime =$just_over_16_days_ago .git/sharedindex.* &&
-	: >fourteen &&
+	create_non_racy_file fourteen &&
 	git update-index --add fourteen &&
 	test $(ls .git/sharedindex.* | wc -l) -le 2
 '
@@ -358,13 +365,13 @@ test_expect_success 'check splitIndex.sharedIndexExpire set to "never" and "now"
 	git config splitIndex.sharedIndexExpire never &&
 	just_10_years_ago=$((-365*10*86400)) &&
 	test-tool chmtime =$just_10_years_ago .git/sharedindex.* &&
-	: >fifteen &&
+	create_non_racy_file fifteen &&
 	git update-index --add fifteen &&
 	test $(ls .git/sharedindex.* | wc -l) -gt 2 &&
 	git config splitIndex.sharedIndexExpire now &&
 	just_1_second_ago=-1 &&
 	test-tool chmtime =$just_1_second_ago .git/sharedindex.* &&
-	: >sixteen &&
+	create_non_racy_file sixteen &&
 	git update-index --add sixteen &&
 	test $(ls .git/sharedindex.* | wc -l) -le 2
 '
@@ -379,7 +386,7 @@ do
 		# Create one new shared index file
 		git config core.sharedrepository "$mode" &&
 		git config core.splitIndex true &&
-		: >one &&
+		create_non_racy_file one &&
 		git update-index --add one &&
 		echo "$modebits" >expect &&
 		test_modebits .git/index >actual &&
-- 
2.19.0.361.gafc87ffe72

[PATCH v2 3/5] split-index: count the number of deleted entries

[SZEDER Gábor]

'struct split_index' contains the field 'nr_deletions', whose name
with the 'nr_' prefix suggests that it contains the number of deleted
cache entries.  However, barring its initialization to 0, this field
is only ever set to 1, indicating that there is at least one deleted
entry, but not the number of deleted entries.  Luckily, this doesn't
cause any issues (other than confusing the reader, that is), because
the only place reading this field uses it in the same sense, i.e.: 'if
(si->nr_deletions)'.

To avoid confusion, we could either rename this field to something
like 'has_deletions' to make its name match its role, or make it a
counter of deleted cache entries to match its name.

Let's make it a counter, to keep it in sync with the related field
'nr_replacements', which does contain the number of replaced cache
entries.  This will also give developers debugging the split index
code easy access to the number of deleted cache entries.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 split-index.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/split-index.c b/split-index.c
index 84f067e10d..548272ec33 100644
--- a/split-index.c
+++ b/split-index.c
@@ -111,7 +111,7 @@ static void mark_entry_for_delete(size_t pos, void *data)
 		die("position for delete %d exceeds base index size %d",
 		    (int)pos, istate->cache_nr);
 	istate->cache[pos]->ce_flags |= CE_REMOVE;
-	istate->split_index->nr_deletions = 1;
+	istate->split_index->nr_deletions++;
 }
 
 static void replace_entry(size_t pos, void *data)
-- 
2.19.0.361.gafc87ffe72

[PATCH v2 4/5] split-index: don't compare stat data of entries already marked for split index

[SZEDER Gábor]

When unpack_trees() constructs a new index, it copies cache entries
from the original index [1].  prepare_to_write_split_index() has to
deal with this, and it has a dedicated code path for copied entries
that are present in the shared index, where it compares the cached
data in the corresponding copied and original entries.  If the cached
data matches, then they are considered the same; if it differs, then
the copied entry will be marked for inclusion as a replacement entry
in the just about to be written split index by setting the
CE_UPDATE_IN_BASE flag.

However, a cache entry already has its CE_UPDATE_IN_BASE flag set upon
reading the split index, if the entry already has a replacement entry
there, or upon refreshing the cached stat data, if the corresponding
file was modified.  The state of this flag is then preserved when
unpack_trees() copies a cache entry from the shared index.

So modify prepare_to_write_split_index() to check the copied cache
entries' CE_UPDATE_IN_BASE flag first, and skip the thorough
comparison of cached data if the flag is already set.

Note that comparing the cached data in copied and original entries in
the shared index might actually be entirely unnecessary.  In theory
all code paths refreshing the cached stat data of an entry in the
shared index should set the CE_UPDATE_IN_BASE flag in that entry, and
unpack_trees() should preserve this flag when copying cache entries.
This means that the cached data is only ever changed if the
CE_UPDATE_IN_BASE flag is set as well.  Our test suite seems to
confirm this: instrumenting the conditions in question and running the
test suite repeatedly with 'GIT_TEST_SPLIT_INDEX=yes' showed that the
cached data in a copied entry differs from the data in the shared
entry only if its CE_UPDATE_IN_BASE flag is indeed set.

In practice, however, our test suite doesn't have 100% coverage,
GIT_TEST_SPLIT_INDEX is inherently random, and I certainly can't claim
to possess complete understanding of what goes on in unpack_trees()...
Therefore I kept the comparison of the cached data when
CE_UPDATE_IN_BASE is not set, just in case that an unnoticed or future
code path were to accidentally miss setting this flag upon refreshing
the cached stat data or unpack_trees() were to drop this flag while
copying a cache entry.

[1] Note that when unpack_trees() constructs the new index and decides
    that a cache entry should now refer to different content than what
    was recorded in the original index (e.g. 'git read-tree -m
    HEAD^'), then that can't really be considered a copy of the
    original, but rather the creation of a new entry.  Notably and
    pertinent to the split index feature, such a new entry doesn't
    have a reference to the original's shared index entry anymore,
    i.e. its 'index' field is set to 0.  Consequently, such an entry
    is treated by prepare_to_write_split_index() as an entry not
    present in the shared index and it will be added to the new split
    index, while the original entry will be marked as deleted, and
    neither the above discussion nor the changes in this patch apply
    to them.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 split-index.c | 79 ++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 62 insertions(+), 17 deletions(-)

diff --git a/split-index.c b/split-index.c
index 548272ec33..7d8799f6b7 100644
--- a/split-index.c
+++ b/split-index.c
@@ -204,19 +204,34 @@ void prepare_to_write_split_index(struct index_state *istate)
 		 * that are not marked with either CE_MATCHED or
 		 * CE_UPDATE_IN_BASE. If istate->cache[i] is a
 		 * duplicate, deduplicate it.
 		 */
 		for (i = 0; i < istate->cache_nr; i++) {
 			struct cache_entry *base;
-			/* namelen is checked separately */
-			const unsigned int ondisk_flags =
-				CE_STAGEMASK | CE_VALID | CE_EXTENDED_FLAGS;
-			unsigned int ce_flags, base_flags, ret;
 			ce = istate->cache[i];
-			if (!ce->index)
+			if (!ce->index) {
+				/*
+				 * During simple update index operations this
+				 * is a cache entry that is not present in
+				 * the shared index.  It will be added to the
+				 * split index.
+				 *
+				 * However, it might also represent a file
+				 * that already has a cache entry in the
+				 * shared index, but a new index has just
+				 * been constructed by unpack_trees(), and
+				 * this entry now refers to different content
+				 * than what was recorded in the original
+				 * index, e.g. during 'read-tree -m HEAD^' or
+				 * 'checkout HEAD^'.  In this case the
+				 * original entry in the shared index will be
+				 * marked as deleted, and this entry will be
+				 * added to the split index.
+				 */
 				continue;
+			}
 			if (ce->index > si->base->cache_nr) {
 				ce->index = 0;
 				continue;
 			}
 			ce->ce_flags |= CE_MATCHED; /* or "shared" */
 			base = si->base->cache[ce->index - 1];
@@ -224,24 +239,54 @@ void prepare_to_write_split_index(struct index_state *istate)
 				continue;
 			if (ce->ce_namelen != base->ce_namelen ||
 			    strcmp(ce->name, base->name)) {
 				ce->index = 0;
 				continue;
 			}
-			ce_flags = ce->ce_flags;
-			base_flags = base->ce_flags;
-			/* only on-disk flags matter */
-			ce->ce_flags   &= ondisk_flags;
-			base->ce_flags &= ondisk_flags;
-			ret = memcmp(&ce->ce_stat_data, &base->ce_stat_data,
-				     offsetof(struct cache_entry, name) -
-				     offsetof(struct cache_entry, ce_stat_data));
-			ce->ce_flags = ce_flags;
-			base->ce_flags = base_flags;
-			if (ret)
-				ce->ce_flags |= CE_UPDATE_IN_BASE;
+			/*
+			 * This is the copy of a cache entry that is present
+			 * in the shared index, created by unpack_trees()
+			 * while it constructed a new index.
+			 */
+			if (ce->ce_flags & CE_UPDATE_IN_BASE) {
+				/*
+				 * Already marked for inclusion in the split
+				 * index, either because the corresponding
+				 * file was modified and the cached stat data
+				 * was refreshed, or because the original
+				 * entry already had a replacement entry in
+				 * the split index.
+				 * Nothing to do.
+				 */
+			} else {
+				/*
+				 * Thoroughly compare the cached data to see
+				 * whether it should be marked for inclusion
+				 * in the split index.
+				 *
+				 * This comparison might be unnecessary, as
+				 * code paths modifying the cached data do
+				 * set CE_UPDATE_IN_BASE as well.
+				 */
+				const unsigned int ondisk_flags =
+					CE_STAGEMASK | CE_VALID |
+					CE_EXTENDED_FLAGS;
+				unsigned int ce_flags, base_flags, ret;
+				ce_flags = ce->ce_flags;
+				base_flags = base->ce_flags;
+				/* only on-disk flags matter */
+				ce->ce_flags   &= ondisk_flags;
+				base->ce_flags &= ondisk_flags;
+				ret = memcmp(&ce->ce_stat_data, &base->ce_stat_data,
+					     offsetof(struct cache_entry, name) -
+					     offsetof(struct cache_entry, ce_stat_data));
+				ce->ce_flags = ce_flags;
+				base->ce_flags = base_flags;
+				if (ret)
+					ce->ce_flags |= CE_UPDATE_IN_BASE;
+			}
 			discard_cache_entry(base);
 			si->base->cache[ce->index - 1] = ce;
 		}
 		for (i = 0; i < si->base->cache_nr; i++) {
 			ce = si->base->cache[i];
 			if ((ce->ce_flags & CE_REMOVE) ||
-- 
2.19.0.361.gafc87ffe72

[PATCH v2 5/5] split-index: smudge and add racily clean cache entries to split index

[SZEDER Gábor]

Ever since the split index feature was introduced [1], refreshing a
split index is prone to a variant of the classic racy git problem.

Consider the following sequence of commands updating the split index
when the shared index contains a racily clean cache entry, i.e. an
entry whose cached stat data matches with the corresponding file in
the worktree and the cached mtime matches that of the index:

  echo "cached content" >file
  git update-index --split-index --add file
  echo "dirty worktree" >file    # size stays the same!
  # ... wait ...
  git update-index --add other-file

Normally, when a non-split index is updated, then do_write_index()
(the function responsible for writing all kinds of indexes, "regular",
split, and shared) recognizes racily clean cache entries, and writes
them with smudged stat data, i.e. with file size set to 0.  When
subsequent git commands read the index, they will notice that the
smudged stat data doesn't match with the file in the worktree, and
then go on to check the file's content and notice its dirtiness.

In the above example, however, in the second 'git update-index'
prepare_to_write_split_index() decides which cache entries stored only
in the shared index should be replaced in the new split index.  Alas,
this function never looks out for racily clean cache entries, and
since the file's stat data in the worktree hasn't changed since the
shared index was written, it won't be replaced in the new split index.
Consequently, do_write_index() doesn't even get this racily clean
cache entry, and can't smudge its stat data.  Subsequent git commands
will then see that the index has more recent mtime than the file and
that the (not smudged) cached stat data still matches with the file in
the worktree, and, ultimately, will erroneously consider the file
clean.

Modify prepare_to_write_split_index() to recognize racily clean cache
entries, and mark them to be added to the split index.  Note that
there are two places where it should check raciness: first those cache
entries that are only stored in the shared index, and then those that
have been copied by unpack_trees() from the shared index while it
constructed a new index.  This way do_write_index() will get these
racily clean cache entries as well, and will then write them with
smudged stat data to the new split index.

Note that after this change if the index is split when it contains a
racily clean cache entry, then a smudged cache entry will be written
both to the new shared and to the new split indexes.  This doesn't
affect regular git commands: as far as they are concerned this is just
an entry in the split index replacing an outdated entry in the shared
index.  It did affect a few tests in 't1700-split-index.sh', though,
because they actually check which entries are stored in the split
index; a previous patch in this series has already made the necessary
adjustments in 't1700'.  And racily clean cache entries and index
splitting are rare enough to not worry about the resulting duplicated
smudged cache entries, and the additional complexity required to
prevent them is not worth it.

Several tests failed occasionally when the test suite was run with
'GIT_TEST_SPLIT_INDEX=yes'.  Here are those that I managed to trace
back to this racy split index problem, starting with those failing
more frequently, with a link to a failing Travis CI build job for
each.  The highlighted line [2] shows when the racy file was written,
which is not always in the failing test but in a preceeding setup
test.

  t3903-stash.sh:
    https://travis-ci.org/git/git/jobs/385542084#L5858

  t4024-diff-optimize-common.sh:
    https://travis-ci.org/git/git/jobs/386531969#L3174

  t4015-diff-whitespace.sh:
    https://travis-ci.org/git/git/jobs/360797600#L8215

  t2200-add-update.sh:
    https://travis-ci.org/git/git/jobs/382543426#L3051

  t0090-cache-tree.sh:
    https://travis-ci.org/git/git/jobs/416583010#L3679

There might be others, e.g. perhaps 't1000-read-tree-m-3way.sh' and
others using 'lib-read-tree-m-3way.sh', but I couldn't confirm yet.

[1] In the branch leading to the merge commit v2.1.0-rc0~45 (Merge
    branch 'nd/split-index', 2014-07-16).

[2] Note that those highlighted lines are in the 'after failure' fold,
    and your browser might unhelpfully fold it up before you could
    take a good look.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 cache.h                     |  2 ++
 read-cache.c                |  2 +-
 split-index.c               | 42 ++++++++++++++++++++++++++++++++++++-
 t/t1701-racy-split-index.sh |  8 ++-----
 4 files changed, 46 insertions(+), 8 deletions(-)

diff --git a/cache.h b/cache.h
index 4d014541ab..3f419b6c79 100644
--- a/cache.h
+++ b/cache.h
@@ -781,6 +781,8 @@ extern void *read_blob_data_from_index(const struct index_state *, const char *,
 #define CE_MATCH_REFRESH		0x10
 /* don't refresh_fsmonitor state or do stat comparison even if CE_FSMONITOR_VALID is true */
 #define CE_MATCH_IGNORE_FSMONITOR 0X20
+extern int is_racy_timestamp(const struct index_state *istate,
+			     const struct cache_entry *ce);
 extern int ie_match_stat(struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
 extern int ie_modified(struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
 
diff --git a/read-cache.c b/read-cache.c
index 7b1354d759..8f644f68b4 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -337,7 +337,7 @@ static int is_racy_stat(const struct index_state *istate,
 		);
 }
 
-static int is_racy_timestamp(const struct index_state *istate,
+int is_racy_timestamp(const struct index_state *istate,
 			     const struct cache_entry *ce)
 {
 	return (!S_ISGITLINK(ce->ce_mode) &&
diff --git a/split-index.c b/split-index.c
index 7d8799f6b7..d989b27286 100644
--- a/split-index.c
+++ b/split-index.c
@@ -235,8 +235,39 @@ void prepare_to_write_split_index(struct index_state *istate)
 			}
 			ce->ce_flags |= CE_MATCHED; /* or "shared" */
 			base = si->base->cache[ce->index - 1];
-			if (ce == base)
+			if (ce == base) {
+				/* The entry is present in the shared index. */
+				if (ce->ce_flags & CE_UPDATE_IN_BASE) {
+					/*
+					 * Already marked for inclusion in
+					 * the split index, either because
+					 * the corresponding file was
+					 * modified and the cached stat data
+					 * was refreshed, or because there
+					 * is already a replacement entry in
+					 * the split index.
+					 * Nothing more to do here.
+					 */
+				} else if (!ce_uptodate(ce) &&
+					   is_racy_timestamp(istate, ce)) {
+					/*
+					 * A racily clean cache entry stored
+					 * only in the shared index: it must
+					 * be added to the split index, so
+					 * the subsequent do_write_index()
+					 * can smudge its stat data.
+					 */
+					ce->ce_flags |= CE_UPDATE_IN_BASE;
+				} else {
+					/*
+					 * The entry is only present in the
+					 * shared index and it was not
+					 * refreshed.
+					 * Just leave it there.
+					 */
+				}
 				continue;
+			}
 			if (ce->ce_namelen != base->ce_namelen ||
 			    strcmp(ce->name, base->name)) {
 				ce->index = 0;
@@ -257,6 +288,15 @@ void prepare_to_write_split_index(struct index_state *istate)
 				 * the split index.
 				 * Nothing to do.
 				 */
+			} else if (!ce_uptodate(ce) &&
+				   is_racy_timestamp(istate, ce)) {
+				/*
+				 * A copy of a racily clean cache entry from
+				 * the shared index.  It must be added to
+				 * the split index, so the subsequent
+				 * do_write_index() can smudge its stat data.
+				 */
+				ce->ce_flags |= CE_UPDATE_IN_BASE;
 			} else {
 				/*
 				 * Thoroughly compare the cached data to see
diff --git a/t/t1701-racy-split-index.sh b/t/t1701-racy-split-index.sh
index ebde418d7e..7f16f5f7a3 100755
--- a/t/t1701-racy-split-index.sh
+++ b/t/t1701-racy-split-index.sh
@@ -148,7 +148,7 @@ done
 
 for trial in $trials
 do
-	test_expect_failure "update the split index when a racily clean cache entry is stored only in the shared index $trial" '
+	test_expect_success "update the split index when a racily clean cache entry is stored only in the shared index #$trial" '
 		rm -f .git/index .git/sharedindex.* &&
 
 		# The next three commands must be run within the same
@@ -170,8 +170,6 @@ do
 		# entry of racy-file is only stored in the shared index.
 		# A corresponding replacement cache entry with smudged
 		# stat data should be added to the new split index.
-		#
-		# Alas, such a smudged replacement entry is not added!
 		git update-index --add other-file &&
 
 		# Subsequent git commands should notice the smudged
@@ -182,7 +180,7 @@ done
 
 for trial in $trials
 do
-	test_expect_failure "update the split index after unpack trees() copied a racily clean cache entry from the shared index $trial" '
+	test_expect_success "update the split index after unpack trees() copied a racily clean cache entry from the shared index #$trial" '
 		rm -f .git/index .git/sharedindex.* &&
 
 		# The next three commands must be run within the same
@@ -205,8 +203,6 @@ do
 		# index.  A corresponding replacement cache entry
 		# with smudged stat data should be added to the new
 		# split index.
-		#
-		# Alas, such a smudged replacement entry is not added!
 		git read-tree -m HEAD &&
 
 		# Subsequent git commands should notice the smudged
-- 
2.19.0.361.gafc87ffe72

Re: [PATCH v2 4/5] split-index: don't compare stat data of entries already marked for split index

[SZEDER Gábor]

On Thu, Sep 27, 2018 at 02:44:33PM +0200, SZEDER Gábor wrote:
>  split-index.c | 79 ++++++++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 62 insertions(+), 17 deletions(-)

I generated this patch with more context lines than usual, so the two
conditions that I didn't add any comments to in this or in the next
patch are fully visible.

> diff --git a/split-index.c b/split-index.c
> index 548272ec33..7d8799f6b7 100644
> --- a/split-index.c
> +++ b/split-index.c
> @@ -204,19 +204,34 @@ void prepare_to_write_split_index(struct index_state *istate)
>  		 * that are not marked with either CE_MATCHED or
>  		 * CE_UPDATE_IN_BASE. If istate->cache[i] is a
>  		 * duplicate, deduplicate it.
>  		 */
>  		for (i = 0; i < istate->cache_nr; i++) {
>  			struct cache_entry *base;
> -			/* namelen is checked separately */
> -			const unsigned int ondisk_flags =
> -				CE_STAGEMASK | CE_VALID | CE_EXTENDED_FLAGS;
> -			unsigned int ce_flags, base_flags, ret;
>  			ce = istate->cache[i];
> -			if (!ce->index)
> +			if (!ce->index) {
> +				/*
> +				 * During simple update index operations this
> +				 * is a cache entry that is not present in
> +				 * the shared index.  It will be added to the
> +				 * split index.
> +				 *
> +				 * However, it might also represent a file
> +				 * that already has a cache entry in the
> +				 * shared index, but a new index has just
> +				 * been constructed by unpack_trees(), and
> +				 * this entry now refers to different content
> +				 * than what was recorded in the original
> +				 * index, e.g. during 'read-tree -m HEAD^' or
> +				 * 'checkout HEAD^'.  In this case the
> +				 * original entry in the shared index will be
> +				 * marked as deleted, and this entry will be
> +				 * added to the split index.
> +				 */
>  				continue;
> +			}
>  			if (ce->index > si->base->cache_nr) {
>  				ce->index = 0;
>  				continue;
>  			}

This condition in the context above checks whether a cache entry
refers to a non-existing entry in the shared index.

I don't understand the role of this condition, for two reasons:

  - Under what circumstances can this condition be ever fulfilled?

    I instrumented it and run the test suite repeatedly with
    'GIT_TEST_SPLIT_INDEX=yes', but it has never been fulfilled.  I
    also tried to come up with all kinds of elaborate scenarios to
    trigger it, but no joy, and code inspection didn't bring anything
    either.

  - There are similar conditions in 'split-index.c' in the functions
    mark_entry_for_delete() and replace_entry(); here is the one from
    the latter, but they only differ in the error message:

      if (pos >= istate->cache_nr)
          die("position for replacement %d exceeds base index size %d",
              (int)pos, istate->cache_nr);

    (Note that this 'istate->cache_nr' here equals
    to 'si->base->cache_nr'; see their caller merge_base_index().)

    The die() clearly indicates that fulfilling this condition is a
    Bad Thing.  These two functions are invoked to create a unified
    view of the just read split and shared indexes, so the fulfillment
    of this condition could indicate a corrupt index file, and
    die()ing right away seems to be justified.

    Then why doesn't the condition in prepare_to_write_split_index()
    die() as well?!  After all if it were fulfilled, then it would
    indicate a corruption in the current index_state, and writing a
    new split index from corrupt data doesn't seem like a particularly
    good idea.


>  			ce->ce_flags |= CE_MATCHED; /* or "shared" */
>  			base = si->base->cache[ce->index - 1];
> @@ -224,24 +239,54 @@ void prepare_to_write_split_index(struct index_state *istate)
>  				continue;
>  			if (ce->ce_namelen != base->ce_namelen ||
>  			    strcmp(ce->name, base->name)) {
>  				ce->index = 0;
>  				continue;
>  			}

I don't understand the role of this condition either, and just like
the one discussed above, the test suite with
'GIT_TEST_SPLIT_INDEX=yes' seems to never fulfill it.

> -			ce_flags = ce->ce_flags;
> -			base_flags = base->ce_flags;
> -			/* only on-disk flags matter */
> -			ce->ce_flags   &= ondisk_flags;
> -			base->ce_flags &= ondisk_flags;
> -			ret = memcmp(&ce->ce_stat_data, &base->ce_stat_data,
> -				     offsetof(struct cache_entry, name) -
> -				     offsetof(struct cache_entry, ce_stat_data));
> -			ce->ce_flags = ce_flags;
> -			base->ce_flags = base_flags;
> -			if (ret)
> -				ce->ce_flags |= CE_UPDATE_IN_BASE;
> +			/*
> +			 * This is the copy of a cache entry that is present
> +			 * in the shared index, created by unpack_trees()
> +			 * while it constructed a new index.
> +			 */
> +			if (ce->ce_flags & CE_UPDATE_IN_BASE) {
> +				/*
> +				 * Already marked for inclusion in the split
> +				 * index, either because the corresponding
> +				 * file was modified and the cached stat data
> +				 * was refreshed, or because the original
> +				 * entry already had a replacement entry in
> +				 * the split index.
> +				 * Nothing to do.
> +				 */
> +			} else {
> +				/*
> +				 * Thoroughly compare the cached data to see
> +				 * whether it should be marked for inclusion
> +				 * in the split index.
> +				 *
> +				 * This comparison might be unnecessary, as
> +				 * code paths modifying the cached data do
> +				 * set CE_UPDATE_IN_BASE as well.
> +				 */
> +				const unsigned int ondisk_flags =
> +					CE_STAGEMASK | CE_VALID |
> +					CE_EXTENDED_FLAGS;
> +				unsigned int ce_flags, base_flags, ret;
> +				ce_flags = ce->ce_flags;
> +				base_flags = base->ce_flags;
> +				/* only on-disk flags matter */
> +				ce->ce_flags   &= ondisk_flags;
> +				base->ce_flags &= ondisk_flags;
> +				ret = memcmp(&ce->ce_stat_data, &base->ce_stat_data,
> +					     offsetof(struct cache_entry, name) -
> +					     offsetof(struct cache_entry, ce_stat_data));
> +				ce->ce_flags = ce_flags;
> +				base->ce_flags = base_flags;
> +				if (ret)
> +					ce->ce_flags |= CE_UPDATE_IN_BASE;
> +			}
>  			discard_cache_entry(base);
>  			si->base->cache[ce->index - 1] = ce;
>  		}
>  		for (i = 0; i < si->base->cache_nr; i++) {
>  			ce = si->base->cache[i];
>  			if ((ce->ce_flags & CE_REMOVE) ||
> -- 
> 2.19.0.361.gafc87ffe72
> 

Re: [PATCH v2 0/5] Fix the racy split index problem

[Ævar Arnfjörð Bjarmason]

On Thu, Sep 27 2018, SZEDER Gábor wrote:

> This is the second attempt to fix the racy split index problem, which
> causes occasional failures in several random test scripts when run
> with 'GIT_TEST_SPLIT_INDEX=yes'.  The important details are in patches
> 1 and 5 (corresponding to v1's 3 and 5).

Thanks. I'm running the same sorts of tests I noted in
https://public-inbox.org/git/87va7ireuu.fsf@evledraar.gmail.com/ on
this. The fix Jeff had that you noted in
https://public-inbox.org/git/20180906151439.GA8016@localhost/ is now in
"master".

I take it your
https://github.com/szeder/git/commits/racy-split-index-fix is the same
as this submission? Anyway, I'm testing that cherry-picked on top of the
latest master.

Unfortunate that we couldn't get the isolated test you made in
https://public-inbox.org/git/20180907034942.GA10370@localhost/ but I
don't see how it could be added without some very liberal
getenv("GIT_TEST_blahblah"), so it's probably best to not add it,
particularly with the C rewrite of git-stash in-flight.

I'll report back when I have enough test data to say how these patches
affect the intermittent test failures under GIT_TEST_SPLIT_INDEX=yes.

Re: [PATCH v2 0/5] Fix the racy split index problem

[SZEDER Gábor]

On Thu, Sep 27, 2018 at 03:53:24PM +0200, Ævar Arnfjörð Bjarmason wrote:
> 
> On Thu, Sep 27 2018, SZEDER Gábor wrote:
> 
> > This is the second attempt to fix the racy split index problem, which
> > causes occasional failures in several random test scripts when run
> > with 'GIT_TEST_SPLIT_INDEX=yes'.  The important details are in patches
> > 1 and 5 (corresponding to v1's 3 and 5).
> 
> Thanks. I'm running the same sorts of tests I noted in
> https://public-inbox.org/git/87va7ireuu.fsf@evledraar.gmail.com/ on
> this. The fix Jeff had that you noted in
> https://public-inbox.org/git/20180906151439.GA8016@localhost/ is now in
> "master".
> 
> I take it your
> https://github.com/szeder/git/commits/racy-split-index-fix is the same
> as this submission?

Yes.

> Anyway, I'm testing that cherry-picked on top of the
> latest master.
> 
> Unfortunate that we couldn't get the isolated test you made in
> https://public-inbox.org/git/20180907034942.GA10370@localhost/

Nah, that's not an isolated test case, that's only a somewhat
narrowed-down, but rather reliable reproduction recipe while I still
had no idea what was going on :)

The _real_ isolated test is the last test in t1701, that's what it
eventually boiled down to.

> but I
> don't see how it could be added without some very liberal
> getenv("GIT_TEST_blahblah"), so it's probably best to not add it,
> particularly with the C rewrite of git-stash in-flight.
> 
> I'll report back when I have enough test data to say how these patches
> affect the intermittent test failures under GIT_TEST_SPLIT_INDEX=yes.

Re: [PATCH v2 0/5] Fix the racy split index problem

[Ævar Arnfjörð Bjarmason]

On Thu, Sep 27 2018, SZEDER Gábor wrote:

> On Thu, Sep 27, 2018 at 03:53:24PM +0200, Ævar Arnfjörð Bjarmason wrote:
>>
>> On Thu, Sep 27 2018, SZEDER Gábor wrote:
>>
>> > This is the second attempt to fix the racy split index problem, which
>> > causes occasional failures in several random test scripts when run
>> > with 'GIT_TEST_SPLIT_INDEX=yes'.  The important details are in patches
>> > 1 and 5 (corresponding to v1's 3 and 5).
>>
>> Thanks. I'm running the same sorts of tests I noted in
>> https://public-inbox.org/git/87va7ireuu.fsf@evledraar.gmail.com/ on
>> this. The fix Jeff had that you noted in
>> https://public-inbox.org/git/20180906151439.GA8016@localhost/ is now in
>> "master".
>>
>> I take it your
>> https://github.com/szeder/git/commits/racy-split-index-fix is the same
>> as this submission?
>
> Yes.
>
>> Anyway, I'm testing that cherry-picked on top of the
>> latest master.
>>
>> Unfortunate that we couldn't get the isolated test you made in
>> https://public-inbox.org/git/20180907034942.GA10370@localhost/
>
> Nah, that's not an isolated test case, that's only a somewhat
> narrowed-down, but rather reliable reproduction recipe while I still
> had no idea what was going on :)
>
> The _real_ isolated test is the last test in t1701, that's what it
> eventually boiled down to.

Thanks. I had ~400 runs of the tests I ran before and they were all
OK. Now trying also with t1701 (which I hadn't noticed was a new
test...).


>> but I
>> don't see how it could be added without some very liberal
>> getenv("GIT_TEST_blahblah"), so it's probably best to not add it,
>> particularly with the C rewrite of git-stash in-flight.
>>
>> I'll report back when I have enough test data to say how these patches
>> affect the intermittent test failures under GIT_TEST_SPLIT_INDEX=yes.

Re: [PATCH v2 1/5] split-index: add tests to demonstrate the racy split index problem

[SZEDER Gábor]

Junio,

On Thu, Sep 27, 2018 at 02:44:30PM +0200, SZEDER Gábor wrote:
> diff --git a/t/t1701-racy-split-index.sh b/t/t1701-racy-split-index.sh
> new file mode 100755
> index 0000000000..ebde418d7e
> --- /dev/null
> +++ b/t/t1701-racy-split-index.sh
> @@ -0,0 +1,218 @@
> +#!/bin/sh
> +
> +# This test can give false success if your machine is sufficiently
> +# slow or all trials happened to happen on second boundaries.
> +
> +test_description='racy split index'
> +
> +. ./test-lib.sh
> +
> +test_expect_success 'setup' '
> +	# Only split the index when the test explicitly says so.
> +	sane_unset GIT_TEST_SPLIT_INDEX GIT_FSMONITOR_TEST &&

Please note that this patch adds another use of the environment
variable GIT_FSMONITOR_TEST, while the topic 'bp/rename-test-env-var'
is about to rename that variable to GIT_TEST_FSMONITOR.

Re: [PATCH v2 1/5] split-index: add tests to demonstrate the racy split index problem

[SZEDER Gábor]

On Fri, Sep 28, 2018 at 02:48:43AM +0200, SZEDER Gábor wrote:
> Junio,
> 
> On Thu, Sep 27, 2018 at 02:44:30PM +0200, SZEDER Gábor wrote:
> > diff --git a/t/t1701-racy-split-index.sh b/t/t1701-racy-split-index.sh
> > new file mode 100755
> > index 0000000000..ebde418d7e
> > --- /dev/null
> > +++ b/t/t1701-racy-split-index.sh
> > @@ -0,0 +1,218 @@
> > +#!/bin/sh
> > +
> > +# This test can give false success if your machine is sufficiently
> > +# slow or all trials happened to happen on second boundaries.
> > +
> > +test_description='racy split index'
> > +
> > +. ./test-lib.sh
> > +
> > +test_expect_success 'setup' '
> > +	# Only split the index when the test explicitly says so.
> > +	sane_unset GIT_TEST_SPLIT_INDEX GIT_FSMONITOR_TEST &&
> 
> Please note that this patch adds another use of the environment
> variable GIT_FSMONITOR_TEST, while the topic 'bp/rename-test-env-var'
> is about to rename that variable to GIT_TEST_FSMONITOR.

Hang on for a sec.  I unset GIT_FSMONITOR_TEST in this test, because I
saw that 't1700-split-index.sh' unsets it, and I just followed suit.
But come to think of it, t1700 has to unset it, because some of its
tests check the index's SHA-1 checksum, and the FSMN extension would
interfere with that, of course.  However, that's not an issue for
t1701, because none of its tests care about the index's checksum, so
unsetting GIT_FSMONITOR_TEST is actually unnecessary here...  unless
it could have other side effects that I'm not aware of.

Re: [PATCH v2 0/5] Fix the racy split index problem

[Ævar Arnfjörð Bjarmason]

On Thu, Sep 27 2018, Ævar Arnfjörð Bjarmason wrote:

> On Thu, Sep 27 2018, SZEDER Gábor wrote:
>
>> On Thu, Sep 27, 2018 at 03:53:24PM +0200, Ævar Arnfjörð Bjarmason wrote:
>>>
>>> On Thu, Sep 27 2018, SZEDER Gábor wrote:
>>>
>>> > This is the second attempt to fix the racy split index problem, which
>>> > causes occasional failures in several random test scripts when run
>>> > with 'GIT_TEST_SPLIT_INDEX=yes'.  The important details are in patches
>>> > 1 and 5 (corresponding to v1's 3 and 5).
>>>
>>> Thanks. I'm running the same sorts of tests I noted in
>>> https://public-inbox.org/git/87va7ireuu.fsf@evledraar.gmail.com/ on
>>> this. The fix Jeff had that you noted in
>>> https://public-inbox.org/git/20180906151439.GA8016@localhost/ is now in
>>> "master".
>>>
>>> I take it your
>>> https://github.com/szeder/git/commits/racy-split-index-fix is the same
>>> as this submission?
>>
>> Yes.
>>
>>> Anyway, I'm testing that cherry-picked on top of the
>>> latest master.
>>>
>>> Unfortunate that we couldn't get the isolated test you made in
>>> https://public-inbox.org/git/20180907034942.GA10370@localhost/
>>
>> Nah, that's not an isolated test case, that's only a somewhat
>> narrowed-down, but rather reliable reproduction recipe while I still
>> had no idea what was going on :)
>>
>> The _real_ isolated test is the last test in t1701, that's what it
>> eventually boiled down to.
>
> Thanks. I had ~400 runs of the tests I ran before and they were all
> OK. Now trying also with t1701 (which I hadn't noticed was a new
> test...).

Ran that overnight with the same conditions as before. 2683 OK runs and
0 failures (and counting). So it seems like the combination of the two
fixed the split index bugs.

>>> but I
>>> don't see how it could be added without some very liberal
>>> getenv("GIT_TEST_blahblah"), so it's probably best to not add it,
>>> particularly with the C rewrite of git-stash in-flight.
>>>
>>> I'll report back when I have enough test data to say how these patches
>>> affect the intermittent test failures under GIT_TEST_SPLIT_INDEX=yes.

Re: [PATCH v2 0/5] Fix the racy split index problem

[SZEDER Gábor]

On Fri, Sep 28, 2018 at 08:57:53AM +0200, Ævar Arnfjörð Bjarmason wrote:
> > Thanks. I had ~400 runs of the tests I ran before and they were all
> > OK. Now trying also with t1701 (which I hadn't noticed was a new
> > test...).
> 
> Ran that overnight with the same conditions as before. 2683 OK runs and
> 0 failures (and counting). So it seems like the combination of the two
> fixed the split index bugs.

Yeah, I thought they would.  If you look at the first loop of
prepare_to_write_split_index() classifying which cache entries should
be included in the new split index, you'll see only two code paths
that could leave out an entry from the split index, i.e. where an
entry could be left with a non-zero 'ce->index' and without its
CE_UPDATE_IN_BASE flag set.  Now, with the fix in patch 5/5 both of
those code paths have the is_race_timestamp() check.

[PATCH v3 0/6] Fix the racy split index problem

[SZEDER Gábor]

Third round of fixing occasional test failures when run with
'GIT_TEST_SPLIT_INDEX=yes', with only two rather minor changes since the
previous version; just look at the trivial interdiff below.

SZEDER Gábor (6):
  t1700-split-index: document why FSMONITOR is disabled in this test
    script
  split-index: add tests to demonstrate the racy split index problem
  t1700-split-index: date back files to avoid racy situations
  split-index: count the number of deleted entries
  split-index: don't compare stat data of entries already marked for
    split index
  split-index: smudge and add racily clean cache entries to split index

 cache.h                     |   2 +
 read-cache.c                |   2 +-
 split-index.c               | 121 +++++++++++++++++---
 t/t1700-split-index.sh      |  52 +++++----
 t/t1701-racy-split-index.sh | 214 ++++++++++++++++++++++++++++++++++++
 5 files changed, 351 insertions(+), 40 deletions(-)
 create mode 100755 t/t1701-racy-split-index.sh

Interdiff:
diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
index c8acbc50d0..f053bf83eb 100755
--- a/t/t1700-split-index.sh
+++ b/t/t1700-split-index.sh
@@ -6,6 +6,9 @@ test_description='split index mode tests'
 
 # We need total control of index splitting here
 sane_unset GIT_TEST_SPLIT_INDEX
+# A couple of tests expect the index to have a specific checksum,
+# but the presence of the optional FSMN extension would interfere
+# with those checks, so disable it in this test script.
 sane_unset GIT_FSMONITOR_TEST
 
 # Create a file named as $1 with content read from stdin.
diff --git a/t/t1701-racy-split-index.sh b/t/t1701-racy-split-index.sh
index 7f16f5f7a3..5dc221ef38 100755
--- a/t/t1701-racy-split-index.sh
+++ b/t/t1701-racy-split-index.sh
@@ -9,7 +9,7 @@ test_description='racy split index'
 
 test_expect_success 'setup' '
 	# Only split the index when the test explicitly says so.
-	sane_unset GIT_TEST_SPLIT_INDEX GIT_FSMONITOR_TEST &&
+	sane_unset GIT_TEST_SPLIT_INDEX &&
 	git config splitIndex.maxPercentChange 100 &&
 
 	echo "cached content" >racy-file &&
-- 
2.19.0.361.gafc87ffe72

[PATCH v3 1/6] t1700-split-index: document why FSMONITOR is disabled in this test script

[SZEDER Gábor]

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 t/t1700-split-index.sh | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
index be22398a85..822257ff7d 100755
--- a/t/t1700-split-index.sh
+++ b/t/t1700-split-index.sh
@@ -6,6 +6,9 @@ test_description='split index mode tests'
 
 # We need total control of index splitting here
 sane_unset GIT_TEST_SPLIT_INDEX
+# A couple of tests expect the index to have a specific checksum,
+# but the presence of the optional FSMN extension would interfere
+# with those checks, so disable it in this test script.
 sane_unset GIT_FSMONITOR_TEST
 
 test_expect_success 'enable split index' '
-- 
2.19.0.361.gafc87ffe72

[PATCH v3 2/6] split-index: add tests to demonstrate the racy split index problem

[SZEDER Gábor]

Ever since the split index feature was introduced [1], refreshing a
split index is prone to a variant of the classic racy git problem.
There are a couple of unrelated tests in the test suite that
occasionally fail when run with 'GIT_TEST_SPLIT_INDEX=yes', but
't1700-split-index.sh', the only test script focusing solely on split
index, has never noticed this issue, because it only cares about how
the index is split under various circumstances and all the different
ways to turn the split index feature on and off.

Add a dedicated test script 't1701-racy-split-index.sh' to exercise
the split index feature in racy situations as well; kind of a
"t0010-racy-git.sh for split index" but with modern style (the tests
do everything in &&-chained list of commands in 'test_expect_...'
blocks, and use 'test_cmp' for more informative output on failure).

The tests cover the following sequences of index splitting, updating,
and racy file modifications, with the last two cases demonstrating the
racy split index problem:

  1. Split the index while adding a racily clean file:

       echo "cached content" >file
       git update-index --split-index --add file
       echo "dirty worktree" >file    # size stays the same

     This case already works properly.  Even though the cache entry's
     stat data matches with the modifid file in the worktree,
     subsequent git commands will notice that the (split) index and
     the file have the same mtime, and then will go on to check the
     file's content and notice its dirtiness.

  2. Add a racily clean file to an already split index:

       git update-index --split-index
       echo "cached content" >file
       git update-index --add file
       echo "dirty worktree" >file

     This case already works properly.  After the second 'git
     update-index' writes the newly added file's cache entry to the
     new split index, it basically works in the same way as case #1.

  3. Split the index when it (i.e. the not yet splitted index)
     contains a racily clean cache entry, i.e. an entry whose cached
     stat data matches with the corresponding file in the worktree and
     the cached mtime matches that of the index:

       echo "cached content" >file
       git update-index --add file
       echo "dirty worktree" >file
       # ... wait ...
       git update-index --split-index --add other-file

     This case already works properly.  The shared index is written by
     do_write_index(), i.e. the same function that is responsible for
     writing "regular" and split indexes as well.  This function
     cleverly notices the racily clean cache entry, and writes the
     entry to the new shared index with smudged stat data, i.e. file
     size set to 0.  When subsequent git commands read the index, they
     will notice that the smudged stat data doesn't match with the
     file in the worktree, and then go on to check the file's content
     and notice its dirtiness.

  4. Update the split index when it contains a racily clean cache
     entry:

       git update-index --split-index
       echo "cached content" >file
       git update-index --add file
       echo "dirty worktree" >file
       # ... wait ...
       git update-index --add other-file

     This case already works properly.  After the second 'git
     update-index' the newly added file's cache entry is only stored
     in the split index.  If a cache entry is present in the split
     index (even if it is a replacement of an outdated entry in the
     shared index), then it will always be included in the new split
     index on subsequent split index updates (until the file is
     removed or a new shared index is written), independently from
     whether the entry is racily clean or not.  When do_write_index()
     writes the new split index, it notices the racily clean cache
     entry, and smudges its stat date.  Subsequent git commands
     reading the index will notice the smudged stat data and then go
     on to check the file's content and notice its dirtiness.

  5. Update the split index when a racily clean cache entry is stored
     only in the shared index:

       echo "cached content" >file
       git update-index --split-index --add file
       echo "dirty worktree" >file
       # ... wait ...
       git update-index --add other-file

     This case fails due to the racy split index problem.  In the
     second 'git update-index' prepare_to_write_split_index() decides,
     among other things, which cache entries stored only in the shared
     index should be replaced in the new split index.  Alas, this
     function never looks out for racily clean cache entries, and
     since the file's stat data in the worktree hasn't changed since
     the shared index was written, the entry won't be replaced in the
     new split index.  Consequently, do_write_index() doesn't even get
     this racily clean cache entry, and can't smudge its stat data.
     Subsequent git commands will then see that the index has more
     recent mtime than the file and that the (not smudged) cached stat
     data still matches with the file in the worktree, and,
     ultimately, will erroneously consider the file clean.

  6. Update the split index after unpack_trees() copied a racily clean
     cache entry from the shared index:

       echo "cached content" >file
       git update-index --split-index --add file
       echo "dirty worktree" >file
       # ... wait ...
       git read-tree -m HEAD

     This case fails due to the racy split index problem.  This
     basically fails for the same reason as case #5 above, but there
     is one important difference, which warrants the dedicated test.
     While that second 'git update-index' in case #5 updates
     index_state in place, in this case 'git read-tree -m' calls
     unpack_trees(), which throws out the entire index, and constructs
     a new one from the (potentially updated) copies of the original's
     cache entries.  Consequently, when prepare_to_write_split_index()
     gets to work on this reconstructed index, it takes a different
     code path than in case #5 when deciding which cache entries in
     the shared index should be replaced.  The result is the same,
     though: the racily clean cache entry goes unnoticed, it isn't
     added to the split index with smudged stat data, and subsequent
     git commands will then erroneously consider the file clean.

Note that in the last two 'test_expect_failure' cases I omitted the
'#' (as in nr. of trial) from the tests' name on purpose for now, as
it confuses 'prove' into thinking that those tests failed
unexpectedly.

[1] In the branch leading to the merge commit v2.1.0-rc0~45 (Merge
    branch 'nd/split-index', 2014-07-16).

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 t/t1701-racy-split-index.sh | 218 ++++++++++++++++++++++++++++++++++++
 1 file changed, 218 insertions(+)
 create mode 100755 t/t1701-racy-split-index.sh

diff --git a/t/t1701-racy-split-index.sh b/t/t1701-racy-split-index.sh
new file mode 100755
index 0000000000..fbb77046da
--- /dev/null
+++ b/t/t1701-racy-split-index.sh
@@ -0,0 +1,218 @@
+#!/bin/sh
+
+# This test can give false success if your machine is sufficiently
+# slow or all trials happened to happen on second boundaries.
+
+test_description='racy split index'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	# Only split the index when the test explicitly says so.
+	sane_unset GIT_TEST_SPLIT_INDEX &&
+	git config splitIndex.maxPercentChange 100 &&
+
+	echo "cached content" >racy-file &&
+	git add racy-file &&
+	git commit -m initial &&
+
+	echo something >other-file &&
+	# No raciness with this file.
+	test-tool chmtime =-20 other-file &&
+
+	echo "+cached content" >expect
+'
+
+check_cached_diff () {
+	git diff-index --patch --cached $EMPTY_TREE racy-file >diff &&
+	tail -1 diff >actual &&
+	test_cmp expect actual
+}
+
+trials="0 1 2 3 4"
+for trial in $trials
+do
+	test_expect_success "split the index while adding a racily clean file #$trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		# The next three commands must be run within the same
+		# second (so both writes to racy-file result in the same
+		# mtime) to create the interesting racy situation.
+		echo "cached content" >racy-file &&
+
+		# Update and split the index.  The cache entry of
+		# racy-file will be stored only in the shared index.
+		git update-index --split-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Subsequent git commands should notice that racy-file
+		# and the split index have the same mtime, and check
+		# the content of the file to see if it is actually
+		# clean.
+		check_cached_diff
+	'
+done
+
+for trial in $trials
+do
+	test_expect_success "add a racily clean file to an already split index #$trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		git update-index --split-index &&
+
+		# The next three commands must be run within the same
+		# second.
+		echo "cached content" >racy-file &&
+
+		# Update the split index.  The cache entry of racy-file
+		# will be stored only in the split index.
+		git update-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Subsequent git commands should notice that racy-file
+		# and the split index have the same mtime, and check
+		# the content of the file to see if it is actually
+		# clean.
+		check_cached_diff
+	'
+done
+
+for trial in $trials
+do
+	test_expect_success "split the index when the index contains a racily clean cache entry #$trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		# The next three commands must be run within the same
+		# second.
+		echo "cached content" >racy-file &&
+
+		git update-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Now wait a bit to ensure that the split index written
+		# below will get a more recent mtime than racy-file.
+		sleep 1 &&
+
+		# Update and split the index when the index contains
+		# the racily clean cache entry of racy-file.
+		# A corresponding replacement cache entry with smudged
+		# stat data should be added to the new split index.
+		git update-index --split-index --add other-file &&
+
+		# Subsequent git commands should notice the smudged
+		# stat data in the replacement cache entry and that it
+		# doesnt match with the file the worktree.
+		check_cached_diff
+	'
+done
+
+for trial in $trials
+do
+	test_expect_success "update the split index when it contains a new racily clean cache entry #$trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		git update-index --split-index &&
+
+		# The next three commands must be run within the same
+		# second.
+		echo "cached content" >racy-file &&
+
+		# Update the split index.  The cache entry of racy-file
+		# will be stored only in the split index.
+		git update-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Now wait a bit to ensure that the split index written
+		# below will get a more recent mtime than racy-file.
+		sleep 1 &&
+
+		# Update the split index when the racily clean cache
+		# entry of racy-file is only stored in the split index.
+		# An updated cache entry with smudged stat data should
+		# be added to the new split index.
+		git update-index --add other-file &&
+
+		# Subsequent git commands should notice the smudged
+		# stat data.
+		check_cached_diff
+	'
+done
+
+for trial in $trials
+do
+	test_expect_failure "update the split index when a racily clean cache entry is stored only in the shared index $trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		# The next three commands must be run within the same
+		# second.
+		echo "cached content" >racy-file &&
+
+		# Update and split the index.  The cache entry of
+		# racy-file will be stored only in the shared index.
+		git update-index --split-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Now wait a bit to ensure that the split index written
+		# below will get a more recent mtime than racy-file.
+		sleep 1 &&
+
+		# Update the split index when the racily clean cache
+		# entry of racy-file is only stored in the shared index.
+		# A corresponding replacement cache entry with smudged
+		# stat data should be added to the new split index.
+		#
+		# Alas, such a smudged replacement entry is not added!
+		git update-index --add other-file &&
+
+		# Subsequent git commands should notice the smudged
+		# stat data.
+		check_cached_diff
+	'
+done
+
+for trial in $trials
+do
+	test_expect_failure "update the split index after unpack trees() copied a racily clean cache entry from the shared index $trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		# The next three commands must be run within the same
+		# second.
+		echo "cached content" >racy-file &&
+
+		# Update and split the index.  The cache entry of
+		# racy-file will be stored only in the shared index.
+		git update-index --split-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Now wait a bit to ensure that the split index written
+		# below will get a more recent mtime than racy-file.
+		sleep 1 &&
+
+		# Update the split index after unpack_trees() copied the
+		# racily clean cache entry of racy-file from the shared
+		# index.  A corresponding replacement cache entry
+		# with smudged stat data should be added to the new
+		# split index.
+		#
+		# Alas, such a smudged replacement entry is not added!
+		git read-tree -m HEAD &&
+
+		# Subsequent git commands should notice the smudged
+		# stat data.
+		check_cached_diff
+	'
+done
+
+test_done
-- 
2.19.0.361.gafc87ffe72

[PATCH v3 3/6] t1700-split-index: date back files to avoid racy situations

[SZEDER Gábor]

't1700-split-index.sh' checks that the index was split correctly under
various circumstances and that all the different ways to turn the
split index feature on and off work correctly.  To do so, most of its
tests use 'test-tool dump-split-index' to see which files have their
cache entries in the split index.  All these tests assume that all
cache entries are written to the shared index (called "base"
throughout these tests) when a new shared index is created.  This is
an implementation detail: most git commands (basically all except 'git
update-index') don't care or know at all about split index or whether
a cache entry is stored in the split or shared index.

As demonstrated in the previous patch, refreshing a split index is
prone to a variant of the classic racy git issue.  The next patch will
fix this issue, but while doing so it will also slightly change this
behaviour: only cache entries with mtime in the past will be written
only to the newly created shared index, but racily clean cache entries
will be written to the new split index (with smudged stat data).

While this upcoming change won't at all affect any git commands, it
will violate the above mentioned assumption of 't1700's tests.  Since
these tests create or modify files and create or refresh the split
index in rapid succession, there are plenty of racily clean cache
entries to be dealt with, which will then be written to the new split
indexes, and, ultimately, will cause several tests in 't1700' to fail.

Let's prepare 't1700-split-index.sh' for this upcoming change and
modify its tests to avoid racily clean files by backdating the mtime
of any file modifications (and since a lot of tests create or modify
files, encapsulate it into a helper function).

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 t/t1700-split-index.sh | 49 ++++++++++++++++++++++++------------------
 1 file changed, 28 insertions(+), 21 deletions(-)

diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
index 822257ff7d..f053bf83eb 100755
--- a/t/t1700-split-index.sh
+++ b/t/t1700-split-index.sh
@@ -11,6 +11,13 @@ sane_unset GIT_TEST_SPLIT_INDEX
 # with those checks, so disable it in this test script.
 sane_unset GIT_FSMONITOR_TEST
 
+# Create a file named as $1 with content read from stdin.
+# Set the file's mtime to a few seconds in the past to avoid racy situations.
+create_non_racy_file () {
+	cat >"$1" &&
+	test-tool chmtime =-5 "$1"
+}
+
 test_expect_success 'enable split index' '
 	git config splitIndex.maxPercentChange 100 &&
 	git update-index --split-index &&
@@ -34,7 +41,7 @@ test_expect_success 'enable split index' '
 '
 
 test_expect_success 'add one file' '
-	: >one &&
+	create_non_racy_file one &&
 	git update-index --add one &&
 	git ls-files --stage >ls-files.actual &&
 	cat >ls-files.expect <<-EOF &&
@@ -86,7 +93,7 @@ test_expect_success 'enable split index again, "one" now belongs to base index"'
 '
 
 test_expect_success 'modify original file, base index untouched' '
-	echo modified >one &&
+	echo modified | create_non_racy_file one &&
 	git update-index one &&
 	git ls-files --stage >ls-files.actual &&
 	cat >ls-files.expect <<-EOF &&
@@ -105,7 +112,7 @@ test_expect_success 'modify original file, base index untouched' '
 '
 
 test_expect_success 'add another file, which stays index' '
-	: >two &&
+	create_non_racy_file two &&
 	git update-index --add two &&
 	git ls-files --stage >ls-files.actual &&
 	cat >ls-files.expect <<-EOF &&
@@ -158,7 +165,7 @@ test_expect_success 'remove file in base index' '
 '
 
 test_expect_success 'add original file back' '
-	: >one &&
+	create_non_racy_file one &&
 	git update-index --add one &&
 	git ls-files --stage >ls-files.actual &&
 	cat >ls-files.expect <<-EOF &&
@@ -177,7 +184,7 @@ test_expect_success 'add original file back' '
 '
 
 test_expect_success 'add new file' '
-	: >two &&
+	create_non_racy_file two &&
 	git update-index --add two &&
 	git ls-files --stage >actual &&
 	cat >expect <<-EOF &&
@@ -221,7 +228,7 @@ test_expect_success 'rev-parse --shared-index-path' '
 
 test_expect_success 'set core.splitIndex config variable to true' '
 	git config core.splitIndex true &&
-	: >three &&
+	create_non_racy_file three &&
 	git update-index --add three &&
 	git ls-files --stage >ls-files.actual &&
 	cat >ls-files.expect <<-EOF &&
@@ -256,9 +263,9 @@ test_expect_success 'set core.splitIndex config variable to false' '
 	test_cmp expect actual
 '
 
-test_expect_success 'set core.splitIndex config variable to true' '
+test_expect_success 'set core.splitIndex config variable back to true' '
 	git config core.splitIndex true &&
-	: >three &&
+	create_non_racy_file three &&
 	git update-index --add three &&
 	BASE=$(test-tool dump-split-index .git/index | grep "^base") &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
@@ -268,7 +275,7 @@ test_expect_success 'set core.splitIndex config variable to true' '
 	deletions:
 	EOF
 	test_cmp expect actual &&
-	: >four &&
+	create_non_racy_file four &&
 	git update-index --add four &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
 	cat >expect <<-EOF &&
@@ -282,7 +289,7 @@ test_expect_success 'set core.splitIndex config variable to true' '
 
 test_expect_success 'check behavior with splitIndex.maxPercentChange unset' '
 	git config --unset splitIndex.maxPercentChange &&
-	: >five &&
+	create_non_racy_file five &&
 	git update-index --add five &&
 	BASE=$(test-tool dump-split-index .git/index | grep "^base") &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
@@ -292,7 +299,7 @@ test_expect_success 'check behavior with splitIndex.maxPercentChange unset' '
 	deletions:
 	EOF
 	test_cmp expect actual &&
-	: >six &&
+	create_non_racy_file six &&
 	git update-index --add six &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
 	cat >expect <<-EOF &&
@@ -306,7 +313,7 @@ test_expect_success 'check behavior with splitIndex.maxPercentChange unset' '
 
 test_expect_success 'check splitIndex.maxPercentChange set to 0' '
 	git config splitIndex.maxPercentChange 0 &&
-	: >seven &&
+	create_non_racy_file seven &&
 	git update-index --add seven &&
 	BASE=$(test-tool dump-split-index .git/index | grep "^base") &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
@@ -316,7 +323,7 @@ test_expect_success 'check splitIndex.maxPercentChange set to 0' '
 	deletions:
 	EOF
 	test_cmp expect actual &&
-	: >eight &&
+	create_non_racy_file eight &&
 	git update-index --add eight &&
 	BASE=$(test-tool dump-split-index .git/index | grep "^base") &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
@@ -329,17 +336,17 @@ test_expect_success 'check splitIndex.maxPercentChange set to 0' '
 '
 
 test_expect_success 'shared index files expire after 2 weeks by default' '
-	: >ten &&
+	create_non_racy_file ten &&
 	git update-index --add ten &&
 	test $(ls .git/sharedindex.* | wc -l) -gt 2 &&
 	just_under_2_weeks_ago=$((5-14*86400)) &&
 	test-tool chmtime =$just_under_2_weeks_ago .git/sharedindex.* &&
-	: >eleven &&
+	create_non_racy_file eleven &&
 	git update-index --add eleven &&
 	test $(ls .git/sharedindex.* | wc -l) -gt 2 &&
 	just_over_2_weeks_ago=$((-1-14*86400)) &&
 	test-tool chmtime =$just_over_2_weeks_ago .git/sharedindex.* &&
-	: >twelve &&
+	create_non_racy_file twelve &&
 	git update-index --add twelve &&
 	test $(ls .git/sharedindex.* | wc -l) -le 2
 '
@@ -347,12 +354,12 @@ test_expect_success 'shared index files expire after 2 weeks by default' '
 test_expect_success 'check splitIndex.sharedIndexExpire set to 16 days' '
 	git config splitIndex.sharedIndexExpire "16.days.ago" &&
 	test-tool chmtime =$just_over_2_weeks_ago .git/sharedindex.* &&
-	: >thirteen &&
+	create_non_racy_file thirteen &&
 	git update-index --add thirteen &&
 	test $(ls .git/sharedindex.* | wc -l) -gt 2 &&
 	just_over_16_days_ago=$((-1-16*86400)) &&
 	test-tool chmtime =$just_over_16_days_ago .git/sharedindex.* &&
-	: >fourteen &&
+	create_non_racy_file fourteen &&
 	git update-index --add fourteen &&
 	test $(ls .git/sharedindex.* | wc -l) -le 2
 '
@@ -361,13 +368,13 @@ test_expect_success 'check splitIndex.sharedIndexExpire set to "never" and "now"
 	git config splitIndex.sharedIndexExpire never &&
 	just_10_years_ago=$((-365*10*86400)) &&
 	test-tool chmtime =$just_10_years_ago .git/sharedindex.* &&
-	: >fifteen &&
+	create_non_racy_file fifteen &&
 	git update-index --add fifteen &&
 	test $(ls .git/sharedindex.* | wc -l) -gt 2 &&
 	git config splitIndex.sharedIndexExpire now &&
 	just_1_second_ago=-1 &&
 	test-tool chmtime =$just_1_second_ago .git/sharedindex.* &&
-	: >sixteen &&
+	create_non_racy_file sixteen &&
 	git update-index --add sixteen &&
 	test $(ls .git/sharedindex.* | wc -l) -le 2
 '
@@ -382,7 +389,7 @@ do
 		# Create one new shared index file
 		git config core.sharedrepository "$mode" &&
 		git config core.splitIndex true &&
-		: >one &&
+		create_non_racy_file one &&
 		git update-index --add one &&
 		echo "$modebits" >expect &&
 		test_modebits .git/index >actual &&
-- 
2.19.0.361.gafc87ffe72

[PATCH v3 4/6] split-index: count the number of deleted entries

[SZEDER Gábor]

'struct split_index' contains the field 'nr_deletions', whose name
with the 'nr_' prefix suggests that it contains the number of deleted
cache entries.  However, barring its initialization to 0, this field
is only ever set to 1, indicating that there is at least one deleted
entry, but not the number of deleted entries.  Luckily, this doesn't
cause any issues (other than confusing the reader, that is), because
the only place reading this field uses it in the same sense, i.e.: 'if
(si->nr_deletions)'.

To avoid confusion, we could either rename this field to something
like 'has_deletions' to make its name match its role, or make it a
counter of deleted cache entries to match its name.

Let's make it a counter, to keep it in sync with the related field
'nr_replacements', which does contain the number of replaced cache
entries.  This will also give developers debugging the split index
code easy access to the number of deleted cache entries.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 split-index.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/split-index.c b/split-index.c
index 84f067e10d..548272ec33 100644
--- a/split-index.c
+++ b/split-index.c
@@ -111,7 +111,7 @@ static void mark_entry_for_delete(size_t pos, void *data)
 		die("position for delete %d exceeds base index size %d",
 		    (int)pos, istate->cache_nr);
 	istate->cache[pos]->ce_flags |= CE_REMOVE;
-	istate->split_index->nr_deletions = 1;
+	istate->split_index->nr_deletions++;
 }
 
 static void replace_entry(size_t pos, void *data)
-- 
2.19.0.361.gafc87ffe72

[PATCH v3 5/6] split-index: don't compare stat data of entries already marked for split index

[SZEDER Gábor]

When unpack_trees() constructs a new index, it copies cache entries
from the original index [1].  prepare_to_write_split_index() has to
deal with this, and it has a dedicated code path for copied entries
that are present in the shared index, where it compares the cached
data in the corresponding copied and original entries.  If the cached
data matches, then they are considered the same; if it differs, then
the copied entry will be marked for inclusion as a replacement entry
in the just about to be written split index by setting the
CE_UPDATE_IN_BASE flag.

However, a cache entry already has its CE_UPDATE_IN_BASE flag set upon
reading the split index, if the entry already has a replacement entry
there, or upon refreshing the cached stat data, if the corresponding
file was modified.  The state of this flag is then preserved when
unpack_trees() copies a cache entry from the shared index.

So modify prepare_to_write_split_index() to check the copied cache
entries' CE_UPDATE_IN_BASE flag first, and skip the thorough
comparison of cached data if the flag is already set.

Note that comparing the cached data in copied and original entries in
the shared index might actually be entirely unnecessary.  In theory
all code paths refreshing the cached stat data of an entry in the
shared index should set the CE_UPDATE_IN_BASE flag in that entry, and
unpack_trees() should preserve this flag when copying cache entries.
This means that the cached data is only ever changed if the
CE_UPDATE_IN_BASE flag is set as well.  Our test suite seems to
confirm this: instrumenting the conditions in question and running the
test suite repeatedly with 'GIT_TEST_SPLIT_INDEX=yes' showed that the
cached data in a copied entry differs from the data in the shared
entry only if its CE_UPDATE_IN_BASE flag is indeed set.

In practice, however, our test suite doesn't have 100% coverage,
GIT_TEST_SPLIT_INDEX is inherently random, and I certainly can't claim
to possess complete understanding of what goes on in unpack_trees()...
Therefore I kept the comparison of the cached data when
CE_UPDATE_IN_BASE is not set, just in case that an unnoticed or future
code path were to accidentally miss setting this flag upon refreshing
the cached stat data or unpack_trees() were to drop this flag while
copying a cache entry.

[1] Note that when unpack_trees() constructs the new index and decides
    that a cache entry should now refer to different content than what
    was recorded in the original index (e.g. 'git read-tree -m
    HEAD^'), then that can't really be considered a copy of the
    original, but rather the creation of a new entry.  Notably and
    pertinent to the split index feature, such a new entry doesn't
    have a reference to the original's shared index entry anymore,
    i.e. its 'index' field is set to 0.  Consequently, such an entry
    is treated by prepare_to_write_split_index() as an entry not
    present in the shared index and it will be added to the new split
    index, while the original entry will be marked as deleted, and
    neither the above discussion nor the changes in this patch apply
    to them.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 split-index.c | 79 ++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 62 insertions(+), 17 deletions(-)

diff --git a/split-index.c b/split-index.c
index 548272ec33..7d8799f6b7 100644
--- a/split-index.c
+++ b/split-index.c
@@ -207,13 +207,28 @@ void prepare_to_write_split_index(struct index_state *istate)
 		 */
 		for (i = 0; i < istate->cache_nr; i++) {
 			struct cache_entry *base;
-			/* namelen is checked separately */
-			const unsigned int ondisk_flags =
-				CE_STAGEMASK | CE_VALID | CE_EXTENDED_FLAGS;
-			unsigned int ce_flags, base_flags, ret;
 			ce = istate->cache[i];
-			if (!ce->index)
+			if (!ce->index) {
+				/*
+				 * During simple update index operations this
+				 * is a cache entry that is not present in
+				 * the shared index.  It will be added to the
+				 * split index.
+				 *
+				 * However, it might also represent a file
+				 * that already has a cache entry in the
+				 * shared index, but a new index has just
+				 * been constructed by unpack_trees(), and
+				 * this entry now refers to different content
+				 * than what was recorded in the original
+				 * index, e.g. during 'read-tree -m HEAD^' or
+				 * 'checkout HEAD^'.  In this case the
+				 * original entry in the shared index will be
+				 * marked as deleted, and this entry will be
+				 * added to the split index.
+				 */
 				continue;
+			}
 			if (ce->index > si->base->cache_nr) {
 				ce->index = 0;
 				continue;
@@ -227,18 +242,48 @@ void prepare_to_write_split_index(struct index_state *istate)
 				ce->index = 0;
 				continue;
 			}
-			ce_flags = ce->ce_flags;
-			base_flags = base->ce_flags;
-			/* only on-disk flags matter */
-			ce->ce_flags   &= ondisk_flags;
-			base->ce_flags &= ondisk_flags;
-			ret = memcmp(&ce->ce_stat_data, &base->ce_stat_data,
-				     offsetof(struct cache_entry, name) -
-				     offsetof(struct cache_entry, ce_stat_data));
-			ce->ce_flags = ce_flags;
-			base->ce_flags = base_flags;
-			if (ret)
-				ce->ce_flags |= CE_UPDATE_IN_BASE;
+			/*
+			 * This is the copy of a cache entry that is present
+			 * in the shared index, created by unpack_trees()
+			 * while it constructed a new index.
+			 */
+			if (ce->ce_flags & CE_UPDATE_IN_BASE) {
+				/*
+				 * Already marked for inclusion in the split
+				 * index, either because the corresponding
+				 * file was modified and the cached stat data
+				 * was refreshed, or because the original
+				 * entry already had a replacement entry in
+				 * the split index.
+				 * Nothing to do.
+				 */
+			} else {
+				/*
+				 * Thoroughly compare the cached data to see
+				 * whether it should be marked for inclusion
+				 * in the split index.
+				 *
+				 * This comparison might be unnecessary, as
+				 * code paths modifying the cached data do
+				 * set CE_UPDATE_IN_BASE as well.
+				 */
+				const unsigned int ondisk_flags =
+					CE_STAGEMASK | CE_VALID |
+					CE_EXTENDED_FLAGS;
+				unsigned int ce_flags, base_flags, ret;
+				ce_flags = ce->ce_flags;
+				base_flags = base->ce_flags;
+				/* only on-disk flags matter */
+				ce->ce_flags   &= ondisk_flags;
+				base->ce_flags &= ondisk_flags;
+				ret = memcmp(&ce->ce_stat_data, &base->ce_stat_data,
+					     offsetof(struct cache_entry, name) -
+					     offsetof(struct cache_entry, ce_stat_data));
+				ce->ce_flags = ce_flags;
+				base->ce_flags = base_flags;
+				if (ret)
+					ce->ce_flags |= CE_UPDATE_IN_BASE;
+			}
 			discard_cache_entry(base);
 			si->base->cache[ce->index - 1] = ce;
 		}
-- 
2.19.0.361.gafc87ffe72

[PATCH v3 6/6] split-index: smudge and add racily clean cache entries to split index

[SZEDER Gábor]

Ever since the split index feature was introduced [1], refreshing a
split index is prone to a variant of the classic racy git problem.

Consider the following sequence of commands updating the split index
when the shared index contains a racily clean cache entry, i.e. an
entry whose cached stat data matches with the corresponding file in
the worktree and the cached mtime matches that of the index:

  echo "cached content" >file
  git update-index --split-index --add file
  echo "dirty worktree" >file    # size stays the same!
  # ... wait ...
  git update-index --add other-file

Normally, when a non-split index is updated, then do_write_index()
(the function responsible for writing all kinds of indexes, "regular",
split, and shared) recognizes racily clean cache entries, and writes
them with smudged stat data, i.e. with file size set to 0.  When
subsequent git commands read the index, they will notice that the
smudged stat data doesn't match with the file in the worktree, and
then go on to check the file's content and notice its dirtiness.

In the above example, however, in the second 'git update-index'
prepare_to_write_split_index() decides which cache entries stored only
in the shared index should be replaced in the new split index.  Alas,
this function never looks out for racily clean cache entries, and
since the file's stat data in the worktree hasn't changed since the
shared index was written, it won't be replaced in the new split index.
Consequently, do_write_index() doesn't even get this racily clean
cache entry, and can't smudge its stat data.  Subsequent git commands
will then see that the index has more recent mtime than the file and
that the (not smudged) cached stat data still matches with the file in
the worktree, and, ultimately, will erroneously consider the file
clean.

Modify prepare_to_write_split_index() to recognize racily clean cache
entries, and mark them to be added to the split index.  Note that
there are two places where it should check raciness: first those cache
entries that are only stored in the shared index, and then those that
have been copied by unpack_trees() from the shared index while it
constructed a new index.  This way do_write_index() will get these
racily clean cache entries as well, and will then write them with
smudged stat data to the new split index.

Note that after this change if the index is split when it contains a
racily clean cache entry, then a smudged cache entry will be written
both to the new shared and to the new split indexes.  This doesn't
affect regular git commands: as far as they are concerned this is just
an entry in the split index replacing an outdated entry in the shared
index.  It did affect a few tests in 't1700-split-index.sh', though,
because they actually check which entries are stored in the split
index; a previous patch in this series has already made the necessary
adjustments in 't1700'.  And racily clean cache entries and index
splitting are rare enough to not worry about the resulting duplicated
smudged cache entries, and the additional complexity required to
prevent them is not worth it.

Several tests failed occasionally when the test suite was run with
'GIT_TEST_SPLIT_INDEX=yes'.  Here are those that I managed to trace
back to this racy split index problem, starting with those failing
more frequently, with a link to a failing Travis CI build job for
each.  The highlighted line [2] shows when the racy file was written,
which is not always in the failing test but in a preceeding setup
test.

  t3903-stash.sh:
    https://travis-ci.org/git/git/jobs/385542084#L5858

  t4024-diff-optimize-common.sh:
    https://travis-ci.org/git/git/jobs/386531969#L3174

  t4015-diff-whitespace.sh:
    https://travis-ci.org/git/git/jobs/360797600#L8215

  t2200-add-update.sh:
    https://travis-ci.org/git/git/jobs/382543426#L3051

  t0090-cache-tree.sh:
    https://travis-ci.org/git/git/jobs/416583010#L3679

There might be others, e.g. perhaps 't1000-read-tree-m-3way.sh' and
others using 'lib-read-tree-m-3way.sh', but I couldn't confirm yet.

[1] In the branch leading to the merge commit v2.1.0-rc0~45 (Merge
    branch 'nd/split-index', 2014-07-16).

[2] Note that those highlighted lines are in the 'after failure' fold,
    and your browser might unhelpfully fold it up before you could
    take a good look.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 cache.h                     |  2 ++
 read-cache.c                |  2 +-
 split-index.c               | 42 ++++++++++++++++++++++++++++++++++++-
 t/t1701-racy-split-index.sh |  8 ++-----
 4 files changed, 46 insertions(+), 8 deletions(-)

diff --git a/cache.h b/cache.h
index 4d014541ab..3f419b6c79 100644
--- a/cache.h
+++ b/cache.h
@@ -781,6 +781,8 @@ extern void *read_blob_data_from_index(const struct index_state *, const char *,
 #define CE_MATCH_REFRESH		0x10
 /* don't refresh_fsmonitor state or do stat comparison even if CE_FSMONITOR_VALID is true */
 #define CE_MATCH_IGNORE_FSMONITOR 0X20
+extern int is_racy_timestamp(const struct index_state *istate,
+			     const struct cache_entry *ce);
 extern int ie_match_stat(struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
 extern int ie_modified(struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
 
diff --git a/read-cache.c b/read-cache.c
index 7b1354d759..8f644f68b4 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -337,7 +337,7 @@ static int is_racy_stat(const struct index_state *istate,
 		);
 }
 
-static int is_racy_timestamp(const struct index_state *istate,
+int is_racy_timestamp(const struct index_state *istate,
 			     const struct cache_entry *ce)
 {
 	return (!S_ISGITLINK(ce->ce_mode) &&
diff --git a/split-index.c b/split-index.c
index 7d8799f6b7..d989b27286 100644
--- a/split-index.c
+++ b/split-index.c
@@ -235,8 +235,39 @@ void prepare_to_write_split_index(struct index_state *istate)
 			}
 			ce->ce_flags |= CE_MATCHED; /* or "shared" */
 			base = si->base->cache[ce->index - 1];
-			if (ce == base)
+			if (ce == base) {
+				/* The entry is present in the shared index. */
+				if (ce->ce_flags & CE_UPDATE_IN_BASE) {
+					/*
+					 * Already marked for inclusion in
+					 * the split index, either because
+					 * the corresponding file was
+					 * modified and the cached stat data
+					 * was refreshed, or because there
+					 * is already a replacement entry in
+					 * the split index.
+					 * Nothing more to do here.
+					 */
+				} else if (!ce_uptodate(ce) &&
+					   is_racy_timestamp(istate, ce)) {
+					/*
+					 * A racily clean cache entry stored
+					 * only in the shared index: it must
+					 * be added to the split index, so
+					 * the subsequent do_write_index()
+					 * can smudge its stat data.
+					 */
+					ce->ce_flags |= CE_UPDATE_IN_BASE;
+				} else {
+					/*
+					 * The entry is only present in the
+					 * shared index and it was not
+					 * refreshed.
+					 * Just leave it there.
+					 */
+				}
 				continue;
+			}
 			if (ce->ce_namelen != base->ce_namelen ||
 			    strcmp(ce->name, base->name)) {
 				ce->index = 0;
@@ -257,6 +288,15 @@ void prepare_to_write_split_index(struct index_state *istate)
 				 * the split index.
 				 * Nothing to do.
 				 */
+			} else if (!ce_uptodate(ce) &&
+				   is_racy_timestamp(istate, ce)) {
+				/*
+				 * A copy of a racily clean cache entry from
+				 * the shared index.  It must be added to
+				 * the split index, so the subsequent
+				 * do_write_index() can smudge its stat data.
+				 */
+				ce->ce_flags |= CE_UPDATE_IN_BASE;
 			} else {
 				/*
 				 * Thoroughly compare the cached data to see
diff --git a/t/t1701-racy-split-index.sh b/t/t1701-racy-split-index.sh
index fbb77046da..5dc221ef38 100755
--- a/t/t1701-racy-split-index.sh
+++ b/t/t1701-racy-split-index.sh
@@ -148,7 +148,7 @@ done
 
 for trial in $trials
 do
-	test_expect_failure "update the split index when a racily clean cache entry is stored only in the shared index $trial" '
+	test_expect_success "update the split index when a racily clean cache entry is stored only in the shared index #$trial" '
 		rm -f .git/index .git/sharedindex.* &&
 
 		# The next three commands must be run within the same
@@ -170,8 +170,6 @@ do
 		# entry of racy-file is only stored in the shared index.
 		# A corresponding replacement cache entry with smudged
 		# stat data should be added to the new split index.
-		#
-		# Alas, such a smudged replacement entry is not added!
 		git update-index --add other-file &&
 
 		# Subsequent git commands should notice the smudged
@@ -182,7 +180,7 @@ done
 
 for trial in $trials
 do
-	test_expect_failure "update the split index after unpack trees() copied a racily clean cache entry from the shared index $trial" '
+	test_expect_success "update the split index after unpack trees() copied a racily clean cache entry from the shared index #$trial" '
 		rm -f .git/index .git/sharedindex.* &&
 
 		# The next three commands must be run within the same
@@ -205,8 +203,6 @@ do
 		# index.  A corresponding replacement cache entry
 		# with smudged stat data should be added to the new
 		# split index.
-		#
-		# Alas, such a smudged replacement entry is not added!
 		git read-tree -m HEAD &&
 
 		# Subsequent git commands should notice the smudged
-- 
2.19.0.361.gafc87ffe72

Re: [PATCH v2 1/5] split-index: add tests to demonstrate the racy split index problem

[Junio C Hamano]

SZEDER Gábor <szeder.dev@gmail.com> writes:

> Junio,
>
> On Thu, Sep 27, 2018 at 02:44:30PM +0200, SZEDER Gábor wrote:
>> diff --git a/t/t1701-racy-split-index.sh b/t/t1701-racy-split-index.sh
>> new file mode 100755
>> index 0000000000..ebde418d7e
>> --- /dev/null
>> +++ b/t/t1701-racy-split-index.sh
>> @@ -0,0 +1,218 @@
>> +#!/bin/sh
>> +
>> +# This test can give false success if your machine is sufficiently
>> +# slow or all trials happened to happen on second boundaries.
>> +
>> +test_description='racy split index'
>> +
>> +. ./test-lib.sh
>> +
>> +test_expect_success 'setup' '
>> +	# Only split the index when the test explicitly says so.
>> +	sane_unset GIT_TEST_SPLIT_INDEX GIT_FSMONITOR_TEST &&
>
> Please note that this patch adds another use of the environment
> variable GIT_FSMONITOR_TEST, while the topic 'bp/rename-test-env-var'
> is about to rename that variable to GIT_TEST_FSMONITOR.

I think the most sensible thing to do during transition is to set
(or unset) both consistently (and leave a note in t/test-lib.sh near
check_var_migration that t1701 has done so and the dup should be
corrected when transition is over---but that is optional).

Re: [PATCH v3 6/6] split-index: smudge and add racily clean cache entries to split index

[Duy Nguyen]

On Fri, Sep 28, 2018 at 06:24:59PM +0200, SZEDER Gábor wrote:
> diff --git a/t/t1701-racy-split-index.sh b/t/t1701-racy-split-index.sh
> index fbb77046da..5dc221ef38 100755
> --- a/t/t1701-racy-split-index.sh
> +++ b/t/t1701-racy-split-index.sh
> @@ -148,7 +148,7 @@ done
>  
>  for trial in $trials
>  do
> -	test_expect_failure "update the split index when a racily clean cache entry is stored only in the shared index $trial" '
> +	test_expect_success "update the split index when a racily clean cache entry is stored only in the shared index #$trial" '

The the new '#' before '$trial' intended?

>  		rm -f .git/index .git/sharedindex.* &&
>  
>  		# The next three commands must be run within the same
> @@ -170,8 +170,6 @@ do
>  		# entry of racy-file is only stored in the shared index.
>  		# A corresponding replacement cache entry with smudged
>  		# stat data should be added to the new split index.
> -		#
> -		# Alas, such a smudged replacement entry is not added!
>  		git update-index --add other-file &&
>  
>  		# Subsequent git commands should notice the smudged
> @@ -182,7 +180,7 @@ done
>  
>  for trial in $trials
>  do
> -	test_expect_failure "update the split index after unpack trees() copied a racily clean cache entry from the shared index $trial" '
> +	test_expect_success "update the split index after unpack trees() copied a racily clean cache entry from the shared index #$trial" '
>  		rm -f .git/index .git/sharedindex.* &&
>  
>  		# The next three commands must be run within the same
--
Duy

Re: [PATCH v3 5/6] split-index: don't compare stat data of entries already marked for split index

[Duy Nguyen]

On Fri, Sep 28, 2018 at 06:24:58PM +0200, SZEDER Gábor wrote:
> When unpack_trees() constructs a new index, it copies cache entries
> from the original index [1].  prepare_to_write_split_index() has to
> deal with this, and it has a dedicated code path for copied entries
> that are present in the shared index, where it compares the cached
> data in the corresponding copied and original entries.  If the cached
> data matches, then they are considered the same; if it differs, then
> the copied entry will be marked for inclusion as a replacement entry
> in the just about to be written split index by setting the
> CE_UPDATE_IN_BASE flag.
> 
> However, a cache entry already has its CE_UPDATE_IN_BASE flag set upon
> reading the split index, if the entry already has a replacement entry
> there, or upon refreshing the cached stat data, if the corresponding
> file was modified.  The state of this flag is then preserved when
> unpack_trees() copies a cache entry from the shared index.
> 
> So modify prepare_to_write_split_index() to check the copied cache
> entries' CE_UPDATE_IN_BASE flag first, and skip the thorough
> comparison of cached data if the flag is already set.

OK so this is an optimization, not a bug fix. Right?

> Note that comparing the cached data in copied and original entries in

s/cached data/cached stat data/ ? I was confused for a bit.

> the shared index might actually be entirely unnecessary.  In theory
> all code paths refreshing the cached stat data of an entry in the
> shared index should set the CE_UPDATE_IN_BASE flag in that entry, and
> unpack_trees() should preserve this flag when copying cache entries.
> This means that the cached data is only ever changed if the
> CE_UPDATE_IN_BASE flag is set as well.  Our test suite seems to
> confirm this: instrumenting the conditions in question and running the
> test suite repeatedly with 'GIT_TEST_SPLIT_INDEX=yes' showed that the
> cached data in a copied entry differs from the data in the shared
> entry only if its CE_UPDATE_IN_BASE flag is indeed set.

Yes I was probably just being paranoid (or sticking to simpler
checks). I was told that split index is computation expensive and not
doing unnecesary/expensive checks may help. But let's leave it for
later.

> +			} else {
> +				/*
> +				 * Thoroughly compare the cached data to see
> +				 * whether it should be marked for inclusion
> +				 * in the split index.
> +				 *
> +				 * This comparison might be unnecessary, as
> +				 * code paths modifying the cached data do
> +				 * set CE_UPDATE_IN_BASE as well.
> +				 */
> +				const unsigned int ondisk_flags =
> +					CE_STAGEMASK | CE_VALID |
> +					CE_EXTENDED_FLAGS;
> +				unsigned int ce_flags, base_flags, ret;
> +				ce_flags = ce->ce_flags;
> +				base_flags = base->ce_flags;
> +				/* only on-disk flags matter */
> +				ce->ce_flags   &= ondisk_flags;
> +				base->ce_flags &= ondisk_flags;
> +				ret = memcmp(&ce->ce_stat_data, &base->ce_stat_data,
> +					     offsetof(struct cache_entry, name) -
> +					     offsetof(struct cache_entry, ce_stat_data));
> +				ce->ce_flags = ce_flags;
> +				base->ce_flags = base_flags;

Maybe make this block a separate function (compare_ce_content or
something). The amount of indentation is getting too high.

> +				if (ret)
> +					ce->ce_flags |= CE_UPDATE_IN_BASE;
> +			}
>  			discard_cache_entry(base);
>  			si->base->cache[ce->index - 1] = ce;
>  		}
> -- 
> 2.19.0.361.gafc87ffe72
> 

Re: [PATCH v3 6/6] split-index: smudge and add racily clean cache entries to split index

[SZEDER Gábor]

On Sat, Sep 29, 2018 at 07:21:36AM +0200, Duy Nguyen wrote:
> On Fri, Sep 28, 2018 at 06:24:59PM +0200, SZEDER Gábor wrote:
> > diff --git a/t/t1701-racy-split-index.sh b/t/t1701-racy-split-index.sh
> > index fbb77046da..5dc221ef38 100755
> > --- a/t/t1701-racy-split-index.sh
> > +++ b/t/t1701-racy-split-index.sh
> > @@ -148,7 +148,7 @@ done
> >  
> >  for trial in $trials
> >  do
> > -	test_expect_failure "update the split index when a racily clean cache entry is stored only in the shared index $trial" '
> > +	test_expect_success "update the split index when a racily clean cache entry is stored only in the shared index #$trial" '
> 
> The the new '#' before '$trial' intended?

Yes, they only cause troubles for 'prove' (or in the TAP output in
general) in 'test_expect_failure', all the previous tests already have
'#$trial', just like the related tests in 't0010-racy-git.sh'.  And
many more tests have '#' in their names:

  $ git grep 'test_expect_success.*#' master |wc -l
  121


> 
> >  		rm -f .git/index .git/sharedindex.* &&
> >  
> >  		# The next three commands must be run within the same
> > @@ -170,8 +170,6 @@ do
> >  		# entry of racy-file is only stored in the shared index.
> >  		# A corresponding replacement cache entry with smudged
> >  		# stat data should be added to the new split index.
> > -		#
> > -		# Alas, such a smudged replacement entry is not added!
> >  		git update-index --add other-file &&
> >  
> >  		# Subsequent git commands should notice the smudged
> > @@ -182,7 +180,7 @@ done
> >  
> >  for trial in $trials
> >  do
> > -	test_expect_failure "update the split index after unpack trees() copied a racily clean cache entry from the shared index $trial" '
> > +	test_expect_success "update the split index after unpack trees() copied a racily clean cache entry from the shared index #$trial" '
> >  		rm -f .git/index .git/sharedindex.* &&
> >  
> >  		# The next three commands must be run within the same
> --
> Duy

Re: [PATCH v3 5/6] split-index: don't compare stat data of entries already marked for split index

[SZEDER Gábor]

On Sat, Sep 29, 2018 at 07:36:08AM +0200, Duy Nguyen wrote:
> On Fri, Sep 28, 2018 at 06:24:58PM +0200, SZEDER Gábor wrote:
> > When unpack_trees() constructs a new index, it copies cache entries
> > from the original index [1].  prepare_to_write_split_index() has to
> > deal with this, and it has a dedicated code path for copied entries
> > that are present in the shared index, where it compares the cached
> > data in the corresponding copied and original entries.  If the cached
> > data matches, then they are considered the same; if it differs, then
> > the copied entry will be marked for inclusion as a replacement entry
> > in the just about to be written split index by setting the
> > CE_UPDATE_IN_BASE flag.
> > 
> > However, a cache entry already has its CE_UPDATE_IN_BASE flag set upon
> > reading the split index, if the entry already has a replacement entry
> > there, or upon refreshing the cached stat data, if the corresponding
> > file was modified.  The state of this flag is then preserved when
> > unpack_trees() copies a cache entry from the shared index.
> > 
> > So modify prepare_to_write_split_index() to check the copied cache
> > entries' CE_UPDATE_IN_BASE flag first, and skip the thorough
> > comparison of cached data if the flag is already set.
> 
> OK so this is an optimization, not a bug fix. Right?

Well, a microoptimization at most: with all what's going on in
unpack_trees() I seriously doubt that it's effect is measurable.

> > Note that comparing the cached data in copied and original entries in
> 
> s/cached data/cached stat data/ ? I was confused for a bit.

No, it's indeed cached data, but now that you mention it, the subject
line does need a s/stat //.

The comparison is done with this call:

  ret = memcmp(&ce->ce_stat_data, &base->ce_stat_data,
               offsetof(struct cache_entry, name) -
               offsetof(struct cache_entry, ce_stat_data));

i.e. it starts at the stat data and ends just before the cache entry's
name, and 'struct cache_entry' has several other fields between these
two, including e.g. the cached oid:

  struct cache_entry {
          struct hashmap_entry ent;
          struct stat_data ce_stat_data;
          unsigned int ce_mode;
          unsigned int ce_flags;
          unsigned int mem_pool_allocated;
          unsigned int ce_namelen;
          unsigned int index;     /* for link extension */
          struct object_id oid;
          char name[FLEX_ARRAY]; /* more */
  };

However, to me it's mostly about clarity of the code, and about
documenting that CE_UPDATE_IN_BASE might have already been set at that
point and why, so the next dev diving in to debug the split index
doesn't have to figure this out himself.

> > the shared index might actually be entirely unnecessary.  In theory
> > all code paths refreshing the cached stat data of an entry in the
> > shared index should set the CE_UPDATE_IN_BASE flag in that entry, and
> > unpack_trees() should preserve this flag when copying cache entries.
> > This means that the cached data is only ever changed if the
> > CE_UPDATE_IN_BASE flag is set as well.  Our test suite seems to
> > confirm this: instrumenting the conditions in question and running the
> > test suite repeatedly with 'GIT_TEST_SPLIT_INDEX=yes' showed that the
> > cached data in a copied entry differs from the data in the shared
> > entry only if its CE_UPDATE_IN_BASE flag is indeed set.
> 
> Yes I was probably just being paranoid (or sticking to simpler
> checks). I was told that split index is computation expensive and not
> doing unnecesary/expensive checks may help. But let's leave it for
> later.
> 
> > +			} else {
> > +				/*
> > +				 * Thoroughly compare the cached data to see
> > +				 * whether it should be marked for inclusion
> > +				 * in the split index.
> > +				 *
> > +				 * This comparison might be unnecessary, as
> > +				 * code paths modifying the cached data do
> > +				 * set CE_UPDATE_IN_BASE as well.
> > +				 */
> > +				const unsigned int ondisk_flags =
> > +					CE_STAGEMASK | CE_VALID |
> > +					CE_EXTENDED_FLAGS;
> > +				unsigned int ce_flags, base_flags, ret;
> > +				ce_flags = ce->ce_flags;
> > +				base_flags = base->ce_flags;
> > +				/* only on-disk flags matter */
> > +				ce->ce_flags   &= ondisk_flags;
> > +				base->ce_flags &= ondisk_flags;
> > +				ret = memcmp(&ce->ce_stat_data, &base->ce_stat_data,
> > +					     offsetof(struct cache_entry, name) -
> > +					     offsetof(struct cache_entry, ce_stat_data));
> > +				ce->ce_flags = ce_flags;
> > +				base->ce_flags = base_flags;
> 
> Maybe make this block a separate function (compare_ce_content or
> something). The amount of indentation is getting too high.

Ah, I was secretly hoping for something along the lines of "your
analysis is correct, we can safely drop this comparison" :)

Btw, I thought about extracing this whole loop into a separate
function like mark_updated_entries_for_split_index() or something, but
there are other things going on in this loop as well, e.g. marking
with CE_MATCHED and deduplicating copied entries, not to mention the
conditions that set 'ce->index = 0', which I think should die() or
BUG() or are unnecessary, see my followup email to this patch in v4:

  https://public-inbox.org/git/20180927134324.GI27036@localhost/

> > +				if (ret)
> > +					ce->ce_flags |= CE_UPDATE_IN_BASE;
> > +			}
> >  			discard_cache_entry(base);
> >  			si->base->cache[ce->index - 1] = ce;
> >  		}
> > -- 
> > 2.19.0.361.gafc87ffe72
> > 

Re: [PATCH v3 5/6] split-index: don't compare stat data of entries already marked for split index

[SZEDER Gábor]

On Sat, Sep 29, 2018 at 11:14:29AM +0200, SZEDER Gábor wrote:
> > > Note that comparing the cached data in copied and original entries in
> > 
> > s/cached data/cached stat data/ ? I was confused for a bit.
> 
> No, it's indeed cached data, but now that you mention it, the subject
> line does need a s/stat //.

Or rather s/stat/cached/

Re: [PATCH v3 0/6] Fix the racy split index problem

[SZEDER Gábor]

Junio,


On Fri, Sep 28, 2018 at 06:24:53PM +0200, SZEDER Gábor wrote:
> Interdiff:
> diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
> index c8acbc50d0..f053bf83eb 100755
> --- a/t/t1700-split-index.sh
> +++ b/t/t1700-split-index.sh
> @@ -6,6 +6,9 @@ test_description='split index mode tests'
>  
>  # We need total control of index splitting here
>  sane_unset GIT_TEST_SPLIT_INDEX

The conflict resolution around here in 3f725b07d3 (Merge branch
'sg/split-index-racefix' into jch, 2018-09-29) accidentally removed
the above line, which makes the test fail when run with
'GIT_TEST_SPLIT_INDEX=yes', e.g.:

  https://travis-ci.org/git/git/jobs/435077629#L3389

> +# A couple of tests expect the index to have a specific checksum,
> +# but the presence of the optional FSMN extension would interfere
> +# with those checks, so disable it in this test script.
>  sane_unset GIT_FSMONITOR_TEST
>  
>  # Create a file named as $1 with content read from stdin.

Re: [PATCH v3 0/6] Fix the racy split index problem

[Junio C Hamano]

SZEDER Gábor <szeder.dev@gmail.com> writes:

> On Fri, Sep 28, 2018 at 06:24:53PM +0200, SZEDER Gábor wrote:
>> Interdiff:
>> diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
>> index c8acbc50d0..f053bf83eb 100755
>> --- a/t/t1700-split-index.sh
>> +++ b/t/t1700-split-index.sh
>> @@ -6,6 +6,9 @@ test_description='split index mode tests'
>>  
>>  # We need total control of index splitting here
>>  sane_unset GIT_TEST_SPLIT_INDEX
>
> The conflict resolution around here in 3f725b07d3 (Merge branch
> 'sg/split-index-racefix' into jch, 2018-09-29) accidentally removed
> the above line, ...

Yeah, I see it in "git show --cc".  Will fix the rerere database
entry to prevent the mismerge from recurring..

Sorry, and thanks for helping me correcting the mismerge.

Re: [PATCH v2 0/5] Fix the racy split index problem

[Ævar Arnfjörð Bjarmason]

On Fri, Sep 28 2018, Ævar Arnfjörð Bjarmason wrote:

> On Thu, Sep 27 2018, Ævar Arnfjörð Bjarmason wrote:
>
>> On Thu, Sep 27 2018, SZEDER Gábor wrote:
>>
>>> On Thu, Sep 27, 2018 at 03:53:24PM +0200, Ævar Arnfjörð Bjarmason wrote:
>>>>
>>>> On Thu, Sep 27 2018, SZEDER Gábor wrote:
>>>>
>>>> > This is the second attempt to fix the racy split index problem, which
>>>> > causes occasional failures in several random test scripts when run
>>>> > with 'GIT_TEST_SPLIT_INDEX=yes'.  The important details are in patches
>>>> > 1 and 5 (corresponding to v1's 3 and 5).
>>>>
>>>> Thanks. I'm running the same sorts of tests I noted in
>>>> https://public-inbox.org/git/87va7ireuu.fsf@evledraar.gmail.com/ on
>>>> this. The fix Jeff had that you noted in
>>>> https://public-inbox.org/git/20180906151439.GA8016@localhost/ is now in
>>>> "master".
>>>>
>>>> I take it your
>>>> https://github.com/szeder/git/commits/racy-split-index-fix is the same
>>>> as this submission?
>>>
>>> Yes.
>>>
>>>> Anyway, I'm testing that cherry-picked on top of the
>>>> latest master.
>>>>
>>>> Unfortunate that we couldn't get the isolated test you made in
>>>> https://public-inbox.org/git/20180907034942.GA10370@localhost/
>>>
>>> Nah, that's not an isolated test case, that's only a somewhat
>>> narrowed-down, but rather reliable reproduction recipe while I still
>>> had no idea what was going on :)
>>>
>>> The _real_ isolated test is the last test in t1701, that's what it
>>> eventually boiled down to.
>>
>> Thanks. I had ~400 runs of the tests I ran before and they were all
>> OK. Now trying also with t1701 (which I hadn't noticed was a new
>> test...).
>
> Ran that overnight with the same conditions as before. 2683 OK runs and
> 0 failures (and counting). So it seems like the combination of the two
> fixed the split index bugs.

I forgot I ad this running, and got up to 45482 OKs and 0 FAILs before
finally Ctrl+C-ing it now :)

>>>> but I
>>>> don't see how it could be added without some very liberal
>>>> getenv("GIT_TEST_blahblah"), so it's probably best to not add it,
>>>> particularly with the C rewrite of git-stash in-flight.
>>>>
>>>> I'll report back when I have enough test data to say how these patches
>>>> affect the intermittent test failures under GIT_TEST_SPLIT_INDEX=yes.

Re: [PATCH v2 0/5] Fix the racy split index problem

[SZEDER Gábor]

On Mon, Oct 08, 2018 at 04:54:51PM +0200, Ævar Arnfjörð Bjarmason wrote:

> >> Thanks. I had ~400 runs of the tests I ran before and they were all
> >> OK. Now trying also with t1701 (which I hadn't noticed was a new
> >> test...).
> >
> > Ran that overnight with the same conditions as before. 2683 OK runs and
> > 0 failures (and counting). So it seems like the combination of the two
> > fixed the split index bugs.
> 
> I forgot I ad this running, and got up to 45482 OKs and 0 FAILs before
> finally Ctrl+C-ing it now :)

Yay! \o/

Thanks for testing, and I feel sorry for your poor machine...

I will send an updated version to address the (minor) points raised in
[1]...  well, most likely not today, but hopefully soon-ish.

1 - https://public-inbox.org/git/20180929091429.GF23446@localhost/

[PATCH v4 0/6] Fix the racy split index problem

[SZEDER Gábor]

Fourth and hopefully final round of fixing occasional test failures when
run with 'GIT_TEST_SPLIT_INDEX=yes'.  The only code change is the
extraction of a helper function to compare two cache entries' content,
and then a couple of minor log message clarifications.  The range-diff
below is rather clear on that.

I will send a 7/6 follow-up patch shortly as well.

SZEDER Gábor (6):
  t1700-split-index: document why FSMONITOR is disabled in this test
    script
  split-index: add tests to demonstrate the racy split index problem
  t1700-split-index: date back files to avoid racy situations
  split-index: count the number of deleted entries
  split-index: don't compare cached data of entries already marked for
    split index
  split-index: smudge and add racily clean cache entries to split index

 cache.h                     |   2 +
 read-cache.c                |   2 +-
 split-index.c               | 131 +++++++++++++++++++---
 t/t1700-split-index.sh      |  52 +++++----
 t/t1701-racy-split-index.sh | 214 ++++++++++++++++++++++++++++++++++++
 5 files changed, 361 insertions(+), 40 deletions(-)
 create mode 100755 t/t1701-racy-split-index.sh

Range-diff:
1:  ba2b1bdf16 = 1:  ba2b1bdf16 t1700-split-index: document why FSMONITOR is disabled in this test script
2:  bf1b038f10 ! 2:  c7cb9d9115 split-index: add tests to demonstrate the racy split index problem
    @@ -136,13 +136,20 @@
              git commands will then erroneously consider the file clean.
     
         Note that in the last two 'test_expect_failure' cases I omitted the
    -    '#' (as in nr. of trial) from the tests' name on purpose for now, as
    -    it confuses 'prove' into thinking that those tests failed
    -    unexpectedly.
    +    '#' (as in nr. of trial) from the tests' description on purpose for
    +    now, as it breakes the TAP output [2]; it will be added at the end of
    +    the series, when those two tests will be flipped to
    +    'test_expect_success'.
     
         [1] In the branch leading to the merge commit v2.1.0-rc0~45 (Merge
             branch 'nd/split-index', 2014-07-16).
     
    +    [2] In the TAP output a '#' should separate the test's description
    +        from the TODO directive emitted by 'test_expect_failure'.  The
    +        additional '#' in "#$trial" interferes with this, the test harness
    +        won't recognize the TODO directive, and will report that those
    +        tests failed unexpectedly.
    +
         Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
     
      diff --git a/t/t1701-racy-split-index.sh b/t/t1701-racy-split-index.sh
3:  e7f7fb6e2d = 3:  ce803d8064 t1700-split-index: date back files to avoid racy situations
4:  6dc0b01ad3 = 4:  1d12d718d1 split-index: count the number of deleted entries
5:  9c420f9c66 ! 5:  0dd448c707 split-index: don't compare stat data of entries already marked for split index
    @@ -1,6 +1,6 @@
     Author: SZEDER Gábor <szeder.dev@gmail.com>
     
    -    split-index: don't compare stat data of entries already marked for split index
    +    split-index: don't compare cached data of entries already marked for split index
     
         When unpack_trees() constructs a new index, it copies cache entries
         from the original index [1].  prepare_to_write_split_index() has to
    @@ -20,7 +20,9 @@
     
         So modify prepare_to_write_split_index() to check the copied cache
         entries' CE_UPDATE_IN_BASE flag first, and skip the thorough
    -    comparison of cached data if the flag is already set.
    +    comparison of cached data if the flag is already set.  Those couple of
    +    lines comparing the cached data would then have too many levels of
    +    indentation, so extract them into a helper function.
     
         Note that comparing the cached data in copied and original entries in
         the shared index might actually be entirely unnecessary.  In theory
    @@ -62,6 +64,37 @@
      diff --git a/split-index.c b/split-index.c
      --- a/split-index.c
      +++ b/split-index.c
    +@@
    + 	si->saved_cache_nr = 0;
    + }
    + 
    ++/*
    ++ * Compare most of the fields in two cache entries, i.e. all except the
    ++ * hashmap_entry and the name.
    ++ */
    ++static int compare_ce_content(struct cache_entry *a, struct cache_entry *b)
    ++{
    ++	const unsigned int ondisk_flags = CE_STAGEMASK | CE_VALID |
    ++					  CE_EXTENDED_FLAGS;
    ++	unsigned int ce_flags = a->ce_flags;
    ++	unsigned int base_flags = b->ce_flags;
    ++	int ret;
    ++
    ++	/* only on-disk flags matter */
    ++	a->ce_flags &= ondisk_flags;
    ++	b->ce_flags &= ondisk_flags;
    ++	ret = memcmp(&a->ce_stat_data, &b->ce_stat_data,
    ++		     offsetof(struct cache_entry, name) -
    ++		     offsetof(struct cache_entry, ce_stat_data));
    ++	a->ce_flags = ce_flags;
    ++	b->ce_flags = base_flags;
    ++
    ++	return ret;
    ++}
    ++
    + void prepare_to_write_split_index(struct index_state *istate)
    + {
    + 	struct split_index *si = init_split_index(istate);
     @@
      		 */
      		for (i = 0; i < istate->cache_nr; i++) {
    @@ -137,21 +170,7 @@
     +				 * code paths modifying the cached data do
     +				 * set CE_UPDATE_IN_BASE as well.
     +				 */
    -+				const unsigned int ondisk_flags =
    -+					CE_STAGEMASK | CE_VALID |
    -+					CE_EXTENDED_FLAGS;
    -+				unsigned int ce_flags, base_flags, ret;
    -+				ce_flags = ce->ce_flags;
    -+				base_flags = base->ce_flags;
    -+				/* only on-disk flags matter */
    -+				ce->ce_flags   &= ondisk_flags;
    -+				base->ce_flags &= ondisk_flags;
    -+				ret = memcmp(&ce->ce_stat_data, &base->ce_stat_data,
    -+					     offsetof(struct cache_entry, name) -
    -+					     offsetof(struct cache_entry, ce_stat_data));
    -+				ce->ce_flags = ce_flags;
    -+				base->ce_flags = base_flags;
    -+				if (ret)
    ++				if (compare_ce_content(ce, base))
     +					ce->ce_flags |= CE_UPDATE_IN_BASE;
     +			}
      			discard_cache_entry(base);
6:  52c755f210 ! 6:  384b440345 split-index: smudge and add racily clean cache entries to split index
    @@ -46,6 +46,11 @@
         racily clean cache entries as well, and will then write them with
         smudged stat data to the new split index.
     
    +    This change makes all tests in 't1701-racy-split-index.sh' pass, so
    +    flip the two 'test_expect_failure' tests to success.  Also add the '#'
    +    (as in nr. of trial) to those tests' description that were omitted
    +    when the tests expected failure.
    +
         Note that after this change if the index is split when it contains a
         racily clean cache entry, then a smudged cache entry will be written
         both to the new shared and to the new split indexes.  This doesn't
-- 
2.19.1.465.gaff195083f

[PATCH v4 1/6] t1700-split-index: document why FSMONITOR is disabled in this test script

[SZEDER Gábor]

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 t/t1700-split-index.sh | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
index be22398a85..822257ff7d 100755
--- a/t/t1700-split-index.sh
+++ b/t/t1700-split-index.sh
@@ -6,6 +6,9 @@ test_description='split index mode tests'
 
 # We need total control of index splitting here
 sane_unset GIT_TEST_SPLIT_INDEX
+# A couple of tests expect the index to have a specific checksum,
+# but the presence of the optional FSMN extension would interfere
+# with those checks, so disable it in this test script.
 sane_unset GIT_FSMONITOR_TEST
 
 test_expect_success 'enable split index' '
-- 
2.19.1.465.gaff195083f

[PATCH v4 2/6] split-index: add tests to demonstrate the racy split index problem

[SZEDER Gábor]

Ever since the split index feature was introduced [1], refreshing a
split index is prone to a variant of the classic racy git problem.
There are a couple of unrelated tests in the test suite that
occasionally fail when run with 'GIT_TEST_SPLIT_INDEX=yes', but
't1700-split-index.sh', the only test script focusing solely on split
index, has never noticed this issue, because it only cares about how
the index is split under various circumstances and all the different
ways to turn the split index feature on and off.

Add a dedicated test script 't1701-racy-split-index.sh' to exercise
the split index feature in racy situations as well; kind of a
"t0010-racy-git.sh for split index" but with modern style (the tests
do everything in &&-chained list of commands in 'test_expect_...'
blocks, and use 'test_cmp' for more informative output on failure).

The tests cover the following sequences of index splitting, updating,
and racy file modifications, with the last two cases demonstrating the
racy split index problem:

  1. Split the index while adding a racily clean file:

       echo "cached content" >file
       git update-index --split-index --add file
       echo "dirty worktree" >file    # size stays the same

     This case already works properly.  Even though the cache entry's
     stat data matches with the modifid file in the worktree,
     subsequent git commands will notice that the (split) index and
     the file have the same mtime, and then will go on to check the
     file's content and notice its dirtiness.

  2. Add a racily clean file to an already split index:

       git update-index --split-index
       echo "cached content" >file
       git update-index --add file
       echo "dirty worktree" >file

     This case already works properly.  After the second 'git
     update-index' writes the newly added file's cache entry to the
     new split index, it basically works in the same way as case #1.

  3. Split the index when it (i.e. the not yet splitted index)
     contains a racily clean cache entry, i.e. an entry whose cached
     stat data matches with the corresponding file in the worktree and
     the cached mtime matches that of the index:

       echo "cached content" >file
       git update-index --add file
       echo "dirty worktree" >file
       # ... wait ...
       git update-index --split-index --add other-file

     This case already works properly.  The shared index is written by
     do_write_index(), i.e. the same function that is responsible for
     writing "regular" and split indexes as well.  This function
     cleverly notices the racily clean cache entry, and writes the
     entry to the new shared index with smudged stat data, i.e. file
     size set to 0.  When subsequent git commands read the index, they
     will notice that the smudged stat data doesn't match with the
     file in the worktree, and then go on to check the file's content
     and notice its dirtiness.

  4. Update the split index when it contains a racily clean cache
     entry:

       git update-index --split-index
       echo "cached content" >file
       git update-index --add file
       echo "dirty worktree" >file
       # ... wait ...
       git update-index --add other-file

     This case already works properly.  After the second 'git
     update-index' the newly added file's cache entry is only stored
     in the split index.  If a cache entry is present in the split
     index (even if it is a replacement of an outdated entry in the
     shared index), then it will always be included in the new split
     index on subsequent split index updates (until the file is
     removed or a new shared index is written), independently from
     whether the entry is racily clean or not.  When do_write_index()
     writes the new split index, it notices the racily clean cache
     entry, and smudges its stat date.  Subsequent git commands
     reading the index will notice the smudged stat data and then go
     on to check the file's content and notice its dirtiness.

  5. Update the split index when a racily clean cache entry is stored
     only in the shared index:

       echo "cached content" >file
       git update-index --split-index --add file
       echo "dirty worktree" >file
       # ... wait ...
       git update-index --add other-file

     This case fails due to the racy split index problem.  In the
     second 'git update-index' prepare_to_write_split_index() decides,
     among other things, which cache entries stored only in the shared
     index should be replaced in the new split index.  Alas, this
     function never looks out for racily clean cache entries, and
     since the file's stat data in the worktree hasn't changed since
     the shared index was written, the entry won't be replaced in the
     new split index.  Consequently, do_write_index() doesn't even get
     this racily clean cache entry, and can't smudge its stat data.
     Subsequent git commands will then see that the index has more
     recent mtime than the file and that the (not smudged) cached stat
     data still matches with the file in the worktree, and,
     ultimately, will erroneously consider the file clean.

  6. Update the split index after unpack_trees() copied a racily clean
     cache entry from the shared index:

       echo "cached content" >file
       git update-index --split-index --add file
       echo "dirty worktree" >file
       # ... wait ...
       git read-tree -m HEAD

     This case fails due to the racy split index problem.  This
     basically fails for the same reason as case #5 above, but there
     is one important difference, which warrants the dedicated test.
     While that second 'git update-index' in case #5 updates
     index_state in place, in this case 'git read-tree -m' calls
     unpack_trees(), which throws out the entire index, and constructs
     a new one from the (potentially updated) copies of the original's
     cache entries.  Consequently, when prepare_to_write_split_index()
     gets to work on this reconstructed index, it takes a different
     code path than in case #5 when deciding which cache entries in
     the shared index should be replaced.  The result is the same,
     though: the racily clean cache entry goes unnoticed, it isn't
     added to the split index with smudged stat data, and subsequent
     git commands will then erroneously consider the file clean.

Note that in the last two 'test_expect_failure' cases I omitted the
'#' (as in nr. of trial) from the tests' description on purpose for
now, as it breakes the TAP output [2]; it will be added at the end of
the series, when those two tests will be flipped to
'test_expect_success'.

[1] In the branch leading to the merge commit v2.1.0-rc0~45 (Merge
    branch 'nd/split-index', 2014-07-16).

[2] In the TAP output a '#' should separate the test's description
    from the TODO directive emitted by 'test_expect_failure'.  The
    additional '#' in "#$trial" interferes with this, the test harness
    won't recognize the TODO directive, and will report that those
    tests failed unexpectedly.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 t/t1701-racy-split-index.sh | 218 ++++++++++++++++++++++++++++++++++++
 1 file changed, 218 insertions(+)
 create mode 100755 t/t1701-racy-split-index.sh

diff --git a/t/t1701-racy-split-index.sh b/t/t1701-racy-split-index.sh
new file mode 100755
index 0000000000..fbb77046da
--- /dev/null
+++ b/t/t1701-racy-split-index.sh
@@ -0,0 +1,218 @@
+#!/bin/sh
+
+# This test can give false success if your machine is sufficiently
+# slow or all trials happened to happen on second boundaries.
+
+test_description='racy split index'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	# Only split the index when the test explicitly says so.
+	sane_unset GIT_TEST_SPLIT_INDEX &&
+	git config splitIndex.maxPercentChange 100 &&
+
+	echo "cached content" >racy-file &&
+	git add racy-file &&
+	git commit -m initial &&
+
+	echo something >other-file &&
+	# No raciness with this file.
+	test-tool chmtime =-20 other-file &&
+
+	echo "+cached content" >expect
+'
+
+check_cached_diff () {
+	git diff-index --patch --cached $EMPTY_TREE racy-file >diff &&
+	tail -1 diff >actual &&
+	test_cmp expect actual
+}
+
+trials="0 1 2 3 4"
+for trial in $trials
+do
+	test_expect_success "split the index while adding a racily clean file #$trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		# The next three commands must be run within the same
+		# second (so both writes to racy-file result in the same
+		# mtime) to create the interesting racy situation.
+		echo "cached content" >racy-file &&
+
+		# Update and split the index.  The cache entry of
+		# racy-file will be stored only in the shared index.
+		git update-index --split-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Subsequent git commands should notice that racy-file
+		# and the split index have the same mtime, and check
+		# the content of the file to see if it is actually
+		# clean.
+		check_cached_diff
+	'
+done
+
+for trial in $trials
+do
+	test_expect_success "add a racily clean file to an already split index #$trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		git update-index --split-index &&
+
+		# The next three commands must be run within the same
+		# second.
+		echo "cached content" >racy-file &&
+
+		# Update the split index.  The cache entry of racy-file
+		# will be stored only in the split index.
+		git update-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Subsequent git commands should notice that racy-file
+		# and the split index have the same mtime, and check
+		# the content of the file to see if it is actually
+		# clean.
+		check_cached_diff
+	'
+done
+
+for trial in $trials
+do
+	test_expect_success "split the index when the index contains a racily clean cache entry #$trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		# The next three commands must be run within the same
+		# second.
+		echo "cached content" >racy-file &&
+
+		git update-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Now wait a bit to ensure that the split index written
+		# below will get a more recent mtime than racy-file.
+		sleep 1 &&
+
+		# Update and split the index when the index contains
+		# the racily clean cache entry of racy-file.
+		# A corresponding replacement cache entry with smudged
+		# stat data should be added to the new split index.
+		git update-index --split-index --add other-file &&
+
+		# Subsequent git commands should notice the smudged
+		# stat data in the replacement cache entry and that it
+		# doesnt match with the file the worktree.
+		check_cached_diff
+	'
+done
+
+for trial in $trials
+do
+	test_expect_success "update the split index when it contains a new racily clean cache entry #$trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		git update-index --split-index &&
+
+		# The next three commands must be run within the same
+		# second.
+		echo "cached content" >racy-file &&
+
+		# Update the split index.  The cache entry of racy-file
+		# will be stored only in the split index.
+		git update-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Now wait a bit to ensure that the split index written
+		# below will get a more recent mtime than racy-file.
+		sleep 1 &&
+
+		# Update the split index when the racily clean cache
+		# entry of racy-file is only stored in the split index.
+		# An updated cache entry with smudged stat data should
+		# be added to the new split index.
+		git update-index --add other-file &&
+
+		# Subsequent git commands should notice the smudged
+		# stat data.
+		check_cached_diff
+	'
+done
+
+for trial in $trials
+do
+	test_expect_failure "update the split index when a racily clean cache entry is stored only in the shared index $trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		# The next three commands must be run within the same
+		# second.
+		echo "cached content" >racy-file &&
+
+		# Update and split the index.  The cache entry of
+		# racy-file will be stored only in the shared index.
+		git update-index --split-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Now wait a bit to ensure that the split index written
+		# below will get a more recent mtime than racy-file.
+		sleep 1 &&
+
+		# Update the split index when the racily clean cache
+		# entry of racy-file is only stored in the shared index.
+		# A corresponding replacement cache entry with smudged
+		# stat data should be added to the new split index.
+		#
+		# Alas, such a smudged replacement entry is not added!
+		git update-index --add other-file &&
+
+		# Subsequent git commands should notice the smudged
+		# stat data.
+		check_cached_diff
+	'
+done
+
+for trial in $trials
+do
+	test_expect_failure "update the split index after unpack trees() copied a racily clean cache entry from the shared index $trial" '
+		rm -f .git/index .git/sharedindex.* &&
+
+		# The next three commands must be run within the same
+		# second.
+		echo "cached content" >racy-file &&
+
+		# Update and split the index.  The cache entry of
+		# racy-file will be stored only in the shared index.
+		git update-index --split-index --add racy-file &&
+
+		# File size must stay the same.
+		echo "dirty worktree" >racy-file &&
+
+		# Now wait a bit to ensure that the split index written
+		# below will get a more recent mtime than racy-file.
+		sleep 1 &&
+
+		# Update the split index after unpack_trees() copied the
+		# racily clean cache entry of racy-file from the shared
+		# index.  A corresponding replacement cache entry
+		# with smudged stat data should be added to the new
+		# split index.
+		#
+		# Alas, such a smudged replacement entry is not added!
+		git read-tree -m HEAD &&
+
+		# Subsequent git commands should notice the smudged
+		# stat data.
+		check_cached_diff
+	'
+done
+
+test_done
-- 
2.19.1.465.gaff195083f

[PATCH v4 3/6] t1700-split-index: date back files to avoid racy situations

[SZEDER Gábor]

't1700-split-index.sh' checks that the index was split correctly under
various circumstances and that all the different ways to turn the
split index feature on and off work correctly.  To do so, most of its
tests use 'test-tool dump-split-index' to see which files have their
cache entries in the split index.  All these tests assume that all
cache entries are written to the shared index (called "base"
throughout these tests) when a new shared index is created.  This is
an implementation detail: most git commands (basically all except 'git
update-index') don't care or know at all about split index or whether
a cache entry is stored in the split or shared index.

As demonstrated in the previous patch, refreshing a split index is
prone to a variant of the classic racy git issue.  The next patch will
fix this issue, but while doing so it will also slightly change this
behaviour: only cache entries with mtime in the past will be written
only to the newly created shared index, but racily clean cache entries
will be written to the new split index (with smudged stat data).

While this upcoming change won't at all affect any git commands, it
will violate the above mentioned assumption of 't1700's tests.  Since
these tests create or modify files and create or refresh the split
index in rapid succession, there are plenty of racily clean cache
entries to be dealt with, which will then be written to the new split
indexes, and, ultimately, will cause several tests in 't1700' to fail.

Let's prepare 't1700-split-index.sh' for this upcoming change and
modify its tests to avoid racily clean files by backdating the mtime
of any file modifications (and since a lot of tests create or modify
files, encapsulate it into a helper function).

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 t/t1700-split-index.sh | 49 ++++++++++++++++++++++++------------------
 1 file changed, 28 insertions(+), 21 deletions(-)

diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
index 822257ff7d..f053bf83eb 100755
--- a/t/t1700-split-index.sh
+++ b/t/t1700-split-index.sh
@@ -11,6 +11,13 @@ sane_unset GIT_TEST_SPLIT_INDEX
 # with those checks, so disable it in this test script.
 sane_unset GIT_FSMONITOR_TEST
 
+# Create a file named as $1 with content read from stdin.
+# Set the file's mtime to a few seconds in the past to avoid racy situations.
+create_non_racy_file () {
+	cat >"$1" &&
+	test-tool chmtime =-5 "$1"
+}
+
 test_expect_success 'enable split index' '
 	git config splitIndex.maxPercentChange 100 &&
 	git update-index --split-index &&
@@ -34,7 +41,7 @@ test_expect_success 'enable split index' '
 '
 
 test_expect_success 'add one file' '
-	: >one &&
+	create_non_racy_file one &&
 	git update-index --add one &&
 	git ls-files --stage >ls-files.actual &&
 	cat >ls-files.expect <<-EOF &&
@@ -86,7 +93,7 @@ test_expect_success 'enable split index again, "one" now belongs to base index"'
 '
 
 test_expect_success 'modify original file, base index untouched' '
-	echo modified >one &&
+	echo modified | create_non_racy_file one &&
 	git update-index one &&
 	git ls-files --stage >ls-files.actual &&
 	cat >ls-files.expect <<-EOF &&
@@ -105,7 +112,7 @@ test_expect_success 'modify original file, base index untouched' '
 '
 
 test_expect_success 'add another file, which stays index' '
-	: >two &&
+	create_non_racy_file two &&
 	git update-index --add two &&
 	git ls-files --stage >ls-files.actual &&
 	cat >ls-files.expect <<-EOF &&
@@ -158,7 +165,7 @@ test_expect_success 'remove file in base index' '
 '
 
 test_expect_success 'add original file back' '
-	: >one &&
+	create_non_racy_file one &&
 	git update-index --add one &&
 	git ls-files --stage >ls-files.actual &&
 	cat >ls-files.expect <<-EOF &&
@@ -177,7 +184,7 @@ test_expect_success 'add original file back' '
 '
 
 test_expect_success 'add new file' '
-	: >two &&
+	create_non_racy_file two &&
 	git update-index --add two &&
 	git ls-files --stage >actual &&
 	cat >expect <<-EOF &&
@@ -221,7 +228,7 @@ test_expect_success 'rev-parse --shared-index-path' '
 
 test_expect_success 'set core.splitIndex config variable to true' '
 	git config core.splitIndex true &&
-	: >three &&
+	create_non_racy_file three &&
 	git update-index --add three &&
 	git ls-files --stage >ls-files.actual &&
 	cat >ls-files.expect <<-EOF &&
@@ -256,9 +263,9 @@ test_expect_success 'set core.splitIndex config variable to false' '
 	test_cmp expect actual
 '
 
-test_expect_success 'set core.splitIndex config variable to true' '
+test_expect_success 'set core.splitIndex config variable back to true' '
 	git config core.splitIndex true &&
-	: >three &&
+	create_non_racy_file three &&
 	git update-index --add three &&
 	BASE=$(test-tool dump-split-index .git/index | grep "^base") &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
@@ -268,7 +275,7 @@ test_expect_success 'set core.splitIndex config variable to true' '
 	deletions:
 	EOF
 	test_cmp expect actual &&
-	: >four &&
+	create_non_racy_file four &&
 	git update-index --add four &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
 	cat >expect <<-EOF &&
@@ -282,7 +289,7 @@ test_expect_success 'set core.splitIndex config variable to true' '
 
 test_expect_success 'check behavior with splitIndex.maxPercentChange unset' '
 	git config --unset splitIndex.maxPercentChange &&
-	: >five &&
+	create_non_racy_file five &&
 	git update-index --add five &&
 	BASE=$(test-tool dump-split-index .git/index | grep "^base") &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
@@ -292,7 +299,7 @@ test_expect_success 'check behavior with splitIndex.maxPercentChange unset' '
 	deletions:
 	EOF
 	test_cmp expect actual &&
-	: >six &&
+	create_non_racy_file six &&
 	git update-index --add six &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
 	cat >expect <<-EOF &&
@@ -306,7 +313,7 @@ test_expect_success 'check behavior with splitIndex.maxPercentChange unset' '
 
 test_expect_success 'check splitIndex.maxPercentChange set to 0' '
 	git config splitIndex.maxPercentChange 0 &&
-	: >seven &&
+	create_non_racy_file seven &&
 	git update-index --add seven &&
 	BASE=$(test-tool dump-split-index .git/index | grep "^base") &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
@@ -316,7 +323,7 @@ test_expect_success 'check splitIndex.maxPercentChange set to 0' '
 	deletions:
 	EOF
 	test_cmp expect actual &&
-	: >eight &&
+	create_non_racy_file eight &&
 	git update-index --add eight &&
 	BASE=$(test-tool dump-split-index .git/index | grep "^base") &&
 	test-tool dump-split-index .git/index | sed "/^own/d" >actual &&
@@ -329,17 +336,17 @@ test_expect_success 'check splitIndex.maxPercentChange set to 0' '
 '
 
 test_expect_success 'shared index files expire after 2 weeks by default' '
-	: >ten &&
+	create_non_racy_file ten &&
 	git update-index --add ten &&
 	test $(ls .git/sharedindex.* | wc -l) -gt 2 &&
 	just_under_2_weeks_ago=$((5-14*86400)) &&
 	test-tool chmtime =$just_under_2_weeks_ago .git/sharedindex.* &&
-	: >eleven &&
+	create_non_racy_file eleven &&
 	git update-index --add eleven &&
 	test $(ls .git/sharedindex.* | wc -l) -gt 2 &&
 	just_over_2_weeks_ago=$((-1-14*86400)) &&
 	test-tool chmtime =$just_over_2_weeks_ago .git/sharedindex.* &&
-	: >twelve &&
+	create_non_racy_file twelve &&
 	git update-index --add twelve &&
 	test $(ls .git/sharedindex.* | wc -l) -le 2
 '
@@ -347,12 +354,12 @@ test_expect_success 'shared index files expire after 2 weeks by default' '
 test_expect_success 'check splitIndex.sharedIndexExpire set to 16 days' '
 	git config splitIndex.sharedIndexExpire "16.days.ago" &&
 	test-tool chmtime =$just_over_2_weeks_ago .git/sharedindex.* &&
-	: >thirteen &&
+	create_non_racy_file thirteen &&
 	git update-index --add thirteen &&
 	test $(ls .git/sharedindex.* | wc -l) -gt 2 &&
 	just_over_16_days_ago=$((-1-16*86400)) &&
 	test-tool chmtime =$just_over_16_days_ago .git/sharedindex.* &&
-	: >fourteen &&
+	create_non_racy_file fourteen &&
 	git update-index --add fourteen &&
 	test $(ls .git/sharedindex.* | wc -l) -le 2
 '
@@ -361,13 +368,13 @@ test_expect_success 'check splitIndex.sharedIndexExpire set to "never" and "now"
 	git config splitIndex.sharedIndexExpire never &&
 	just_10_years_ago=$((-365*10*86400)) &&
 	test-tool chmtime =$just_10_years_ago .git/sharedindex.* &&
-	: >fifteen &&
+	create_non_racy_file fifteen &&
 	git update-index --add fifteen &&
 	test $(ls .git/sharedindex.* | wc -l) -gt 2 &&
 	git config splitIndex.sharedIndexExpire now &&
 	just_1_second_ago=-1 &&
 	test-tool chmtime =$just_1_second_ago .git/sharedindex.* &&
-	: >sixteen &&
+	create_non_racy_file sixteen &&
 	git update-index --add sixteen &&
 	test $(ls .git/sharedindex.* | wc -l) -le 2
 '
@@ -382,7 +389,7 @@ do
 		# Create one new shared index file
 		git config core.sharedrepository "$mode" &&
 		git config core.splitIndex true &&
-		: >one &&
+		create_non_racy_file one &&
 		git update-index --add one &&
 		echo "$modebits" >expect &&
 		test_modebits .git/index >actual &&
-- 
2.19.1.465.gaff195083f

[PATCH v4 4/6] split-index: count the number of deleted entries

[SZEDER Gábor]

'struct split_index' contains the field 'nr_deletions', whose name
with the 'nr_' prefix suggests that it contains the number of deleted
cache entries.  However, barring its initialization to 0, this field
is only ever set to 1, indicating that there is at least one deleted
entry, but not the number of deleted entries.  Luckily, this doesn't
cause any issues (other than confusing the reader, that is), because
the only place reading this field uses it in the same sense, i.e.: 'if
(si->nr_deletions)'.

To avoid confusion, we could either rename this field to something
like 'has_deletions' to make its name match its role, or make it a
counter of deleted cache entries to match its name.

Let's make it a counter, to keep it in sync with the related field
'nr_replacements', which does contain the number of replaced cache
entries.  This will also give developers debugging the split index
code easy access to the number of deleted cache entries.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 split-index.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/split-index.c b/split-index.c
index 84f067e10d..548272ec33 100644
--- a/split-index.c
+++ b/split-index.c
@@ -111,7 +111,7 @@ static void mark_entry_for_delete(size_t pos, void *data)
 		die("position for delete %d exceeds base index size %d",
 		    (int)pos, istate->cache_nr);
 	istate->cache[pos]->ce_flags |= CE_REMOVE;
-	istate->split_index->nr_deletions = 1;
+	istate->split_index->nr_deletions++;
 }
 
 static void replace_entry(size_t pos, void *data)
-- 
2.19.1.465.gaff195083f

[PATCH v4 5/6] split-index: don't compare cached data of entries already marked for split index

[SZEDER Gábor]

When unpack_trees() constructs a new index, it copies cache entries
from the original index [1].  prepare_to_write_split_index() has to
deal with this, and it has a dedicated code path for copied entries
that are present in the shared index, where it compares the cached
data in the corresponding copied and original entries.  If the cached
data matches, then they are considered the same; if it differs, then
the copied entry will be marked for inclusion as a replacement entry
in the just about to be written split index by setting the
CE_UPDATE_IN_BASE flag.

However, a cache entry already has its CE_UPDATE_IN_BASE flag set upon
reading the split index, if the entry already has a replacement entry
there, or upon refreshing the cached stat data, if the corresponding
file was modified.  The state of this flag is then preserved when
unpack_trees() copies a cache entry from the shared index.

So modify prepare_to_write_split_index() to check the copied cache
entries' CE_UPDATE_IN_BASE flag first, and skip the thorough
comparison of cached data if the flag is already set.  Those couple of
lines comparing the cached data would then have too many levels of
indentation, so extract them into a helper function.

Note that comparing the cached data in copied and original entries in
the shared index might actually be entirely unnecessary.  In theory
all code paths refreshing the cached stat data of an entry in the
shared index should set the CE_UPDATE_IN_BASE flag in that entry, and
unpack_trees() should preserve this flag when copying cache entries.
This means that the cached data is only ever changed if the
CE_UPDATE_IN_BASE flag is set as well.  Our test suite seems to
confirm this: instrumenting the conditions in question and running the
test suite repeatedly with 'GIT_TEST_SPLIT_INDEX=yes' showed that the
cached data in a copied entry differs from the data in the shared
entry only if its CE_UPDATE_IN_BASE flag is indeed set.

In practice, however, our test suite doesn't have 100% coverage,
GIT_TEST_SPLIT_INDEX is inherently random, and I certainly can't claim
to possess complete understanding of what goes on in unpack_trees()...
Therefore I kept the comparison of the cached data when
CE_UPDATE_IN_BASE is not set, just in case that an unnoticed or future
code path were to accidentally miss setting this flag upon refreshing
the cached stat data or unpack_trees() were to drop this flag while
copying a cache entry.

[1] Note that when unpack_trees() constructs the new index and decides
    that a cache entry should now refer to different content than what
    was recorded in the original index (e.g. 'git read-tree -m
    HEAD^'), then that can't really be considered a copy of the
    original, but rather the creation of a new entry.  Notably and
    pertinent to the split index feature, such a new entry doesn't
    have a reference to the original's shared index entry anymore,
    i.e. its 'index' field is set to 0.  Consequently, such an entry
    is treated by prepare_to_write_split_index() as an entry not
    present in the shared index and it will be added to the new split
    index, while the original entry will be marked as deleted, and
    neither the above discussion nor the changes in this patch apply
    to them.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 split-index.c | 89 +++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 72 insertions(+), 17 deletions(-)

diff --git a/split-index.c b/split-index.c
index 548272ec33..187b910f5b 100644
--- a/split-index.c
+++ b/split-index.c
@@ -188,6 +188,30 @@ void merge_base_index(struct index_state *istate)
 	si->saved_cache_nr = 0;
 }
 
+/*
+ * Compare most of the fields in two cache entries, i.e. all except the
+ * hashmap_entry and the name.
+ */
+static int compare_ce_content(struct cache_entry *a, struct cache_entry *b)
+{
+	const unsigned int ondisk_flags = CE_STAGEMASK | CE_VALID |
+					  CE_EXTENDED_FLAGS;
+	unsigned int ce_flags = a->ce_flags;
+	unsigned int base_flags = b->ce_flags;
+	int ret;
+
+	/* only on-disk flags matter */
+	a->ce_flags &= ondisk_flags;
+	b->ce_flags &= ondisk_flags;
+	ret = memcmp(&a->ce_stat_data, &b->ce_stat_data,
+		     offsetof(struct cache_entry, name) -
+		     offsetof(struct cache_entry, ce_stat_data));
+	a->ce_flags = ce_flags;
+	b->ce_flags = base_flags;
+
+	return ret;
+}
+
 void prepare_to_write_split_index(struct index_state *istate)
 {
 	struct split_index *si = init_split_index(istate);
@@ -207,13 +231,28 @@ void prepare_to_write_split_index(struct index_state *istate)
 		 */
 		for (i = 0; i < istate->cache_nr; i++) {
 			struct cache_entry *base;
-			/* namelen is checked separately */
-			const unsigned int ondisk_flags =
-				CE_STAGEMASK | CE_VALID | CE_EXTENDED_FLAGS;
-			unsigned int ce_flags, base_flags, ret;
 			ce = istate->cache[i];
-			if (!ce->index)
+			if (!ce->index) {
+				/*
+				 * During simple update index operations this
+				 * is a cache entry that is not present in
+				 * the shared index.  It will be added to the
+				 * split index.
+				 *
+				 * However, it might also represent a file
+				 * that already has a cache entry in the
+				 * shared index, but a new index has just
+				 * been constructed by unpack_trees(), and
+				 * this entry now refers to different content
+				 * than what was recorded in the original
+				 * index, e.g. during 'read-tree -m HEAD^' or
+				 * 'checkout HEAD^'.  In this case the
+				 * original entry in the shared index will be
+				 * marked as deleted, and this entry will be
+				 * added to the split index.
+				 */
 				continue;
+			}
 			if (ce->index > si->base->cache_nr) {
 				ce->index = 0;
 				continue;
@@ -227,18 +266,34 @@ void prepare_to_write_split_index(struct index_state *istate)
 				ce->index = 0;
 				continue;
 			}
-			ce_flags = ce->ce_flags;
-			base_flags = base->ce_flags;
-			/* only on-disk flags matter */
-			ce->ce_flags   &= ondisk_flags;
-			base->ce_flags &= ondisk_flags;
-			ret = memcmp(&ce->ce_stat_data, &base->ce_stat_data,
-				     offsetof(struct cache_entry, name) -
-				     offsetof(struct cache_entry, ce_stat_data));
-			ce->ce_flags = ce_flags;
-			base->ce_flags = base_flags;
-			if (ret)
-				ce->ce_flags |= CE_UPDATE_IN_BASE;
+			/*
+			 * This is the copy of a cache entry that is present
+			 * in the shared index, created by unpack_trees()
+			 * while it constructed a new index.
+			 */
+			if (ce->ce_flags & CE_UPDATE_IN_BASE) {
+				/*
+				 * Already marked for inclusion in the split
+				 * index, either because the corresponding
+				 * file was modified and the cached stat data
+				 * was refreshed, or because the original
+				 * entry already had a replacement entry in
+				 * the split index.
+				 * Nothing to do.
+				 */
+			} else {
+				/*
+				 * Thoroughly compare the cached data to see
+				 * whether it should be marked for inclusion
+				 * in the split index.
+				 *
+				 * This comparison might be unnecessary, as
+				 * code paths modifying the cached data do
+				 * set CE_UPDATE_IN_BASE as well.
+				 */
+				if (compare_ce_content(ce, base))
+					ce->ce_flags |= CE_UPDATE_IN_BASE;
+			}
 			discard_cache_entry(base);
 			si->base->cache[ce->index - 1] = ce;
 		}
-- 
2.19.1.465.gaff195083f

[PATCH v4 6/6] split-index: smudge and add racily clean cache entries to split index

[SZEDER Gábor]

Ever since the split index feature was introduced [1], refreshing a
split index is prone to a variant of the classic racy git problem.

Consider the following sequence of commands updating the split index
when the shared index contains a racily clean cache entry, i.e. an
entry whose cached stat data matches with the corresponding file in
the worktree and the cached mtime matches that of the index:

  echo "cached content" >file
  git update-index --split-index --add file
  echo "dirty worktree" >file    # size stays the same!
  # ... wait ...
  git update-index --add other-file

Normally, when a non-split index is updated, then do_write_index()
(the function responsible for writing all kinds of indexes, "regular",
split, and shared) recognizes racily clean cache entries, and writes
them with smudged stat data, i.e. with file size set to 0.  When
subsequent git commands read the index, they will notice that the
smudged stat data doesn't match with the file in the worktree, and
then go on to check the file's content and notice its dirtiness.

In the above example, however, in the second 'git update-index'
prepare_to_write_split_index() decides which cache entries stored only
in the shared index should be replaced in the new split index.  Alas,
this function never looks out for racily clean cache entries, and
since the file's stat data in the worktree hasn't changed since the
shared index was written, it won't be replaced in the new split index.
Consequently, do_write_index() doesn't even get this racily clean
cache entry, and can't smudge its stat data.  Subsequent git commands
will then see that the index has more recent mtime than the file and
that the (not smudged) cached stat data still matches with the file in
the worktree, and, ultimately, will erroneously consider the file
clean.

Modify prepare_to_write_split_index() to recognize racily clean cache
entries, and mark them to be added to the split index.  Note that
there are two places where it should check raciness: first those cache
entries that are only stored in the shared index, and then those that
have been copied by unpack_trees() from the shared index while it
constructed a new index.  This way do_write_index() will get these
racily clean cache entries as well, and will then write them with
smudged stat data to the new split index.

This change makes all tests in 't1701-racy-split-index.sh' pass, so
flip the two 'test_expect_failure' tests to success.  Also add the '#'
(as in nr. of trial) to those tests' description that were omitted
when the tests expected failure.

Note that after this change if the index is split when it contains a
racily clean cache entry, then a smudged cache entry will be written
both to the new shared and to the new split indexes.  This doesn't
affect regular git commands: as far as they are concerned this is just
an entry in the split index replacing an outdated entry in the shared
index.  It did affect a few tests in 't1700-split-index.sh', though,
because they actually check which entries are stored in the split
index; a previous patch in this series has already made the necessary
adjustments in 't1700'.  And racily clean cache entries and index
splitting are rare enough to not worry about the resulting duplicated
smudged cache entries, and the additional complexity required to
prevent them is not worth it.

Several tests failed occasionally when the test suite was run with
'GIT_TEST_SPLIT_INDEX=yes'.  Here are those that I managed to trace
back to this racy split index problem, starting with those failing
more frequently, with a link to a failing Travis CI build job for
each.  The highlighted line [2] shows when the racy file was written,
which is not always in the failing test but in a preceeding setup
test.

  t3903-stash.sh:
    https://travis-ci.org/git/git/jobs/385542084#L5858

  t4024-diff-optimize-common.sh:
    https://travis-ci.org/git/git/jobs/386531969#L3174

  t4015-diff-whitespace.sh:
    https://travis-ci.org/git/git/jobs/360797600#L8215

  t2200-add-update.sh:
    https://travis-ci.org/git/git/jobs/382543426#L3051

  t0090-cache-tree.sh:
    https://travis-ci.org/git/git/jobs/416583010#L3679

There might be others, e.g. perhaps 't1000-read-tree-m-3way.sh' and
others using 'lib-read-tree-m-3way.sh', but I couldn't confirm yet.

[1] In the branch leading to the merge commit v2.1.0-rc0~45 (Merge
    branch 'nd/split-index', 2014-07-16).

[2] Note that those highlighted lines are in the 'after failure' fold,
    and your browser might unhelpfully fold it up before you could
    take a good look.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 cache.h                     |  2 ++
 read-cache.c                |  2 +-
 split-index.c               | 42 ++++++++++++++++++++++++++++++++++++-
 t/t1701-racy-split-index.sh |  8 ++-----
 4 files changed, 46 insertions(+), 8 deletions(-)

diff --git a/cache.h b/cache.h
index 4d014541ab..3f419b6c79 100644
--- a/cache.h
+++ b/cache.h
@@ -781,6 +781,8 @@ extern void *read_blob_data_from_index(const struct index_state *, const char *,
 #define CE_MATCH_REFRESH		0x10
 /* don't refresh_fsmonitor state or do stat comparison even if CE_FSMONITOR_VALID is true */
 #define CE_MATCH_IGNORE_FSMONITOR 0X20
+extern int is_racy_timestamp(const struct index_state *istate,
+			     const struct cache_entry *ce);
 extern int ie_match_stat(struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
 extern int ie_modified(struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
 
diff --git a/read-cache.c b/read-cache.c
index 7b1354d759..8f644f68b4 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -337,7 +337,7 @@ static int is_racy_stat(const struct index_state *istate,
 		);
 }
 
-static int is_racy_timestamp(const struct index_state *istate,
+int is_racy_timestamp(const struct index_state *istate,
 			     const struct cache_entry *ce)
 {
 	return (!S_ISGITLINK(ce->ce_mode) &&
diff --git a/split-index.c b/split-index.c
index 187b910f5b..875f538802 100644
--- a/split-index.c
+++ b/split-index.c
@@ -259,8 +259,39 @@ void prepare_to_write_split_index(struct index_state *istate)
 			}
 			ce->ce_flags |= CE_MATCHED; /* or "shared" */
 			base = si->base->cache[ce->index - 1];
-			if (ce == base)
+			if (ce == base) {
+				/* The entry is present in the shared index. */
+				if (ce->ce_flags & CE_UPDATE_IN_BASE) {
+					/*
+					 * Already marked for inclusion in
+					 * the split index, either because
+					 * the corresponding file was
+					 * modified and the cached stat data
+					 * was refreshed, or because there
+					 * is already a replacement entry in
+					 * the split index.
+					 * Nothing more to do here.
+					 */
+				} else if (!ce_uptodate(ce) &&
+					   is_racy_timestamp(istate, ce)) {
+					/*
+					 * A racily clean cache entry stored
+					 * only in the shared index: it must
+					 * be added to the split index, so
+					 * the subsequent do_write_index()
+					 * can smudge its stat data.
+					 */
+					ce->ce_flags |= CE_UPDATE_IN_BASE;
+				} else {
+					/*
+					 * The entry is only present in the
+					 * shared index and it was not
+					 * refreshed.
+					 * Just leave it there.
+					 */
+				}
 				continue;
+			}
 			if (ce->ce_namelen != base->ce_namelen ||
 			    strcmp(ce->name, base->name)) {
 				ce->index = 0;
@@ -281,6 +312,15 @@ void prepare_to_write_split_index(struct index_state *istate)
 				 * the split index.
 				 * Nothing to do.
 				 */
+			} else if (!ce_uptodate(ce) &&
+				   is_racy_timestamp(istate, ce)) {
+				/*
+				 * A copy of a racily clean cache entry from
+				 * the shared index.  It must be added to
+				 * the split index, so the subsequent
+				 * do_write_index() can smudge its stat data.
+				 */
+				ce->ce_flags |= CE_UPDATE_IN_BASE;
 			} else {
 				/*
 				 * Thoroughly compare the cached data to see
diff --git a/t/t1701-racy-split-index.sh b/t/t1701-racy-split-index.sh
index fbb77046da..5dc221ef38 100755
--- a/t/t1701-racy-split-index.sh
+++ b/t/t1701-racy-split-index.sh
@@ -148,7 +148,7 @@ done
 
 for trial in $trials
 do
-	test_expect_failure "update the split index when a racily clean cache entry is stored only in the shared index $trial" '
+	test_expect_success "update the split index when a racily clean cache entry is stored only in the shared index #$trial" '
 		rm -f .git/index .git/sharedindex.* &&
 
 		# The next three commands must be run within the same
@@ -170,8 +170,6 @@ do
 		# entry of racy-file is only stored in the shared index.
 		# A corresponding replacement cache entry with smudged
 		# stat data should be added to the new split index.
-		#
-		# Alas, such a smudged replacement entry is not added!
 		git update-index --add other-file &&
 
 		# Subsequent git commands should notice the smudged
@@ -182,7 +180,7 @@ done
 
 for trial in $trials
 do
-	test_expect_failure "update the split index after unpack trees() copied a racily clean cache entry from the shared index $trial" '
+	test_expect_success "update the split index after unpack trees() copied a racily clean cache entry from the shared index #$trial" '
 		rm -f .git/index .git/sharedindex.* &&
 
 		# The next three commands must be run within the same
@@ -205,8 +203,6 @@ do
 		# index.  A corresponding replacement cache entry
 		# with smudged stat data should be added to the new
 		# split index.
-		#
-		# Alas, such a smudged replacement entry is not added!
 		git read-tree -m HEAD &&
 
 		# Subsequent git commands should notice the smudged
-- 
2.19.1.465.gaff195083f

[PATCH 7/6] split-index: BUG() when cache entry refers to non-existing shared entry

[SZEDER Gábor]

When the split index feature is in use, then a cache entry is:

  - either only present in the split index, in which case its 'index'
    field must be 0,

  - or it should refer to an existing entry in the shared index, i.e.
    the 'index' field can't be greater than the size of the shared
    index.

If a cache entry were to refer to a non-existing entry in the shared
index, then that's a sign of something being wrong in the index state,
either as a result of a bug in dealing with the split/shared index
entries, or perhaps a (potentially unrelated) memory corruption issue.

prepare_to_write_split_index() already has a condition to catch cache
entries with such bogus 'index' field, but instead of calling BUG() it
just sets cache entry's 'index = 0', and the entry will then be
written to the new split index.

Don't write a new index file from bogus index state, and call BUG()
upon encountering an cache entry referring to a non-existing shared
index entry.

Running the test suite repeatedly with 'GIT_TEST_SPLIT_INDEX=yes'
doesn't trigger this condition.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---

A follow-up to:

  https://public-inbox.org/git/20180927134324.GI27036@localhost/


 split-index.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/split-index.c b/split-index.c
index 875f538802..5820412dc5 100644
--- a/split-index.c
+++ b/split-index.c
@@ -254,8 +254,8 @@ void prepare_to_write_split_index(struct index_state *istate)
 				continue;
 			}
 			if (ce->index > si->base->cache_nr) {
-				ce->index = 0;
-				continue;
+				BUG("ce refers to a shared ce at %d, which is beyond the shared index size %d",
+				    ce->index, si->base->cache_nr);
 			}
 			ce->ce_flags |= CE_MATCHED; /* or "shared" */
 			base = si->base->cache[ce->index - 1];
-- 
2.19.1.465.gaff195083f

Re: [PATCH v4 0/6] Fix the racy split index problem

[Ævar Arnfjörð Bjarmason]

On Thu, Oct 11 2018, SZEDER Gábor wrote:

> Fourth and hopefully final round of fixing occasional test failures when
> run with 'GIT_TEST_SPLIT_INDEX=yes'.  The only code change is the
> extraction of a helper function to compare two cache entries' content,
> and then a couple of minor log message clarifications.  The range-diff
> below is rather clear on that.

Looks good. I'm not going to run the stress test I did on v5 on this
since the changes are just moving existing code into a fuction, unless
you'd like me to or think there's a reason to that is.

Re: [PATCH v4 0/6] Fix the racy split index problem

[SZEDER Gábor]

On Thu, Oct 11, 2018 at 12:36:47PM +0200, Ævar Arnfjörð Bjarmason wrote:
> 
> On Thu, Oct 11 2018, SZEDER Gábor wrote:
> 
> > Fourth and hopefully final round of fixing occasional test failures when
> > run with 'GIT_TEST_SPLIT_INDEX=yes'.  The only code change is the
> > extraction of a helper function to compare two cache entries' content,
> > and then a couple of minor log message clarifications.  The range-diff
> > below is rather clear on that.
> 
> Looks good. I'm not going to run the stress test I did on v5 on this
> since the changes are just moving existing code into a fuction, unless
> you'd like me to or think there's a reason to that is.

FWIW, I intend to carry this patch below and use it in tests both
locally and on Travis CI.  I could never trigger any of those three
conditions by repeated test runs with 'GIT_TEST_SPLIT_INDEX=yes', or
by deliberately constructing tricky scenarios where they might be
triggered.

Perhaps with enough time I'll get lucky eventually :)

If it's not too much trouble, then perhaps you could pick it up as
well?  While testing previous versions of this series it turned out
that your setup has much more "luck" in finding problematic timings
than mine.


diff --git a/split-index.c b/split-index.c
index 5820412dc5..4af535e236 100644
--- a/split-index.c
+++ b/split-index.c
@@ -254,8 +254,8 @@ void prepare_to_write_split_index(struct index_state *istate)
 				continue;
 			}
 			if (ce->index > si->base->cache_nr) {
-				BUG("ce refers to a shared ce at %d, which is beyond the shared index size %d",
-				    ce->index, si->base->cache_nr);
+				BUG("ce of '%s' refers to a shared ce at %d, which is beyond the shared index size %d",
+				    ce->name, ce->index, si->base->cache_nr);
 			}
 			ce->ce_flags |= CE_MATCHED; /* or "shared" */
 			base = si->base->cache[ce->index - 1];
@@ -293,10 +293,9 @@ void prepare_to_write_split_index(struct index_state *istate)
 				continue;
 			}
 			if (ce->ce_namelen != base->ce_namelen ||
-			    strcmp(ce->name, base->name)) {
-				ce->index = 0;
-				continue;
-			}
+			    strcmp(ce->name, base->name))
+				BUG("ce of '%s' refers to the shared ce of a different file '%s'",
+				    ce->name, base->name);
 			/*
 			 * This is the copy of a cache entry that is present
 			 * in the shared index, created by unpack_trees()
@@ -332,7 +331,8 @@ void prepare_to_write_split_index(struct index_state *istate)
 				 * set CE_UPDATE_IN_BASE as well.
 				 */
 				if (compare_ce_content(ce, base))
-					ce->ce_flags |= CE_UPDATE_IN_BASE;
+					BUG("ce of '%s' differs from its shared ce, but the CE_UPDATE_IN_BASE flag was not set",
+					    ce->name);
 			}
 			discard_cache_entry(base);
 			si->base->cache[ce->index - 1] = ce;

Re: [PATCH v4 0/6] Fix the racy split index problem

[Junio C Hamano]

Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:

> On Thu, Oct 11 2018, SZEDER Gábor wrote:
>
>> Fourth and hopefully final round of fixing occasional test failures when
>> run with 'GIT_TEST_SPLIT_INDEX=yes'.  The only code change is the
>> extraction of a helper function to compare two cache entries' content,
>> and then a couple of minor log message clarifications.  The range-diff
>> below is rather clear on that.
>
> Looks good. I'm not going to run the stress test I did on v5 on this
> since the changes are just moving existing code into a fuction, unless
> you'd like me to or think there's a reason to that is.

Thanks, both.  Let's merge this round to 'next'; it would be OK to
do any further tweaks incrementally on top.