[PATCH v2 0/2] Minor fixes to git notes fanout code

[PATCH v2 0/2] Minor fixes to git notes fanout code

[Johan Herland]

No changes from v1 (except cosmetic fixes to commit messages).

Johan Herland (2):
  t3305: check notes fanout more carefully and robustly
  notes.c: fix off-by-one error when decreasing notes fanout

 notes.c                 |  20 +++++---
 t/t3305-notes-fanout.sh | 107 ++++++++++++++++++++++++++++++----------
 2 files changed, 94 insertions(+), 33 deletions(-)

-- 
2.23.1

[PATCH v2 1/2] t3305: check notes fanout more carefully and robustly

[Johan Herland]

In short, before this patch, this test script:
 - creates many notes
 - verifies that all notes in the notes tree has a fanout of 1
 - removes most notes
 - verifies that the notes in the notes tree now has a fanout of 0

The fanout verification only happened twice: after creating all the
notes, and after removing most of them.

This patch strengthens the test by checking the fanout after _each_
added/removed note: We assert that the switch from fanout 0 -> 1
happens exactly once while adding notes (and that the switch pervades
the entire notes tree). Likewise, we assert that the switch from
fanout 1 -> 0 happens exactly once while removing notes.

Additionally, we decrease the number of notes left after removal,
from 50 to 15 notes, in order to ensure that fanout 1 -> 0 transition
keeps happening regardless of external factors[1].

[1]: Currently (with the SHA1 hash function and the deterministic
object ids of the test environment) the fanout heuristic in the notes
code happens to switch from 0 -> 1 at 109 notes, and from 1 -> 0 at
59 notes. However, changing the hash function or other external
factors will vary these numbers, and the latter may - in theory - go
as low as 15. For more details, please see the discussion at
https://public-inbox.org/git/20200125230035.136348-4-sandals@crustytoothpaste.net/

Signed-off-by: Johan Herland <johan@herland.net>
---
 t/t3305-notes-fanout.sh | 101 ++++++++++++++++++++++++++++++----------
 1 file changed, 76 insertions(+), 25 deletions(-)

diff --git a/t/t3305-notes-fanout.sh b/t/t3305-notes-fanout.sh
index 831f83d211..402057c83a 100755
--- a/t/t3305-notes-fanout.sh
+++ b/t/t3305-notes-fanout.sh
@@ -4,6 +4,32 @@ test_description='Test that adding/removing many notes triggers automatic fanout
 
 . ./test-lib.sh
 
+path_has_fanout() {
+	path=$1 &&
+	fanout=$2 &&
+	after_last_slash=$((40 - $fanout * 2)) &&
+	echo $path | grep -q "^\([0-9a-f]\{2\}/\)\{$fanout\}[0-9a-f]\{$after_last_slash\}$"
+}
+
+touched_one_note_with_fanout() {
+	notes_commit=$1 &&
+	modification=$2 &&  # 'A' for addition, 'D' for deletion
+	fanout=$3 &&
+	diff=$(git diff-tree --no-commit-id --name-status --root -r $notes_commit) &&
+	path=$(echo $diff | sed -e "s/^$modification[\t ]//") &&
+	path_has_fanout "$path" $fanout;
+}
+
+all_notes_have_fanout() {
+	notes_commit=$1 &&
+	fanout=$2 &&
+	git ls-tree -r --name-only $notes_commit 2>/dev/null |
+	while read path
+	do
+		path_has_fanout $path $fanout || return 1
+	done
+}
+
 test_expect_success 'creating many notes with git-notes' '
 	num_notes=300 &&
 	i=0 &&
@@ -20,7 +46,7 @@ test_expect_success 'creating many notes with git-notes' '
 
 test_expect_success 'many notes created correctly with git-notes' '
 	git log | grep "^    " > output &&
-	i=300 &&
+	i=$num_notes &&
 	while test $i -gt 0
 	do
 		echo "    commit #$i" &&
@@ -30,34 +56,46 @@ test_expect_success 'many notes created correctly with git-notes' '
 	test_cmp expect output
 '
 
-test_expect_success 'many notes created with git-notes triggers fanout' '
-	# Expect entire notes tree to have a fanout == 1
-	git ls-tree -r --name-only refs/notes/commits |
-	while read path
+test_expect_success 'stable fanout 0 is followed by stable fanout 1' '
+	i=$num_notes &&
+	fanout=0 &&
+	while test $i -gt 0
 	do
-		echo $path | grep "^../[0-9a-f]*$" || {
-			echo "Invalid path \"$path\"" &&
-			return 1;
-		}
-	done
+		i=$(($i - 1)) &&
+		if touched_one_note_with_fanout refs/notes/commits~$i A $fanout
+		then
+			continue
+		elif test $fanout -eq 0
+		then
+			fanout=1 &&
+			if all_notes_have_fanout refs/notes/commits~$i $fanout
+			then
+				echo "Fanout 0 -> 1 at refs/notes/commits~$i" &&
+				continue
+			fi
+		fi &&
+		echo "Failed fanout=$fanout check at refs/notes/commits~$i" &&
+		git ls-tree -r --name-only refs/notes/commits~$i &&
+		return 1
+	done &&
+	all_notes_have_fanout refs/notes/commits 1
 '
 
 test_expect_success 'deleting most notes with git-notes' '
-	num_notes=250 &&
+	remove_notes=285 &&
 	i=0 &&
 	git rev-list HEAD |
-	while test $i -lt $num_notes && read sha1
+	while test $i -lt $remove_notes && read sha1
 	do
 		i=$(($i + 1)) &&
 		test_tick &&
-		git notes remove "$sha1" ||
-		exit 1
+		git notes remove "$sha1" 2>/dev/null || return 1
 	done
 '
 
 test_expect_success 'most notes deleted correctly with git-notes' '
-	git log HEAD~250 | grep "^    " > output &&
-	i=50 &&
+	git log HEAD~$remove_notes | grep "^    " > output &&
+	i=$(($num_notes - $remove_notes)) &&
 	while test $i -gt 0
 	do
 		echo "    commit #$i" &&
@@ -67,16 +105,29 @@ test_expect_success 'most notes deleted correctly with git-notes' '
 	test_cmp expect output
 '
 
-test_expect_success 'deleting most notes triggers fanout consolidation' '
-	# Expect entire notes tree to have a fanout == 0
-	git ls-tree -r --name-only refs/notes/commits |
-	while read path
+test_expect_success 'stable fanout 1 is followed by stable fanout 0' '
+	i=$remove_notes &&
+	fanout=1 &&
+	while test $i -gt 0
 	do
-		echo $path | grep -v "^../.*" || {
-			echo "Invalid path \"$path\"" &&
-			return 1;
-		}
-	done
+		i=$(($i - 1)) &&
+		if touched_one_note_with_fanout refs/notes/commits~$i D $fanout
+		then
+			continue
+		elif test $fanout -eq 1
+		then
+			fanout=0 &&
+			if all_notes_have_fanout refs/notes/commits~$i $fanout
+			then
+				echo "Fanout 1 -> 0 at refs/notes/commits~$i" &&
+				continue
+			fi
+		fi &&
+		echo "Failed fanout=$fanout check at refs/notes/commits~$i" &&
+		git ls-tree -r --name-only refs/notes/commits~$i &&
+		return 1
+	done &&
+	all_notes_have_fanout refs/notes/commits 0
 '
 
 test_done
-- 
2.23.1

[PATCH v2 2/2] notes.c: fix off-by-one error when decreasing notes fanout

[Johan Herland]

As noted in the previous commit, the nature of the fanout heuristic
in the notes code causes the exact point at which we increase or
decrease the notes fanout to vary with the objects being annotated.
Since the object ids generated by the test environment are
deterministic (by design), the notes generated and tested by t3305
are always the same, and we therefore happen to see the same fanout
behavior from one run to the next.

Coincidentally, if we were to change the test environment slightly
(say by making a test commit on an unrelated branch before we start
the t3305 test proper), we not only see the fanout switch happen at
different points, we also manage to trigger a _bug_ in the notes
code where the fanout 1 -> 0 switch is not applied uniformly across
the notes tree, but instead yields a notes tree like this:

  ...
  bdeafb301e44b0e4db0f738a2d2a7beefdb70b70
  bff2d39b4f7122bd4c5caee3de353a774d1e632a
  d3/8ec8f851adf470131178085bfbaab4b12ad2a7
  e0b173960431a3e692ae929736df3c9b73a11d5b
  eb3c3aede523d729990ac25c62a93eb47c21e2e3
  ...

The bug occurs when we are writing out a notes tree with a newly
decreased fanout, and the notes tree contains unexpanded subtrees
that should be consolidated into the parent tree as a consequence of
the decreased fanout):

Subtrees that happen to sit at an _even_ level in the internal notes
16-tree structure (in other words: subtrees whose path - "d3" in the
example above - is unique in the first nibble - i.e. there are no
other note paths that start with "d") are _not_ unpacked as part of
the tree writeout. This error will repeat itself in subsequent note
trees until the subtree is forced to be unpacked. In t3305 this only
happens when the d38ec8f8 note is itself removed from the tree.

The error is not severe (no information is lost, and the notes code
is able to read/decode this tree and manipulate it correctly), but
this is nonetheless a bug in the current implementation that should
be fixed.

That said, fixing the off-by-one error is not without complications:
We must take into account that the load_subtree() call from
for_each_note_helper() (that is now done to correctly unpack the
subtree while we're writing out the notes tree) may end up inserting
unpacked non-notes into the linked list of non_note entries held by
the struct notes_tree. Since we are in the process of writing out the
notes tree, this linked list is currently in the process of being
traversed by write_each_non_note_until(). The unpacked non-notes are
necessarily inserted between the last non-note we wrote out, and the
next non-note to be written. Hence, we cannot simply hold the
next_non_note to write in struct write_each_note_data (as we would
then silently skip these newly inserted notes), but must instead
always follow the ->next pointer from the last non-note we wrote.
(This part was caught by an existing test in t3304.)

Signed-off-by: Johan Herland <johan@herland.net>
---
 notes.c                 | 20 ++++++++++++--------
 t/t3305-notes-fanout.sh |  6 ++++++
 2 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/notes.c b/notes.c
index 0c79964c26..2de7f4bcfb 100644
--- a/notes.c
+++ b/notes.c
@@ -576,16 +576,16 @@ static int for_each_note_helper(struct notes_tree *t, struct int_node *tree,
 			 * the note tree that have not yet been explored. There
 			 * is a direct relationship between subtree entries at
 			 * level 'n' in the tree, and the 'fanout' variable:
-			 * Subtree entries at level 'n <= 2 * fanout' should be
+			 * Subtree entries at level 'n < 2 * fanout' should be
 			 * preserved, since they correspond exactly to a fanout
 			 * directory in the on-disk structure. However, subtree
-			 * entries at level 'n > 2 * fanout' should NOT be
+			 * entries at level 'n >= 2 * fanout' should NOT be
 			 * preserved, but rather consolidated into the above
 			 * notes tree level. We achieve this by unconditionally
 			 * unpacking subtree entries that exist below the
 			 * threshold level at 'n = 2 * fanout'.
 			 */
-			if (n <= 2 * fanout &&
+			if (n < 2 * fanout &&
 			    flags & FOR_EACH_NOTE_YIELD_SUBTREES) {
 				/* invoke callback with subtree */
 				unsigned int path_len =
@@ -602,7 +602,7 @@ static int for_each_note_helper(struct notes_tree *t, struct int_node *tree,
 					 path,
 					 cb_data);
 			}
-			if (n > fanout * 2 ||
+			if (n >= 2 * fanout ||
 			    !(flags & FOR_EACH_NOTE_DONT_UNPACK_SUBTREES)) {
 				/* unpack subtree and resume traversal */
 				tree->a[i] = NULL;
@@ -723,13 +723,15 @@ static int write_each_note_helper(struct tree_write_stack *tws,
 
 struct write_each_note_data {
 	struct tree_write_stack *root;
-	struct non_note *next_non_note;
+	struct non_note **nn_list;
+	struct non_note *nn_prev;
 };
 
 static int write_each_non_note_until(const char *note_path,
 		struct write_each_note_data *d)
 {
-	struct non_note *n = d->next_non_note;
+	struct non_note *p = d->nn_prev;
+	struct non_note *n = p ? p->next : *d->nn_list;
 	int cmp = 0, ret;
 	while (n && (!note_path || (cmp = strcmp(n->path, note_path)) <= 0)) {
 		if (note_path && cmp == 0)
@@ -740,9 +742,10 @@ static int write_each_non_note_until(const char *note_path,
 			if (ret)
 				return ret;
 		}
+		p = n;
 		n = n->next;
 	}
-	d->next_non_note = n;
+	d->nn_prev = p;
 	return 0;
 }
 
@@ -1177,7 +1180,8 @@ int write_notes_tree(struct notes_tree *t, struct object_id *result)
 	strbuf_init(&root.buf, 256 * (32 + the_hash_algo->hexsz)); /* assume 256 entries */
 	root.path[0] = root.path[1] = '\0';
 	cb_data.root = &root;
-	cb_data.next_non_note = t->first_non_note;
+	cb_data.nn_list = &(t->first_non_note);
+	cb_data.nn_prev = NULL;
 
 	/* Write tree objects representing current notes tree */
 	flags = FOR_EACH_NOTE_DONT_UNPACK_SUBTREES |
diff --git a/t/t3305-notes-fanout.sh b/t/t3305-notes-fanout.sh
index 402057c83a..3b4753e1b4 100755
--- a/t/t3305-notes-fanout.sh
+++ b/t/t3305-notes-fanout.sh
@@ -30,6 +30,12 @@ all_notes_have_fanout() {
 	done
 }
 
+test_expect_success 'tweak test environment' '
+	git checkout -b nondeterminism &&
+	test_commit A &&
+	git checkout --orphan with_notes;
+'
+
 test_expect_success 'creating many notes with git-notes' '
 	num_notes=300 &&
 	i=0 &&
-- 
2.23.1