[PATCH v3] read-cache: make the split index obey umask settings

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Make the split index write out its .git/sharedindex_* files with the
same permissions as .git/index. This only changes the behavior when
core.sharedRepository isn't set, i.e. the user's umask settings will
be respected.

This hasn't been the case ever since the split index was originally
implemented in c18b80a0e8 ("update-index: new options to
enable/disable split index mode", 2014-06-13). A mkstemp()-like
function has always been used to create it. First mkstemp() itself,
and then later our own mkstemp()-like in
f6ecc62dbf ("write_shared_index(): use tempfile module", 2015-08-10)

A related bug was fixed in df801f3f9f ("read-cache: use shared perms
when writing shared index", 2017-06-25). Since then the split index
has respected core.sharedRepository.

However, using that setting should not be required simply to make git
obey the user's umask setting. It's intended for the use-case of
overriding whatever that umask is set to. This fixes cases where the
user has e.g. set his umask to 022 on a shared server in anticipation
of other user's needing to run "status", "log" etc. in his repository.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---

Here it is with a rewritten commit message & adjusted comment as
discussed in the v2 discussion.

 read-cache.c           |  3 ++-
 t/t1700-split-index.sh | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/read-cache.c b/read-cache.c
index 4ca81286c0..e7e77e780b 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -3150,7 +3150,8 @@ int write_locked_index(struct index_state *istate, struct lock_file *lock,
 		struct tempfile *temp;
 		int saved_errno;
 
-		temp = mks_tempfile(git_path("sharedindex_XXXXXX"));
+		/* Same initial permissions as the main .git/index file */
+		temp = mks_tempfile_sm(git_path("sharedindex_XXXXXX"), 0, 0666);
 		if (!temp) {
 			oidclr(&si->base_oid);
 			ret = do_write_locked_index(istate, lock, flags);
diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
index 2ac47aa0e4..fa1d3d468b 100755
--- a/t/t1700-split-index.sh
+++ b/t/t1700-split-index.sh
@@ -381,6 +381,26 @@ test_expect_success 'check splitIndex.sharedIndexExpire set to "never" and "now"
 	test $(ls .git/sharedindex.* | wc -l) -le 2
 '
 
+test_expect_success POSIXPERM 'same mode for index & split index' '
+	git init same-mode &&
+	(
+		cd same-mode &&
+		test_commit A &&
+		test_modebits .git/index >index_mode &&
+		test_must_fail git config core.sharedRepository &&
+		git -c core.splitIndex=true status &&
+		shared=$(ls .git/sharedindex.*) &&
+		case "$shared" in
+		*" "*)
+			# we have more than one???
+			false ;;
+		*)
+			test_modebits "$shared" >split_index_mode &&
+			test_cmp index_mode split_index_mode ;;
+		esac
+	)
+'
+
 while read -r mode modebits
 do
 	test_expect_success POSIXPERM "split index respects core.sharedrepository $mode" '
-- 
2.19.1.1182.g4ecb1133ce