[PATCH 3/3] read-cache: don't write index twice if we can't write shared index

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

In a0a967568e ("update-index --split-index: do not split if $GIT_DIR is
read only", 2014-06-13), we tried to make sure we can still write an
index, even if the shared index can not be written.

We did so by just calling 'do_write_locked_index()' just before
'write_shared_index()'.  'do_write_locked_index()' always at least
closes the tempfile nowadays, and used to close or commit the lockfile
if COMMIT_LOCK or CLOSE_LOCK were given at the time this feature was
introduced.  COMMIT_LOCK or CLOSE_LOCK is passed in by most callers of
'write_locked_index()'.

After calling 'write_shared_index()', we call 'write_split_index()',
which calls 'do_write_locked_index()' again, which then tries to use the
closed lockfile again, but in fact fails to do so as it's already
closed. This eventually leads to a segfault.

Make sure to write the main index only once.

[nd: most of the commit message and investigation done by Thomas, I only
tweaked the solution a bit]

Helped-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 read-cache.c           |  5 +++--
 t/t1700-split-index.sh | 19 +++++++++++++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/read-cache.c b/read-cache.c
index c568643f55..c58c0a978a 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -2561,8 +2561,9 @@ int write_locked_index(struct index_state *istate, struct lock_file *lock,
 		if (!temp) {
 			hashclr(si->base_sha1);
 			ret = do_write_locked_index(istate, lock, flags);
-		} else
-			ret = write_shared_index(istate, &temp);
+			goto out;
+		}
+		ret = write_shared_index(istate, &temp);
 
 		saved_errno = errno;
 		if (is_tempfile_active(temp))
diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
index af9b847761..d2a8e0312a 100755
--- a/t/t1700-split-index.sh
+++ b/t/t1700-split-index.sh
@@ -401,4 +401,23 @@ done <<\EOF
 0642 -rw-r---w-
 EOF
 
+test_expect_success SANITY 'graceful handling when splitting index is not allowed' '
+	test_create_repo ro &&
+	(
+		cd ro &&
+		test_commit initial &&
+		git update-index --split-index &&
+		test -f .git/sharedindex.*
+	) &&
+	cp ro/.git/index new-index &&
+	test_when_finished "chmod u+w ro/.git" &&
+	chmod u-w ro/.git &&
+	GIT_INDEX_FILE="$(pwd)/new-index" git -C ro update-index --split-index &&
+	chmod u+w ro/.git &&
+	rm ro/.git/sharedindex.* &&
+	GIT_INDEX_FILE=new-index git ls-files >actual &&
+	echo initial.t >expected &&
+	test_cmp expected actual
+'
+
 test_done
-- 
2.16.0.47.g3d9b0fac3a