Re: [PATCH] cache: add fake_lstat()

[Junio C Hamano <gitster@pobox.com>]

And your diff-lib change may look like this on top.  This one
unfortunately is completely untested, as I do not use macOS.

Reviewing, testing, and improving it is highly appreciated.

----- >8 ---------- >8 ---------- >8 -----
Subject: diff-lib: fix check_removed() when fsmonitor is active

`git diff-index` may return incorrect deleted entries when fsmonitor
is used in a repository with git submodules. This can be observed on
Mac machines, but it can affect all other supported platforms too.

If fsmonitor is used, `stat *st` is left uninitialied if cache_entry
has CE_FSMONITOR_VALID bit set.  But, there are three call sites
that rely on stat afterwards, which can result in incorrect results.

We can fill members of "struct stat" that matters well enough using
the information we have in "struct cache_entry" that fsmonitor told
us is up-to-date to solve this.

Helped-by: Josip Sokcevic <sokcevic@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 diff-lib.c                   | 9 ++++++++-
 t/t7527-builtin-fsmonitor.sh | 5 +++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git c/diff-lib.c w/diff-lib.c
index d8aa777a73..7799747a97 100644
--- c/diff-lib.c
+++ w/diff-lib.c
@@ -38,8 +38,15 @@
  */
 static int check_removed(const struct index_state *istate, const struct cache_entry *ce, struct stat *st)
 {
+	int stat_err;
+
 	assert(is_fsmonitor_refreshed(istate));
-	if (!(ce->ce_flags & CE_FSMONITOR_VALID) && lstat(ce->name, st) < 0) {
+
+	if (!(ce->ce_flags & CE_FSMONITOR_VALID))
+		stat_err = lstat(ce->name, st);
+	else
+		stat_err = fake_lstat(ce, st);
+	if (stat_err < 0) {
 		if (!is_missing_file_error(errno))
 			return -1;
 		return 1;
diff --git c/t/t7527-builtin-fsmonitor.sh w/t/t7527-builtin-fsmonitor.sh
index 0c241d6c14..78503158fd 100755
--- c/t/t7527-builtin-fsmonitor.sh
+++ w/t/t7527-builtin-fsmonitor.sh
@@ -809,6 +809,11 @@ my_match_and_clean () {
 		status --porcelain=v2 >actual.without &&
 	test_cmp actual.with actual.without &&
 
+	git -C super --no-optional-locks diff-index --name-status HEAD >actual.with &&
+	git -C super --no-optional-locks -c core.fsmonitor=false \
+		diff-index --name-status HEAD >actual.without &&
+	test_cmp actual.with actual.without &&
+
 	git -C super/dir_1/dir_2/sub reset --hard &&
 	git -C super/dir_1/dir_2/sub clean -d -f
 }