[PATCH] [diff-lib] Fix check_removed when fsmonitor is on

[Josip Sokcevic <sokcevic@google.com>]

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` may not be initialized. Since `lstat` calls
aren't not desired when fsmonitor is on, skip the entire gitlink check using
the same condition used to initialize `stat *st`.
---
This patch is using jonathantanmy@ test case, which now passes. It's
possible there are similar edge cases with fsmonitor, so I'll do more
extensive e2e testing tomorrow.
 diff-lib.c                   | 2 +-
 t/t7527-builtin-fsmonitor.sh | 6 ++++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/diff-lib.c b/diff-lib.c
index d8aa777a73..c67aa5ff89 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -46,7 +46,7 @@ static int check_removed(const struct index_state *istate, const struct cache_en
 	}
 	if (has_symlink_leading_path(ce->name, ce_namelen(ce)))
 		return 1;
-	if (S_ISDIR(st->st_mode)) {
+	if (!(ce->ce_flags & CE_FSMONITOR_VALID) && S_ISDIR(st->st_mode)) {
 		struct object_id sub;
 
 		/*
diff --git a/t/t7527-builtin-fsmonitor.sh b/t/t7527-builtin-fsmonitor.sh
index 0c241d6c14..894a80fbe8 100755
--- a/t/t7527-builtin-fsmonitor.sh
+++ b/t/t7527-builtin-fsmonitor.sh
@@ -824,6 +824,10 @@ test_expect_success 'submodule always visited' '
 
 	create_super super &&
 	create_sub sub &&
+	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 submodule add ../sub ./dir_1/dir_2/sub &&
 	git -C super commit -m "add sub" &&
@@ -837,6 +841,8 @@ test_expect_success 'submodule always visited' '
 	# some dirt in the submodule and confirm matching output.
 
 	# Completely clean status.
+	echo Now running for clean status &&
+
 	my_match_and_clean &&
 
 	# .M S..U
-- 
2.42.0.283.g2d96d420d3-goog