diff: do not quit early on stat-dirty files

When QUICK is set (i.e. with --quiet) we try to do as little work as
possible, stopping after seeing the first change. stat-dirty is
considered a "change" but it may turn out not, if no actual content is
changed. The actual content test is performed too late in the process
and the shortcut may be taken prematurely, leading to incorrect return
code.

Assume we do "git diff --quiet". If we have a stat-dirty file "a" and
a really dirty file "b". We break the loop in run_diff_files() and
stop after "a" because we have got a "change". Later in
diffcore_skip_stat_unmatch() we find out "a" is actually not
changed. But there's nothing else in the diff queue, we incorrectly
declare "no change", ignoring the fact that "b" is changed.

This also happens to "git diff --quiet HEAD" when it hits
diff_can_quit_early() in oneway_diff().

This patch does the content test earlier in order to keep going if "a"
is unchanged. The test result is cached so that when
diffcore_skip_stat_unmatch() is done in the end, we spend no cycles on
re-testing "a".

Reported-by: IWAMOTO Toshihiro <iwamoto@valinux.co.jp>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -4594,6 +4594,11 @@ static int diff_filespec_is_identical(struct diff_filespec *one,
 
 static int diff_filespec_check_stat_unmatch(struct diff_filepair *p)
 {
+	if (p->done_skip_stat_unmatch)
+		return p->skip_stat_unmatch_result;
+
+	p->done_skip_stat_unmatch = 1;
+	p->skip_stat_unmatch_result = 0;
 	/*
 	 * 1. Entries that come from stat info dirtiness
 	 *    always have both sides (iow, not create/delete),
@@ -4615,8 +4620,8 @@ static int diff_filespec_check_stat_unmatch(struct diff_filepair *p)
 	    diff_populate_filespec(p->two, 1) ||
 	    (p->one->size != p->two->size) ||
 	    !diff_filespec_is_identical(p->one, p->two)) /* (2) */
-		return 1;
-	return 0;
+		p->skip_stat_unmatch_result = 1;
+	return p->skip_stat_unmatch_result;
 }
 
 static void diffcore_skip_stat_unmatch(struct diff_options *diffopt)
@@ -4792,6 +4797,7 @@ void diff_change(struct diff_options *options,
 		 unsigned old_dirty_submodule, unsigned new_dirty_submodule)
 {
 	struct diff_filespec *one, *two;
+	struct diff_filepair *p;
 
 	if (S_ISGITLINK(old_mode) && S_ISGITLINK(new_mode) &&
 	    is_submodule_ignored(concatpath, options))
@@ -4818,10 +4824,16 @@ void diff_change(struct diff_options *options,
 	fill_filespec(two, new_sha1, new_sha1_valid, new_mode);
 	one->dirty_submodule = old_dirty_submodule;
 	two->dirty_submodule = new_dirty_submodule;
+	p = diff_queue(&diff_queued_diff, one, two);
 
-	diff_queue(&diff_queued_diff, one, two);
-	if (!DIFF_OPT_TST(options, DIFF_FROM_CONTENTS))
-		DIFF_OPT_SET(options, HAS_CHANGES);
+	if (DIFF_OPT_TST(options, DIFF_FROM_CONTENTS))
+		return;
+
+	if (DIFF_OPT_TST(options, QUICK) && options->skip_stat_unmatch &&
+	    !diff_filespec_check_stat_unmatch(p))
+		return;
+
+	DIFF_OPT_SET(options, HAS_CHANGES);
 }
 
 struct diff_filepair *diff_unmerge(struct diff_options *options, const char *path)

diffcore.h

@@ -70,6 +70,8 @@ struct diff_filepair {
 	unsigned broken_pair : 1;
 	unsigned renamed_pair : 1;
 	unsigned is_unmerged : 1;
+	unsigned done_skip_stat_unmatch : 1;
+	unsigned skip_stat_unmatch_result : 1;
 };
 #define DIFF_PAIR_UNMERGED(p) ((p)->is_unmerged)
 

t/t4035-diff-quiet.sh

@@ -148,4 +148,10 @@ test_expect_success 'git diff --ignore-all-space, both files outside repo' '
 	)
 '
 
+test_expect_success 'git diff --quiet ignores stat-change only entries' '
+	test-chmtime +10 a &&
+	echo modified >>b &&
+	test_expect_code 1 git diff --quiet
+'
+
 test_done