[PATCH v3] diff: stop output garbled message in dry run mode

[Lidong Yan <yldhome2d2@gmail.com>]

Earlier, b55e6d36 (diff: ensure consistent diff behavior with
ignore options, 2025-08-08) introduced "dry-run" mode to the
diff machinery so that content-based diff filtering (like
ignoring space changes or those that match -I<regex>) can first
try to produce a patch without emitting any output to see if
under the given diff filtering condition we would get any output
lines, and a new helper function diff_flush_patch_quietly() was
introduced to use the mode to see an individual filepair needs
to be shown.

However, the solution was not complete. When files are deleted,
file modes change, or there are unmerged entries in the index,
dry-run mode still produces output because we overlooked these
conditions, and as a result, dry-run mode was not quiet.

Since dry-run mode is only set in diff_flush_patch_quietly(),
setting the output file to "/dev/null" within diff_flush_patch_quietly()
ensures no output is emitted in dry-run mode. To improve performance
of dry-run mode, add a check before outputting to determine if we
should exit early to avoid unnecessary output processing.

Signed-off-by: Lidong Yan <yldhome2d2@gmail.com>
---
 diff.c                  |  8 ++++++--
 t/t4013-diff-various.sh | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/diff.c b/diff.c
index 87fa16b730..3c92f0d806 100644
--- a/diff.c
+++ b/diff.c
@@ -1351,6 +1351,9 @@ static void emit_diff_symbol_from_struct(struct diff_options *o,
 	int len = eds->len;
 	unsigned flags = eds->flags;
 
+	if (o->dry_run)
+		return;
+
 	switch (s) {
 	case DIFF_SYMBOL_NO_LF_EOF:
 		context = diff_get_color_opt(o, DIFF_CONTEXT);
@@ -4420,7 +4423,7 @@ static void run_external_diff(const struct external_diff *pgm,
 {
 	struct child_process cmd = CHILD_PROCESS_INIT;
 	struct diff_queue_struct *q = &diff_queued_diff;
-	int quiet = !(o->output_format & DIFF_FORMAT_PATCH);
+	int quiet = !(o->output_format & DIFF_FORMAT_PATCH) || o->dry_run;
 	int rc;
 
 	/*
@@ -4615,7 +4618,8 @@ static void run_diff_cmd(const struct external_diff *pgm,
 		    p->status == DIFF_STATUS_RENAMED)
 			o->found_changes = 1;
 	} else {
-		fprintf(o->file, "* Unmerged path %s\n", name);
+		if (!o->dry_run)
+			fprintf(o->file, "* Unmerged path %s\n", name);
 		o->found_changes = 1;
 	}
 }
diff --git a/t/t4013-diff-various.sh b/t/t4013-diff-various.sh
index 55a06eadb3..d35695f5b0 100755
--- a/t/t4013-diff-various.sh
+++ b/t/t4013-diff-various.sh
@@ -661,6 +661,43 @@ test_expect_success 'diff -I<regex>: ignore matching file' '
 	test_grep ! "file1" actual
 '
 
+test_expect_success 'diff -I<regex>: ignore all content changes' '
+	test_when_finished "git rm -f file1 file2 file3" &&
+	: >file1 &&
+	git add file1 &&
+	: >file2 &&
+	git add file2 &&
+	: >file3 &&
+	git add file3 &&
+
+	rm -f file1 file2 &&
+	mkdir file2 &&
+	echo "A" >file3 &&
+	A_hash=$(git hash-object -w file3) &&
+	echo "B" >file3 &&
+	B_hash=$(git hash-object -w file3) &&
+	cat <<-EOF | git update-index --index-info &&
+	100644 $A_hash 1	file3
+	100644 $B_hash 2	file3
+	EOF
+
+	test_diff_no_content_changes () {
+		git diff $1 --ignore-blank-lines -I".*" >actual &&
+		test_line_count = 3 actual &&
+		test_grep "file1" actual &&
+		test_grep "file2" actual &&
+		test_grep "file3" actual &&
+		test_grep ! "diff --git" actual
+	} &&
+	test_diff_no_content_changes "--raw" &&
+	test_diff_no_content_changes "--name-only" &&
+	test_diff_no_content_changes "--name-status" &&
+
+	: >actual &&
+	test_must_fail git diff --quiet -I".*" >actual &&
+	test_must_be_empty actual
+'
+
 # check_prefix <patch> <src> <dst>
 # check only lines with paths to avoid dependency on exact oid/contents
 check_prefix () {
-- 
2.50.1 (Apple Git-155)