diff: ensure consistent diff behavior with ignore options

In git-diff, options like `-w` and `-I<regex>`, two files are considered
equivalent under the specified "ignore" rules, even when they are not
bit-for-bit identical. For options like `--raw`, `--name-status`,
and `--name-only`, git-diff deliberately compares only the SHA values
to determine whether two files are equivalent, for performance reasons.
As a result, a file shown in `git diff --name-status` may not appear
in `git diff --patch`.

To quickly determine whether two files are equivalent, add a helper
function diff_flush_patch_quietly() in diff.c. Add `.dry_run` field in
`struct diff_options`. When `.dry_run` is true, builtin_diff() returns
immediately upon finding any change. Call diff_flush_patch_quietly()
to determine if we should flush `--raw`, `--name-only` or `--name-status`
output.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Lidong Yan <yldhome2d2@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 50 insertions, 14 deletions

diff --git a/diff.c b/diff.c
index dca87e164f..cb04a9a6f2 100644
--- a/diff.c
+++ b/diff.c
@@ -2444,6 +2444,15 @@ static int fn_out_consume(void *priv, char *line, unsigned long len)
 	return 0;
 }
 
+static int quick_consume(void *priv, char *line UNUSED, unsigned long len UNUSED)
+{
+	struct emit_callback *ecbdata = priv;
+	struct diff_options *o = ecbdata->opt;
+
+	o->found_changes = 1;
+	return 1;
+}
+
 static void pprint_rename(struct strbuf *name, const char *a, const char *b)
 {
 	const char *old_name = a;
@@ -3759,8 +3768,21 @@ static void builtin_diff(const char *name_a,
 
 		if (o->word_diff)
 			init_diff_words_data(&ecbdata, o, one, two);
-		if (xdi_diff_outf(&mf1, &mf2, NULL, fn_out_consume,
-				  &ecbdata, &xpp, &xecfg))
+		if (o->dry_run) {
+			/*
+			 * Unlike the !dry_run case, we need to ignore the
+			 * return value from xdi_diff_outf() here, because
+			 * xdi_diff_outf() takes non-zero return from its
+			 * callback function as a sign of error and returns
+			 * early (which is why we return non-zero from our
+			 * callback, quick_consume()).  Unfortunately,
+			 * xdi_diff_outf() signals an error by returning
+			 * non-zero.
+			 */
+			xdi_diff_outf(&mf1, &mf2, NULL, quick_consume,
+				      &ecbdata, &xpp, &xecfg);
+		} else if (xdi_diff_outf(&mf1, &mf2, NULL, fn_out_consume,
+					 &ecbdata, &xpp, &xecfg))
 			die("unable to generate diff for %s", one->path);
 		if (o->word_diff)
 			free_diff_words_data(&ecbdata);
@@ -6150,6 +6172,22 @@ static void diff_flush_patch(struct diff_filepair *p, struct diff_options *o)
 	run_diff(p, o);
 }
 
+/* return 1 if any change is found; otherwise, return 0 */
+static int diff_flush_patch_quietly(struct diff_filepair *p, struct diff_options *o)
+{
+	int saved_dry_run = o->dry_run;
+	int saved_found_changes = o->found_changes;
+	int ret;
+
+	o->dry_run = 1;
+	o->found_changes = 0;
+	diff_flush_patch(p, o);
+	ret = o->found_changes;
+	o->dry_run = saved_dry_run;
+	o->found_changes |= saved_found_changes;
+	return ret;
+}
+
 static void diff_flush_stat(struct diff_filepair *p, struct diff_options *o,
 			    struct diffstat_t *diffstat)
 {
@@ -6778,8 +6816,15 @@ void diff_flush(struct diff_options *options)
 			     DIFF_FORMAT_CHECKDIFF)) {
 		for (i = 0; i < q->nr; i++) {
 			struct diff_filepair *p = q->queue[i];
-			if (check_pair_status(p))
-				flush_one_pair(p, options);
+
+			if (!check_pair_status(p))
+				continue;
+
+			if (options->flags.diff_from_contents &&
+			    !diff_flush_patch_quietly(p, options))
+				continue;
+
+			flush_one_pair(p, options);
 		}
 		separator++;
 	}
@@ -6831,19 +6876,10 @@ void diff_flush(struct diff_options *options)
 	if (output_format & DIFF_FORMAT_NO_OUTPUT &&
 	    options->flags.exit_with_status &&
 	    options->flags.diff_from_contents) {
-		/*
-		 * run diff_flush_patch for the exit status. setting
-		 * options->file to /dev/null should be safe, because we
-		 * aren't supposed to produce any output anyway.
-		 */
-		diff_free_file(options);
-		options->file = xfopen("/dev/null", "w");
-		options->close_file = 1;
-		options->color_moved = 0;
 		for (i = 0; i < q->nr; i++) {
 			struct diff_filepair *p = q->queue[i];
 			if (check_pair_status(p))
-				diff_flush_patch(p, options);
+				diff_flush_patch_quietly(p, options);
 			if (options->found_changes)
 				break;
 		}