add -p: gracefully handle unparseable hunk headers in colored diffs

In
https://lore.kernel.org/git/ecf6f5be-22ca-299f-a8f1-bda38e5ca246@gmail.com,
Phillipe Blain reported that the built-in `git add -p` command fails
when asked to use [`diff-so-fancy`][diff-so-fancy] to colorize the diff.

The reason is that this tool produces colored diffs with a hunk header
that does not contain any parseable `@@ ... @@` line range information,
and therefore we cannot detect any part in that header that comes after
the line range.

As proposed by Phillip Wood, let's take that for a clear indicator that
we should show the hunk headers verbatim. This is what the Perl version
of the interactive `add` command did, too.

[diff-so-fancy]: https://github.com/so-fancy/diff-so-fancy

Reported-by: Philippe Blain <levraiphilippeblain@gmail.com>
Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

add-patch.c

@@ -238,6 +238,7 @@ struct hunk_header {
 	 * include the newline.
 	 */
 	size_t extra_start, extra_end, colored_extra_start, colored_extra_end;
+	unsigned suppress_colored_line_range:1;
 };
 
 struct hunk {
@@ -358,15 +359,14 @@ static int parse_hunk_header(struct add_p_state *s, struct hunk *hunk)
 	if (!eol)
 		eol = s->colored.buf + s->colored.len;
 	p = memmem(line, eol - line, "@@ -", 4);
-	if (!p)
-		return error(_("could not parse colored hunk header '%.*s'"),
-			     (int)(eol - line), line);
-	p = memmem(p + 4, eol - p - 4, " @@", 3);
-	if (!p)
-		return error(_("could not parse colored hunk header '%.*s'"),
-			     (int)(eol - line), line);
+	if (p && (p = memmem(p + 4, eol - p - 4, " @@", 3))) {
+		header->colored_extra_start = p + 3 - s->colored.buf;
+	} else {
+		/* could not parse colored hunk header, leave as-is */
+		header->colored_extra_start = hunk->colored_start;
+		header->suppress_colored_line_range = 1;
+	}
 	hunk->colored_start = eol - s->colored.buf + (*eol == '\n');
-	header->colored_extra_start = p + 3 - s->colored.buf;
 	header->colored_extra_end = hunk->colored_start;
 
 	return 0;
@@ -659,6 +659,15 @@ static void render_hunk(struct add_p_state *s, struct hunk *hunk,
 		if (!colored) {
 			p = s->plain.buf + header->extra_start;
 			len = header->extra_end - header->extra_start;
+		} else if (header->suppress_colored_line_range) {
+			strbuf_add(out,
+				   s->colored.buf + header->colored_extra_start,
+				   header->colored_extra_end -
+				   header->colored_extra_start);
+
+			strbuf_add(out, s->colored.buf + hunk->colored_start,
+				   hunk->colored_end - hunk->colored_start);
+			return;
 		} else {
 			strbuf_addstr(out, s->s.fraginfo_color);
 			p = s->colored.buf + header->colored_extra_start;

t/t3701-add-interactive.sh

@@ -767,6 +767,16 @@ test_expect_success 'detect bogus diffFilter output' '
 	grep "mismatched output" output
 '
 
+test_expect_success 'handle iffy colored hunk headers' '
+	git reset --hard &&
+
+	echo content >test &&
+	printf n >n &&
+	force_color git -c interactive.diffFilter="sed s/.*@@.*/XX/" \
+		add -p >output 2>&1 <n &&
+	grep "^XX$" output
+'
+
 test_expect_success 'diff.algorithm is passed to `git diff-files`' '
 	git reset --hard &&