utf8: fix returning negative string width

The `utf8_strnwidth()` function calls `utf8_width()` in a loop and adds
its returned width to the end result. `utf8_width()` can return `-1`
though in case it reads a control character, which means that the
computed string width is going to be wrong. In the worst case where
there are more control characters than non-control characters, we may
even return a negative string width.

Fix this bug by treating control characters as having zero width.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/utf8.c b/utf8.c
index 504e517..6a21fd6 100644
--- a/utf8.c
+++ b/utf8.c
@@ -212,11 +212,15 @@ int utf8_strnwidth(const char *string, size_t len, int skip_ansi)
 	const char *orig = string;
 
 	while (string && string < orig + len) {
-		int skip;
+		int glyph_width, skip;
+
 		while (skip_ansi &&
 		       (skip = display_mode_esc_sequence_len(string)) != 0)
 			string += skip;
-		width += utf8_width(&string, NULL);
+
+		glyph_width = utf8_width(&string, NULL);
+		if (glyph_width > 0)
+			width += glyph_width;
 	}
 	return string ? width : len;
 }