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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

t/t4205-log-pretty-formats.sh

@@ -899,6 +899,12 @@ test_expect_success SIZE_T_IS_64BIT 'log --pretty with overflowing wrapping dire
 	test_cmp expect error
 '
 
+test_expect_success 'log --pretty with padding and preceding control chars' '
+	printf "\20\20   0" >expect &&
+	git log -1 --pretty="format:%x10%x10%>|(4)%x30" >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success EXPENSIVE,SIZE_T_IS_64BIT 'log --pretty with huge commit message' '
 	# We only assert that this command does not crash. This needs to be
 	# executed with the address sanitizer to demonstrate failure.

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;
 }