Check utf8.RuneError result

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

model/labels/regexp.go

@@ -829,7 +829,11 @@ type zeroOrOneCharacterStringMatcher struct {
 
 func (m *zeroOrOneCharacterStringMatcher) Matches(s string) bool {
 	// If there's more than one rune in the string, then it can't match.
-	if _, size := utf8.DecodeRuneInString(s); size < len(s) {
+	if r, size := utf8.DecodeRuneInString(s); r == utf8.RuneError {
+		// Size is 0 for empty strings, 1 for invalid rune.
+		// Empty string matches, invalid rune matches if there isn't anything else.
+		return size == len(s)
+	} else if size < len(s) {
 		return false
 	}
 

model/labels/regexp_test.go

@@ -968,6 +968,23 @@ func TestZeroOrOneCharacterStringMatcher(t *testing.T) {
 		require.False(t, matcher.Matches(emoji1+"x"))
 		require.False(t, matcher.Matches(emoji1+emoji2))
 	})
+
+	t.Run("invalid unicode", func(t *testing.T) {
+		// Just for reference, we also compare to what `^.?$` regular expression matches.
+		re := regexp.MustCompile("^.?$")
+		matcher := &zeroOrOneCharacterStringMatcher{matchNL: true}
+
+		requireMatches := func(s string, expected bool) {
+			t.Helper()
+			require.Equal(t, expected, matcher.Matches(s))
+			require.Equal(t, re.MatchString(s), matcher.Matches(s))
+		}
+
+		requireMatches("\xff", true)
+		requireMatches("x\xff", false)
+		requireMatches("\xffx", false)
+		requireMatches("\xff\xfe", false)
+	})
 }
 
 func BenchmarkZeroOrOneCharacterStringMatcher(b *testing.B) {