lib/transform: fix panic in truncate_keep_extension

Return error when extension is longer than truncation limit.

lib/transform/transform.go

@@ -165,13 +165,13 @@ func transformPathSegment(s string, t transform) (string, error) {
 		if err != nil {
 			return s, err
 		}
-		return truncateChars(s, max, false), nil
+		return truncateChars(s, max, false)
 	case ConvTruncateKeepExtension:
 		max, err := strconv.Atoi(t.value)
 		if err != nil {
 			return s, err
 		}
-		return truncateChars(s, max, true), nil
+		return truncateChars(s, max, true)
 	case ConvTruncateBytes:
 		max, err := strconv.Atoi(t.value)
 		if err != nil {
@@ -266,19 +266,23 @@ func splitExtension(remote string) (base, exts string) {
 	return base, exts
 }
 
-func truncateChars(s string, max int, keepExtension bool) string {
+func truncateChars(s string, max int, keepExtension bool) (string, error) {
 	if max <= 0 {
-		return s
+		return s, nil
 	}
 	if utf8.RuneCountInString(s) <= max {
-		return s
+		return s, nil
 	}
 	exts := ""
 	if keepExtension {
 		s, exts = splitExtension(s)
 	}
+	keep := max - utf8.RuneCountInString(exts)
+	if keep <= 0 {
+		return "", errors.New("extension is longer than the truncation limit")
+	}
 	runes := []rune(s)
-	return string(runes[:max-utf8.RuneCountInString(exts)]) + exts
+	return string(runes[:keep]) + exts, nil
 }
 
 // truncateBytes is like truncateChars but counts the number of bytes, not UTF-8 characters

lib/transform/transform_test.go

@@ -134,6 +134,9 @@ func TestVarious(t *testing.T) {
 		{"stories/Вот русское предложение, в котором байтов больше, чем символов.txt", "stories/Вот русское предложение, в котором бай", []string{"truncate_bytes=70"}},
 		{"stories/Вот русское предложение, в котором байтов больше, чем символов.txt", "stories/Вот русское предложение, в котором байтов больше, чем си.txt", []string{"truncate_keep_extension=60"}},
 		{"stories/Вот русское предложение, в котором байтов больше, чем символов.txt", "stories/Вот русское предложение, в котором б.txt", []string{"truncate_bytes_keep_extension=70"}},
+		// the extension alone exceeds the limit, so there is nothing valid to return
+		{"stories/photo.jpeg", "stories/photo.jpeg", []string{"all,truncate_keep_extension=3"}},
+		{"stories/a.tar.gz", "stories/a.tar.gz", []string{"all,truncate_keep_extension=2"}},
 		{"stories/The Quick Brown Fox!.txt", "stories/The Quick Brown Fox!.txt", []string{"all,command=echo"}},
 		{"stories/The Quick Brown Fox!.txt", "stories/The Quick Brown Fox!.txt-" + time.Now().Local().Format("20060102"), []string{"date=-{YYYYMMDD}"}},
 		{"stories/The Quick Brown Fox!.txt", "stories/The Quick Brown Fox!.txt-" + time.Now().Local().Format("2006-01-02 0304PM"), []string{"date=-{macfriendlytime}"}},