tar/common: get index of first invalid utf-8 char

tar/common/utf8.go

@@ -2,20 +2,21 @@ package common
 
 // IsValidUtf8String checks for in valid UTF-8 characters
 func IsValidUtf8String(s string) bool {
-	for _, r := range s {
-		if int(r) == 0xfffd {
-			return false
-		}
-	}
-	return true
+	return InvalidUtf8Index([]byte(s)) == -1
 }
 
 // IsValidUtf8Btyes checks for in valid UTF-8 characters
 func IsValidUtf8Btyes(b []byte) bool {
-	for _, r := range string(b) {
+	return InvalidUtf8Index(b) == -1
+}
+
+// InvalidUtf8Index returns the offset of the first invalid UTF-8 character.
+// Default is to return -1 for a wholly valid sequence.
+func InvalidUtf8Index(b []byte) int {
+	for i, r := range string(b) {
 		if int(r) == 0xfffd {
-			return false
+			return i
 		}
 	}
-	return true
+	return -1
 }