Skip to modify the file if the updater returned errors (#161)

* Check results are keeping updatable format even after replaced

* Add failing example for bad result

examples/simple.txt

@@ -1,3 +1,4 @@
 will_be_replaced: '0.39.0' # selfup { "regex": "\\d[^']+", "script": "echo 0.76.9" }
 not_be_replacedA: '0.39.0' # selfup { "regex": "\\d[^']+", "script": "echo 0.39.0" }
-not_be_replacedB: ':<' # selfup { "regex": ":<", "script": "echo ':)'" }
+not_be_replacedB: ':<' # selfup { "regex": ":[<\\)]", "script": "echo ':)'" }
+broken: ':<' # selfup { "regex": ":[<\\)]", "script": "this_command_does_not_exist_so_raise_errors_and_do_not_update_this_file" }

internal/selfup.go

@@ -69,9 +69,17 @@ func Update(path string, prefix string, skipBy string, isColor bool) (Result, er
 		}
 		replacer := strings.TrimSuffix(string(out), "\n")
 		extracted := re.FindString(head)
+		replaced := strings.Replace(head, extracted, replacer, 1)
+		if !isChanged {
+			isChanged = replaced != head
+		}
+		extractedToEnsure := re.FindString(replaced)
+		if replacer != extractedToEnsure {
+			return Result{}, xerrors.Errorf("%s:%d: The result of updater command has malformed format: %s", path, lineNumber, replacer)
+		}
 		estimation := " "
 		suffix := ""
-		if extracted != replacer {
+		if replaced != head {
 			replacedCount += 1
 			estimation = "✓"
 			if isColor {
@@ -80,13 +88,8 @@ func Update(path string, prefix string, skipBy string, isColor bool) (Result, er
 			}
 			suffix = fmt.Sprintf(" => %s", replacer)
 		}
-		fmt.Println(fmt.Sprintf("%s %s:%d: %s", estimation, path, lineNumber, extracted) + suffix)
-
-		replaced := strings.Replace(head, extracted, replacer, 1)
-		if !isChanged {
-			isChanged = replaced != head
-		}
 		newLines = append(newLines, replaced+match+tail)
+		fmt.Println(fmt.Sprintf("%s %s:%d: %s", estimation, path, lineNumber, extracted) + suffix)
 	}
 
 	return Result{