packer: MultiErrorAppend helper method.

packer/multi_error.go

@@ -22,3 +22,21 @@ func (e *MultiError) Error() string {
 		"%d error(s) occurred:\n\n%s",
 		len(e.Errors), strings.Join(points, "\n"))
 }
+
+// MultiErrorAppend is a helper function that will append more errors
+// onto a MultiError in order to create a larger multi-error. If the
+// original error is not a MultiError, it will be turned into one.
+func MultiErrorAppend(err error, errs ...error) *MultiError {
+	switch err := err.(type) {
+	case *MultiError:
+		err.Errors = append(err.Errors, errs...)
+		return err
+	default:
+		newErrs := make([]error, len(errs)+1)
+		newErrs[0] = err
+		copy(newErrs[1:], errs)
+		return &MultiError{
+			Errors: newErrs,
+		}
+	}
+}

packer/multi_error_test.go

@@ -30,3 +30,22 @@ func TestMultiErrorError(t *testing.T) {
 	multi := &MultiError{errors}
 	assert.Equal(multi.Error(), expected, "should have proper error")
 }
+
+func TestMultiErrorAppend_MultiError(t *testing.T) {
+	original := &MultiError{
+		Errors: []error{errors.New("foo")},
+	}
+
+	result := MultiErrorAppend(original, errors.New("bar"))
+	if len(result.Errors) != 2 {
+		t.Fatalf("wrong len: %d", len(result.Errors))
+	}
+}
+
+func TestMultiErrorAppend_NonMultiError(t *testing.T) {
+	original := errors.New("foo")
+	result := MultiErrorAppend(original, errors.New("bar"))
+	if len(result.Errors) != 2 {
+		t.Fatalf("wrong len: %d", len(result.Errors))
+	}
+}