retry: Do not retry Load() if file does not exist

changelog/unreleased/issue-4515

@@ -0,0 +1,8 @@
+Change: Don't retry to load files that don't exist
+
+Restic used to always retry to load files. It now only retries to load
+files if they exist.
+
+https://github.com/restic/restic/issues/4515
+https://github.com/restic/restic/issues/1523
+https://github.com/restic/restic/pull/4520

internal/backend/retry/backend_retry.go

@@ -128,7 +128,11 @@ func (be *Backend) Save(ctx context.Context, h restic.Handle, rd restic.RewindRe
 func (be *Backend) Load(ctx context.Context, h restic.Handle, length int, offset int64, consumer func(rd io.Reader) error) (err error) {
 	return be.retry(ctx, fmt.Sprintf("Load(%v, %v, %v)", h, length, offset),
 		func() error {
-			return be.Backend.Load(ctx, h, length, offset, consumer)
+			err := be.Backend.Load(ctx, h, length, offset, consumer)
+			if be.Backend.IsNotExist(err) {
+				return backoff.Permanent(err)
+			}
+			return err
 		})
 }
 

internal/backend/retry/backend_retry_test.go

@@ -274,6 +274,34 @@ func TestBackendLoadRetry(t *testing.T) {
 	test.Equals(t, 2, attempt)
 }
 
+func TestBackendLoadNotExists(t *testing.T) {
+	// load should not retry if the error matches IsNotExist
+	notFound := errors.New("not found")
+	attempt := 0
+
+	be := mock.NewBackend()
+	be.OpenReaderFn = func(ctx context.Context, h restic.Handle, length int, offset int64) (io.ReadCloser, error) {
+		attempt++
+		if attempt > 1 {
+			t.Fail()
+			return nil, errors.New("must not retry")
+		}
+		return nil, notFound
+	}
+	be.IsNotExistFn = func(err error) bool {
+		return errors.Is(err, notFound)
+	}
+
+	TestFastRetries(t)
+	retryBackend := New(be, 10, nil, nil)
+
+	err := retryBackend.Load(context.TODO(), restic.Handle{}, 0, 0, func(rd io.Reader) (err error) {
+		return nil
+	})
+	test.Assert(t, be.IsNotExistFn(err), "unexpected error %v", err)
+	test.Equals(t, 1, attempt)
+}
+
 func TestBackendStatNotExists(t *testing.T) {
 	// stat should not retry if the error matches IsNotExist
 	notFound := errors.New("not found")