Reworked Backend.Load API to retry errors during ongoing download

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>

internal/mock/backend.go

@@ -13,7 +13,7 @@ type Backend struct {
 	CloseFn      func() error
 	IsNotExistFn func(err error) bool
 	SaveFn       func(ctx context.Context, h restic.Handle, rd io.Reader) error
-	LoadFn       func(ctx context.Context, h restic.Handle, length int, offset int64) (io.ReadCloser, error)
+	OpenReaderFn func(ctx context.Context, h restic.Handle, length int, offset int64) (io.ReadCloser, error)
 	StatFn       func(ctx context.Context, h restic.Handle) (restic.FileInfo, error)
 	ListFn       func(ctx context.Context, t restic.FileType, fn func(restic.FileInfo) error) error
 	RemoveFn     func(ctx context.Context, h restic.Handle) error
@@ -22,6 +22,12 @@ type Backend struct {
 	LocationFn   func() string
 }
 
+// NewBackend returns new mock Backend instance
+func NewBackend() *Backend {
+	be := &Backend{}
+	return be
+}
+
 // Close the backend.
 func (m *Backend) Close() error {
 	if m.CloseFn == nil {
@@ -58,13 +64,27 @@ func (m *Backend) Save(ctx context.Context, h restic.Handle, rd io.Reader) error
 	return m.SaveFn(ctx, h, rd)
 }
 
-// Load loads data from the backend.
-func (m *Backend) Load(ctx context.Context, h restic.Handle, length int, offset int64) (io.ReadCloser, error) {
-	if m.LoadFn == nil {
+// Load runs fn with a reader that yields the contents of the file at h at the
+// given offset.
+func (m *Backend) Load(ctx context.Context, h restic.Handle, length int, offset int64, fn func(rd io.Reader) error) error {
+	rd, err := m.openReader(ctx, h, length, offset)
+	if err != nil {
+		return err
+	}
+	err = fn(rd)
+	if err != nil {
+		rd.Close() // ignore secondary errors closing the reader
+		return err
+	}
+	return rd.Close()
+}
+
+func (m *Backend) openReader(ctx context.Context, h restic.Handle, length int, offset int64) (io.ReadCloser, error) {
+	if m.OpenReaderFn == nil {
 		return nil, errors.New("not implemented")
 	}
 
-	return m.LoadFn(ctx, h, length, offset)
+	return m.OpenReaderFn(ctx, h, length, offset)
 }
 
 // Stat an object in the backend.