Reworked Backend.Load API to retry errors during ongoing download

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

internal/backend/utils.go

@@ -10,24 +10,11 @@ import (
 
 // LoadAll reads all data stored in the backend for the handle.
 func LoadAll(ctx context.Context, be restic.Backend, h restic.Handle) (buf []byte, err error) {
-	rd, err := be.Load(ctx, h, 0, 0)
-	if err != nil {
-		return nil, err
-	}
-
-	defer func() {
-		_, e := io.Copy(ioutil.Discard, rd)
-		if err == nil {
-			err = e
-		}
-
-		e = rd.Close()
-		if err == nil {
-			err = e
-		}
-	}()
-
-	return ioutil.ReadAll(rd)
+	err = be.Load(ctx, h, 0, 0, func(rd io.Reader) (ierr error) {
+		buf, ierr = ioutil.ReadAll(rd)
+		return ierr
+	})
+	return buf, err
 }
 
 // LimitedReadCloser wraps io.LimitedReader and exposes the Close() method.
@@ -46,3 +33,19 @@ func (l *LimitedReadCloser) Read(p []byte) (int, error) {
 func LimitReadCloser(r io.ReadCloser, n int64) *LimitedReadCloser {
 	return &LimitedReadCloser{ReadCloser: r, Reader: io.LimitReader(r, n)}
 }
+
+// DefaultLoad implements Backend.Load using lower-level openReader func
+func DefaultLoad(ctx context.Context, h restic.Handle, length int, offset int64,
+	openReader func(ctx context.Context, h restic.Handle, length int, offset int64) (io.ReadCloser, error),
+	fn func(rd io.Reader) error) error {
+	rd, err := 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()
+}