Reworked Backend.Load API to retry errors during ongoing download

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

internal/restic/backend.go

@@ -23,11 +23,15 @@ type Backend interface {
 	// Save stores the data in the backend under the given handle.
 	Save(ctx context.Context, h Handle, rd io.Reader) error
 
-	// Load returns a reader that yields the contents of the file at h at the
+	// Load runs fn with a reader that yields the contents of the file at h at the
 	// given offset. If length is larger than zero, only a portion of the file
-	// is returned. rd must be closed after use. If an error is returned, the
-	// ReadCloser must be nil.
-	Load(ctx context.Context, h Handle, length int, offset int64) (io.ReadCloser, error)
+	// is read.
+	//
+	// The function fn may be called multiple times during the same Load invocation
+	// and therefore must be idempotent.
+	//
+	// Implementations are encouraged to use backend.DefaultLoad
+	Load(ctx context.Context, h Handle, length int, offset int64, fn func(rd io.Reader) error) error
 
 	// Stat returns information about the File identified by h.
 	Stat(ctx context.Context, h Handle) (FileInfo, error)

internal/restic/readerat.go

@@ -25,17 +25,16 @@ func ReaderAt(be Backend, h Handle) io.ReaderAt {
 // ReadAt reads from the backend handle h at the given position.
 func ReadAt(ctx context.Context, be Backend, h Handle, offset int64, p []byte) (n int, err error) {
 	debug.Log("ReadAt(%v) at %v, len %v", h, offset, len(p))
-	rd, err := be.Load(ctx, h, len(p), offset)
+
+	err = be.Load(ctx, h, len(p), offset, func(rd io.Reader) (ierr error) {
+		n, ierr = io.ReadFull(rd, p)
+
+		return ierr
+	})
 	if err != nil {
 		return 0, err
 	}
 
-	n, err = io.ReadFull(rd, p)
-	e := rd.Close()
-	if err == nil {
-		err = e
-	}
-
 	debug.Log("ReadAt(%v) ReadFull returned %v bytes", h, n)
 
 	return n, errors.Wrapf(err, "ReadFull(%v)", h)