Reworked Backend.Load API to retry errors during ongoing download

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

internal/checker/checker.go

@@ -630,14 +630,8 @@ func checkPack(ctx context.Context, r restic.Repository, id restic.ID) error {
 	debug.Log("checking pack %v", id)
 	h := restic.Handle{Type: restic.DataFile, Name: id.String()}
 
-	rd, err := r.Backend().Load(ctx, h, 0, 0)
-	if err != nil {
-		return err
-	}
-
 	packfile, err := fs.TempFile("", "restic-temp-check-")
 	if err != nil {
-		_ = rd.Close()
 		return errors.Wrap(err, "TempFile")
 	}
 
@@ -646,18 +640,25 @@ func checkPack(ctx context.Context, r restic.Repository, id restic.ID) error {
 		_ = os.Remove(packfile.Name())
 	}()
 
-	hrd := hashing.NewReader(rd, sha256.New())
-	size, err := io.Copy(packfile, hrd)
+	var hash restic.ID
+	var size int64
+	err = r.Backend().Load(ctx, h, 0, 0, func(rd io.Reader) (ierr error) {
+		_, ierr = packfile.Seek(0, io.SeekStart)
+		if ierr == nil {
+			ierr = packfile.Truncate(0)
+		}
+		if ierr != nil {
+			return ierr
+		}
+		hrd := hashing.NewReader(rd, sha256.New())
+		size, ierr = io.Copy(packfile, hrd)
+		hash = restic.IDFromHash(hrd.Sum(nil))
+		return ierr
+	})
 	if err != nil {
-		_ = rd.Close()
-		return errors.Wrap(err, "Copy")
+		return errors.Wrap(err, "checkPack")
 	}
 
-	if err = rd.Close(); err != nil {
-		return err
-	}
-
-	hash := restic.IDFromHash(hrd.Sum(nil))
 	debug.Log("hash for pack %v is %v", id, hash)
 
 	if !hash.Equal(id) {

internal/checker/checker_test.go

@@ -195,20 +195,17 @@ func TestModifiedIndex(t *testing.T) {
 		Type: restic.IndexFile,
 		Name: "90f838b4ac28735fda8644fe6a08dbc742e57aaf81b30977b4fefa357010eafd",
 	}
-	f, err := repo.Backend().Load(context.TODO(), h, 0, 0)
+	err := repo.Backend().Load(context.TODO(), h, 0, 0, func(rd io.Reader) error {
+		// save the index again with a modified name so that the hash doesn't match
+		// the content any more
+		h2 := restic.Handle{
+			Type: restic.IndexFile,
+			Name: "80f838b4ac28735fda8644fe6a08dbc742e57aaf81b30977b4fefa357010eafd",
+		}
+		return repo.Backend().Save(context.TODO(), h2, rd)
+	})
 	test.OK(t, err)
 
-	// save the index again with a modified name so that the hash doesn't match
-	// the content any more
-	h2 := restic.Handle{
-		Type: restic.IndexFile,
-		Name: "80f838b4ac28735fda8644fe6a08dbc742e57aaf81b30977b4fefa357010eafd",
-	}
-	err = repo.Backend().Save(context.TODO(), h2, f)
-	test.OK(t, err)
-
-	test.OK(t, f.Close())
-
 	chkr := checker.New(repo)
 	hints, errs := chkr.LoadIndex(context.TODO())
 	if len(errs) == 0 {
@@ -262,35 +259,27 @@ type errorBackend struct {
 	ProduceErrors bool
 }
 
-func (b errorBackend) Load(ctx context.Context, h restic.Handle, length int, offset int64) (io.ReadCloser, error) {
-	rd, err := b.Backend.Load(ctx, h, length, offset)
-	if err != nil {
-		return rd, err
-	}
-
-	if b.ProduceErrors {
-		return errorReadCloser{rd}, err
-	}
-
-	return rd, nil
+func (b errorBackend) Load(ctx context.Context, h restic.Handle, length int, offset int64, consumer func(rd io.Reader) error) error {
+	return b.Backend.Load(ctx, h, length, offset, func(rd io.Reader) error {
+		if b.ProduceErrors {
+			return consumer(errorReadCloser{rd})
+		}
+		return consumer(rd)
+	})
 }
 
 type errorReadCloser struct {
-	io.ReadCloser
+	io.Reader
 }
 
 func (erd errorReadCloser) Read(p []byte) (int, error) {
-	n, err := erd.ReadCloser.Read(p)
+	n, err := erd.Reader.Read(p)
 	if n > 0 {
 		induceError(p[:n])
 	}
 	return n, err
 }
 
-func (erd errorReadCloser) Close() error {
-	return erd.ReadCloser.Close()
-}
-
 // induceError flips a bit in the slice.
 func induceError(data []byte) {
 	if rand.Float32() < 0.2 {