repository: move backend.LoadAll to repository.LoadRaw

LoadRaw also includes improved context cancellation handling similar to the
implementation in repository.LoadUnpacked.

The removed cache backend test will be added again later on.

internal/repository/raw.go

@@ -0,0 +1,63 @@
+package repository
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"io"
+
+	"github.com/restic/restic/internal/backend"
+	"github.com/restic/restic/internal/debug"
+	"github.com/restic/restic/internal/restic"
+)
+
+// LoadRaw reads all data stored in the backend for the file with id and filetype t.
+// If the backend returns data that does not match the id, then the buffer is returned
+// along with an error that is a restic.ErrInvalidData error.
+func (r *Repository) LoadRaw(ctx context.Context, t restic.FileType, id restic.ID) (buf []byte, err error) {
+	h := backend.Handle{Type: t, Name: id.String()}
+
+	ctx, cancel := context.WithCancel(ctx)
+
+	var dataErr error
+	retriedInvalidData := false
+	err = r.be.Load(ctx, h, 0, 0, func(rd io.Reader) error {
+		// make sure this is idempotent, in case an error occurs this function may be called multiple times!
+		wr := bytes.NewBuffer(buf[:0])
+		_, cerr := io.Copy(wr, rd)
+		if cerr != nil {
+			return cerr
+		}
+		buf = wr.Bytes()
+
+		// retry loading damaged data only once. If a file fails to download correctly
+		// the second time, then it  is likely corrupted at the backend.
+		if h.Type != backend.ConfigFile {
+			if id != restic.Hash(buf) {
+				if !retriedInvalidData {
+					debug.Log("retry loading broken blob %v", h)
+					retriedInvalidData = true
+				} else {
+					// with a canceled context there is not guarantee which error will
+					// be returned by `be.Load`.
+					dataErr = fmt.Errorf("loadAll(%v): %w", h, restic.ErrInvalidData)
+					cancel()
+				}
+				return restic.ErrInvalidData
+			}
+		}
+		return nil
+	})
+
+	// Return corrupted data to the caller if it is still broken the second time to
+	// let the caller decide what to do with the data.
+	if dataErr != nil {
+		return buf, dataErr
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	return buf, nil
+}