backend: extract readerat from restic package

internal/backend/readerat.go

@@ -0,0 +1,44 @@
+package backend
+
+import (
+	"context"
+	"io"
+
+	"github.com/restic/restic/internal/debug"
+	"github.com/restic/restic/internal/errors"
+	"github.com/restic/restic/internal/restic"
+)
+
+type backendReaderAt struct {
+	ctx context.Context
+	be  restic.Backend
+	h   restic.Handle
+}
+
+func (brd backendReaderAt) ReadAt(p []byte, offset int64) (n int, err error) {
+	return ReadAt(brd.ctx, brd.be, brd.h, offset, p)
+}
+
+// ReaderAt returns an io.ReaderAt for a file in the backend. The returned reader
+// should not escape the caller function to avoid unexpected interactions with the
+// embedded context
+func ReaderAt(ctx context.Context, be restic.Backend, h restic.Handle) io.ReaderAt {
+	return backendReaderAt{ctx: ctx, be: be, h: h}
+}
+
+// ReadAt reads from the backend handle h at the given position.
+func ReadAt(ctx context.Context, be restic.Backend, h restic.Handle, offset int64, p []byte) (n int, err error) {
+	debug.Log("ReadAt(%v) at %v, len %v", h, offset, len(p))
+
+	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, errors.Wrapf(err, "ReadFull(%v)", h)
+	}
+
+	debug.Log("ReadAt(%v) ReadFull returned %v bytes", h, n)
+	return n, nil
+}