Add []byte to repo.LoadAndDecrypt and utils.LoadAll

This commit changes the signatures for repository.LoadAndDecrypt and
utils.LoadAll to allow passing in a []byte as the buffer to use. This
buffer is enlarged as needed, and returned back to the caller for
further use.

In later commits, this allows reducing allocations by reusing a buffer
for multiple calls, e.g. in a worker function.

internal/backend/utils.go

@@ -1,20 +1,33 @@
 package backend
 
 import (
+	"bytes"
 	"context"
 	"io"
-	"io/ioutil"
 
 	"github.com/restic/restic/internal/restic"
 )
 
-// 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) {
-	err = be.Load(ctx, h, 0, 0, func(rd io.Reader) (ierr error) {
-		buf, ierr = ioutil.ReadAll(rd)
-		return ierr
+// LoadAll reads all data stored in the backend for the handle into the given
+// buffer, which is truncated. If the buffer is not large enough or nil, a new
+// one is allocated.
+func LoadAll(ctx context.Context, buf []byte, be restic.Backend, h restic.Handle) ([]byte, error) {
+	err := 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()
+		return nil
 	})
-	return buf, err
+
+	if err != nil {
+		return nil, err
+	}
+
+	return buf, nil
 }
 
 // LimitedReadCloser wraps io.LimitedReader and exposes the Close() method.