backend: Ensure Reader is closed on error

src/restic/backend.go

@@ -28,7 +28,8 @@ type Backend interface {
 
 	// Get returns a reader that yields the contents of the file at h at the
 	// given offset. If length is nonzero, only a portion of the file is
-	// returned. rd must be closed after use.
+	// returned. rd must be closed after use. If an error is returned, the
+	// ReadCloser must be nil.
 	Get(h Handle, length int, offset int64) (io.ReadCloser, error)
 
 	// Stat returns information about the File identified by h.

src/restic/backend/s3/s3.go

@@ -214,6 +214,7 @@ func (be *s3) Get(h restic.Handle, length int, offset int64) (io.ReadCloser, err
 		debug.Log("Get %v: pass on object", h)
 		_, err = obj.Seek(offset, 0)
 		if err != nil {
+			_ = obj.Close()
 			return nil, errors.Wrap(err, "obj.Seek")
 		}
 
@@ -223,10 +224,12 @@ func (be *s3) Get(h restic.Handle, length int, offset int64) (io.ReadCloser, err
 	// otherwise use a buffer with ReadAt
 	info, err := obj.Stat()
 	if err != nil {
+		_ = obj.Close()
 		return nil, errors.Wrap(err, "obj.Stat")
 	}
 
 	if offset > info.Size {
+		_ = obj.Close()
 		return nil, errors.Errorf("offset larger than file size")
 	}
 
@@ -245,6 +248,7 @@ func (be *s3) Get(h restic.Handle, length int, offset int64) (io.ReadCloser, err
 	}
 
 	if err != nil {
+		_ = obj.Close()
 		return nil, errors.Wrap(err, "obj.ReadAt")
 	}
 

src/restic/backend/sftp/sftp.go

@@ -419,7 +419,7 @@ func (r *SFTP) Get(h restic.Handle, length int, offset int64) (io.ReadCloser, er
 	if offset > 0 {
 		_, err = f.Seek(offset, 0)
 		if err != nil {
-			f.Close()
+			_ = f.Close()
 			return nil, err
 		}
 	}