gcs: fix url parse panic (#664)

pkg/storage/parse.go

@@ -70,7 +70,10 @@ func ParseBackend(rawURL string, options *BackendOptions) (*backup.StorageBacken
 		return &backup.StorageBackend{Backend: &backup.StorageBackend_S3{S3: s3}}, nil
 
 	case "gs", "gcs":
-		prefix := strings.Trim(u.Path[1:], "/")
+		if u.Host == "" {
+			return nil, errors.Annotatef(berrors.ErrStorageInvalidConfig, "please specify the bucket for gcs in %s", rawURL)
+		}
+		prefix := strings.Trim(u.Path, "/")
 		gcs := &backup.GCS{Bucket: u.Host, Prefix: prefix}
 		if options == nil {
 			options = &BackendOptions{}

pkg/storage/parse_test.go

@@ -91,6 +91,15 @@ func (r *testStorageSuite) TestCreateStorage(c *C) {
 	c.Assert(gcs.Endpoint, Equals, "https://gcs.example.com/")
 	c.Assert(gcs.CredentialsBlob, Equals, "")
 
+	s, err = ParseBackend("gcs://bucket2", gcsOpt)
+	c.Assert(err, IsNil)
+	gcs = s.GetGcs()
+	c.Assert(gcs, NotNil)
+	c.Assert(gcs.Bucket, Equals, "bucket2")
+	c.Assert(gcs.Prefix, Equals, "")
+	c.Assert(gcs.Endpoint, Equals, "https://gcs.example.com/")
+	c.Assert(gcs.CredentialsBlob, Equals, "")
+
 	var credFeilPerm os.FileMode = 0o600
 	fakeCredentialsFile := filepath.Join(c.MkDir(), "fakeCredentialsFile")
 	err = ioutil.WriteFile(fakeCredentialsFile, []byte("fakeCredentials"), credFeilPerm)