create a helper method to shorten the implementation

Signed-off-by: Kengo TODA <skypencil+github@gmail.com>

gcs/storage.go

@@ -28,30 +28,18 @@ func NewStorage(config *Config, client Client) (*Storage, error) {
 }
 
 func (s *Storage) Write(ctx context.Context, b []byte) error {
-	if s.client == nil {
-		client, err := gcStorage.NewClient(ctx)
-		if err != nil {
-			return err
-		}
-		s.client = Adapter{
-			config: *s.config,
-			client: client,
-		}
+	err := s.init(ctx)
+	if err != nil {
+		return err
 	}
 
 	return s.client.Write(ctx, b)
 }
 
 func (s *Storage) Read(ctx context.Context) ([]byte, error) {
-	if s.client == nil {
-		client, err := gcStorage.NewClient(ctx)
-		if err != nil {
-			return nil, err
-		}
-		s.client = Adapter{
-			config: *s.config,
-			client: client,
-		}
+	err := s.init(ctx)
+	if err != nil {
+		return nil, err
 	}
 
 	r, err := s.client.Read(ctx)
@@ -62,3 +50,17 @@ func (s *Storage) Read(ctx context.Context) ([]byte, error) {
 	}
 	return r, nil
 }
+
+func (s *Storage) init(ctx context.Context) error {
+	if s.client == nil {
+		client, err := gcStorage.NewClient(ctx)
+		if err != nil {
+			return err
+		}
+		s.client = Adapter{
+			config: *s.config,
+			client: client,
+		}
+	}
+	return nil
+}