handle edge case when a blob has been uploaded earlier

x/imagegen/transfer/upload.go

@@ -155,6 +155,9 @@ func (u *uploader) uploadOnce(ctx context.Context, blob Blob) (int64, error) {
 	if err != nil {
 		return 0, err
 	}
+	if uploadURL == "" {
+		return 0, nil // blob already exists
+	}
 
 	// Open file
 	f, err := os.Open(filepath.Join(u.srcDir, digestToPath(blob.Digest)))
@@ -217,6 +220,10 @@ func (u *uploader) initUpload(ctx context.Context, blob Blob) (string, error) {
 		return u.initUpload(ctx, blob)
 	}
 
+	if resp.StatusCode == http.StatusCreated {
+		return "", nil // blob already exists
+	}
+
 	if resp.StatusCode != http.StatusAccepted {
 		return "", fmt.Errorf("init: status %d", resp.StatusCode)
 	}