Send XML header before the first of whitespace chars (#2046)

* Sent XML header before the first of whitespace chars

XML parsing fails in aws cli due to unexpected whitespace character. To
fix this, we send the xml header before we send the first whitespace
character, if any.

* Fix race between sendWhiteSpaceChars and completeMultiUploadpart

api-response.go

@@ -502,10 +502,11 @@ func writeErrorResponse(w http.ResponseWriter, req *http.Request, errorCode APIE
 	setCommonHeaders(w)
 	// write Header
 	w.WriteHeader(error.HTTPStatusCode)
-	writeErrorResponseNoHeader(w, req, error, resource)
+	writeErrorResponseNoHeader(w, req, errorCode, resource)
 }
 
-func writeErrorResponseNoHeader(w http.ResponseWriter, req *http.Request, error APIError, resource string) {
+func writeErrorResponseNoHeader(w http.ResponseWriter, req *http.Request, errorCode APIErrorCode, resource string) {
+	error := getAPIError(errorCode)
 	// Generate error response.
 	errorResponse := getAPIErrorResponse(error, resource)
 	encodedErrorResponse := encodeResponse(errorResponse)

object-handlers.go

@@ -974,6 +974,14 @@ func (api objectAPIHandlers) CompleteMultipartUploadHandler(w http.ResponseWrite
 	// Send 200 OK
 	setCommonHeaders(w)
 	w.WriteHeader(http.StatusOK)
+	// Xml headers need to be sent before we possibly send whitespace characters
+	// to the client.
+	_, err = w.Write([]byte(xml.Header))
+	if err != nil {
+		errorIf(err, "Unable to write XML header for complete multipart upload")
+		writeErrorResponseNoHeader(w, r, ErrInternalError, r.URL.Path)
+		return
+	}
 
 	doneCh := make(chan struct{})
 	// Signal that completeMultipartUpload is over via doneCh
@@ -992,7 +1000,7 @@ func (api objectAPIHandlers) CompleteMultipartUploadHandler(w http.ResponseWrite
 			writePartSmallErrorResponse(w, r, oErr)
 		default:
 			// Handle all other generic issues.
-			writeErrorResponseNoHeader(w, r, getAPIError(toAPIErrorCode(err)), r.URL.Path)
+			writeErrorResponseNoHeader(w, r, toAPIErrorCode(err), r.URL.Path)
 		}
 		return
 	}
@@ -1001,7 +1009,12 @@ func (api objectAPIHandlers) CompleteMultipartUploadHandler(w http.ResponseWrite
 	location := getLocation(r)
 	// Generate complete multipart response.
 	response := generateCompleteMultpartUploadResponse(bucket, object, location, md5Sum)
-	encodedSuccessResponse := encodeResponse(response)
+	encodedSuccessResponse, err := xml.Marshal(response)
+	if err != nil {
+		errorIf(err, "Unable to parse CompleteMultipartUpload response")
+		writeErrorResponseNoHeader(w, r, ErrInternalError, r.URL.Path)
+		return
+	}
 	// write success response.
 	w.Write(encodedSuccessResponse)
 	w.(http.Flusher).Flush()