[#60] Add *NoHeader functions

Such functions should be used together with periodic white space
XML writer.

Signed-off-by: Alex Vanin <a.vanin@yadro.com>

api/response.go

@@ -133,6 +133,13 @@ func WriteErrorResponse(w http.ResponseWriter, reqInfo *ReqInfo, err error) int
 	return code
 }
 
+// WriteErrorResponseNoHeader writes XML encoded error to the response body.
+func WriteErrorResponseNoHeader(w http.ResponseWriter, reqInfo *ReqInfo, err error) {
+	errorResponse := getAPIErrorResponse(reqInfo, err)
+	encodedErrorResponse := EncodeResponse(errorResponse)
+	WriteResponseBody(w, encodedErrorResponse)
+}
+
 // If none of the http routes match respond with appropriate errors.
 func errorResponseHandler(w http.ResponseWriter, r *http.Request) {
 	desc := fmt.Sprintf("Unknown API request at %s", r.URL.Path)
@@ -172,6 +179,11 @@ func WriteResponse(w http.ResponseWriter, statusCode int, response []byte, mType
 		return
 	}
 
+	WriteResponseBody(w, response)
+}
+
+// WriteResponseBody writes response into w.
+func WriteResponseBody(w http.ResponseWriter, response []byte) {
 	_, _ = w.Write(response)
 	if flusher, ok := w.(http.Flusher); ok {
 		flusher.Flush()
@@ -188,13 +200,30 @@ func EncodeResponse(response interface{}) []byte {
 	return bytesBuffer.Bytes()
 }
 
+// EncodeResponseNoHeader encodes response without setting xml.Header.
+// Should be used with periodicXMLWriter which sends xml.Header to the client
+// with whitespaces to keep connection alive.
+func EncodeResponseNoHeader(response interface{}) []byte {
+	var bytesBuffer bytes.Buffer
+	_ = xml.NewEncoder(&bytesBuffer).Encode(response)
+	return bytesBuffer.Bytes()
+}
+
 // EncodeToResponse encodes the response into ResponseWriter.
 func EncodeToResponse(w http.ResponseWriter, response interface{}) error {
 	w.WriteHeader(http.StatusOK)
 
 	if _, err := w.Write(xmlHeader); err != nil {
 		return fmt.Errorf("write headers: %w", err)
-	} else if err = xml.NewEncoder(w).Encode(response); err != nil {
+	}
+
+	return EncodeToResponseNoHeader(w, response)
+}
+
+// EncodeToResponseNoHeader encodes the response into ResponseWriter without
+// header status.
+func EncodeToResponseNoHeader(w http.ResponseWriter, response interface{}) error {
+	if err := xml.NewEncoder(w).Encode(response); err != nil {
 		return fmt.Errorf("encode xml response: %w", err)
 	}