Refactoring API

- api.Error should implement error interface
- add method that encodes the response into ResponseWriter

Signed-off-by: Evgeniy Kulikov <kim@nspcc.ru>

api/errors.go

@@ -1612,6 +1612,10 @@ func (e errorCodeMap) ToAPIErr(errCode ErrorCode) Error {
 	return e.ToAPIErrWithErr(errCode, nil)
 }
 
+func (e Error) Error() string {
+	return fmt.Sprintf("%s: %d => %s", e.Code, e.HTTPStatusCode, e.Description)
+}
+
 // GetAPIError provides API Error for input API error code.
 func GetAPIError(code ErrorCode) Error {
 	if apiErr, ok := errorCodes[code]; ok {

api/response.go

@@ -68,7 +68,11 @@ const (
 	hdrSSECopyKey = "X-Amz-Copy-Source-Server-Side-Encryption-Customer-Key"
 )
 
-var deploymentID, _ = uuid.NewRandom()
+var (
+	deploymentID, _ = uuid.NewRandom()
+
+	xmlHeader = []byte(xml.Header)
+)
 
 // Non exhaustive list of AWS S3 standard error responses -
 // http://docs.aws.amazon.com/AmazonS3/latest/API/ErrorResponses.html
@@ -181,6 +185,14 @@ func EncodeResponse(response interface{}) []byte {
 	return bytesBuffer.Bytes()
 }
 
+// EncodeToResponse encodes the response into ResponseWriter.
+func EncodeToResponse(w http.ResponseWriter, response interface{}) error {
+	if _, err := w.Write(xmlHeader); err != nil {
+		return err
+	}
+	return xml.NewEncoder(w).Encode(response)
+}
+
 // WriteSuccessResponseXML writes success headers and response if any,
 // with content-type set to `application/xml`.
 func WriteSuccessResponseXML(w http.ResponseWriter, response []byte) {