MM-23131 Include HTTP status code in the metrics (#14240)

* ResponseWriter wrapper to get status code

For our metrics, we need the status code returned by a request
so this wrapper includes a new method StatusCode() that includes
the desired code

* Shadow the responsewriter variable in the handlers

In order to avoid confusion to people deciding what variable to use.
I've also changed the tests to reflect this change and added a new
one that checks the Flush method works

web/response_writer_wrapper_test.go

@@ -0,0 +1,87 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package web
+
+import (
+	"bufio"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+type TestHandler struct {
+	TestFunc func(w http.ResponseWriter, r *http.Request)
+}
+
+func (h *TestHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	h.TestFunc(w, r)
+}
+
+type responseRecorderHijack struct {
+	httptest.ResponseRecorder
+}
+
+func (r *responseRecorderHijack) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+	r.WriteHeader(http.StatusOK)
+	return nil, nil, nil
+}
+
+func newResponseWithHijack(original *httptest.ResponseRecorder) *responseRecorderHijack {
+	return &responseRecorderHijack{*original}
+}
+
+func TestStatusCodeIsAccessible(t *testing.T) {
+	resp := newWrappedWriter(httptest.NewRecorder())
+	req := httptest.NewRequest("GET", "/api/v4/test", nil)
+	handler := TestHandler{func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusBadRequest)
+	}}
+	handler.ServeHTTP(resp, req)
+	assert.Equal(t, http.StatusBadRequest, resp.StatusCode())
+}
+
+func TestStatusCodeShouldBe200IfNotHeaderWritten(t *testing.T) {
+	resp := newWrappedWriter(httptest.NewRecorder())
+	req := httptest.NewRequest("GET", "/api/v4/test", nil)
+	handler := TestHandler{func(w http.ResponseWriter, r *http.Request) {
+		w.Write([]byte{})
+	}}
+	handler.ServeHTTP(resp, req)
+	assert.Equal(t, http.StatusOK, resp.StatusCode())
+}
+
+func TestForUnsupportedHijack(t *testing.T) {
+	resp := newWrappedWriter(httptest.NewRecorder())
+	req := httptest.NewRequest("GET", "/api/v4/test", nil)
+	handler := TestHandler{func(w http.ResponseWriter, r *http.Request) {
+		_, _, err := w.(*responseWriterWrapper).Hijack()
+		assert.NotNil(t, err)
+		assert.Equal(t, "Hijacker interface not supported by the wrapped ResponseWriter", err.Error())
+	}}
+	handler.ServeHTTP(resp, req)
+}
+
+func TestForSupportedHijack(t *testing.T) {
+	resp := newWrappedWriter(newResponseWithHijack(httptest.NewRecorder()))
+	req := httptest.NewRequest("GET", "/api/v4/test", nil)
+	handler := TestHandler{func(w http.ResponseWriter, r *http.Request) {
+		_, _, err := w.(*responseWriterWrapper).Hijack()
+		assert.Nil(t, err)
+	}}
+	handler.ServeHTTP(resp, req)
+}
+
+func TestForSupportedFlush(t *testing.T) {
+	resp := newWrappedWriter(httptest.NewRecorder())
+	req := httptest.NewRequest("GET", "/api/v4/test", nil)
+	handler := TestHandler{func(w http.ResponseWriter, r *http.Request) {
+		w.Write([]byte{})
+		w.(*responseWriterWrapper).Flush()
+	}}
+	handler.ServeHTTP(resp, req)
+	assert.Equal(t, http.StatusOK, resp.StatusCode())
+}