error pages: do not buffer response when it's not an error

middlewares/errorpages/error_pages.go

@@ -74,25 +74,28 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, req *http.Request, next http.
 		return
 	}
 
-	recorder := newResponseRecorder(w)
-	next.ServeHTTP(recorder, req)
+	catcher := newCodeCatcher(w, h.httpCodeRanges)
+	next.ServeHTTP(catcher, req)
+	if !catcher.isError {
+		return
+	}
 
 	// check the recorder code against the configured http status code ranges
 	for _, block := range h.httpCodeRanges {
-		if recorder.GetCode() >= block[0] && recorder.GetCode() <= block[1] {
-			log.Errorf("Caught HTTP Status Code %d, returning error page", recorder.GetCode())
+		if catcher.code >= block[0] && catcher.code <= block[1] {
+			log.Errorf("Caught HTTP Status Code %d, returning error page", catcher.code)
 
 			var query string
 			if len(h.backendQuery) > 0 {
 				query = "/" + strings.TrimPrefix(h.backendQuery, "/")
-				query = strings.Replace(query, "{status}", strconv.Itoa(recorder.GetCode()), -1)
+				query = strings.Replace(query, "{status}", strconv.Itoa(catcher.code), -1)
 			}
 
 			pageReq, err := newRequest(h.backendURL + query)
 			if err != nil {
 				log.Error(err)
-				w.WriteHeader(recorder.GetCode())
-				fmt.Fprint(w, http.StatusText(recorder.GetCode()))
+				w.WriteHeader(catcher.code)
+				fmt.Fprint(w, http.StatusText(catcher.code))
 				return
 			}
 
@@ -102,16 +105,11 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, req *http.Request, next http.
 			h.backendHandler.ServeHTTP(recorderErrorPage, pageReq.WithContext(req.Context()))
 
 			utils.CopyHeaders(w.Header(), recorderErrorPage.Header())
-			w.WriteHeader(recorder.GetCode())
+			w.WriteHeader(catcher.code)
 			w.Write(recorderErrorPage.GetBody().Bytes())
 			return
 		}
 	}
-
-	// did not catch a configured status code so proceed with the request
-	utils.CopyHeaders(w.Header(), recorder.Header())
-	w.WriteHeader(recorder.GetCode())
-	w.Write(recorder.GetBody().Bytes())
 }
 
 func newRequest(baseURL string) (*http.Request, error) {
@@ -129,6 +127,73 @@ func newRequest(baseURL string) (*http.Request, error) {
 	return req, nil
 }
 
+// codeCatcher is a response writer that detects as soon as possible whether the
+// response is a code within the ranges of codes it watches for. If it is, it
+// simply drops the data from the response. Otherwise, it forwards it directly to
+// the original client (its responseWriter) without any buffering.
+type codeCatcher struct {
+	headerMap      http.Header
+	code           int
+	httpCodeRanges types.HTTPCodeRanges
+	firstWrite     bool
+	isError        bool
+	responseWriter http.ResponseWriter
+	err            error
+}
+
+func newCodeCatcher(rw http.ResponseWriter, httpCodeRanges types.HTTPCodeRanges) *codeCatcher {
+	catcher := &codeCatcher{
+		headerMap:      make(http.Header),
+		code:           http.StatusOK, // If backend does not call WriteHeader on us, we consider it's a 200.
+		responseWriter: rw,
+		httpCodeRanges: httpCodeRanges,
+		firstWrite:     true,
+	}
+	return catcher
+}
+
+func (cc *codeCatcher) Header() http.Header {
+	if cc.headerMap == nil {
+		cc.headerMap = make(http.Header)
+	}
+
+	return cc.headerMap
+}
+
+func (cc *codeCatcher) Write(buf []byte) (int, error) {
+	if cc.err != nil {
+		return 0, cc.err
+	}
+	if !cc.firstWrite {
+		if cc.isError {
+			// We don't care about the contents of the response,
+			// since we want to serve the ones from the error page,
+			// so we just drop them.
+			return len(buf), nil
+		}
+		return cc.responseWriter.Write(buf)
+	}
+
+	for _, block := range cc.httpCodeRanges {
+		if cc.code >= block[0] && cc.code <= block[1] {
+			cc.isError = true
+			break
+		}
+	}
+	cc.firstWrite = false
+	if !cc.isError {
+		utils.CopyHeaders(cc.responseWriter.Header(), cc.Header())
+		cc.responseWriter.WriteHeader(cc.code)
+	} else {
+		return len(buf), nil
+	}
+	return cc.responseWriter.Write(buf)
+}
+
+func (cc *codeCatcher) WriteHeader(code int) {
+	cc.code = code
+}
+
 type responseRecorder interface {
 	http.ResponseWriter
 	http.Flusher

middlewares/errorpages/error_pages_test.go

@@ -34,6 +34,18 @@ func TestHandler(t *testing.T) {
 				assert.Contains(t, recorder.Body.String(), http.StatusText(http.StatusOK))
 			},
 		},
+		{
+			desc:        "no error, but not a 200",
+			errorPage:   &types.ErrorPage{Backend: "error", Query: "/test", Status: []string{"500-501", "503-599"}},
+			backendCode: http.StatusPartialContent,
+			backendErrorHandler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				fmt.Fprintln(w, "My error page.")
+			}),
+			validate: func(t *testing.T, recorder *httptest.ResponseRecorder) {
+				assert.Equal(t, http.StatusPartialContent, recorder.Code, "HTTP status")
+				assert.Contains(t, recorder.Body.String(), http.StatusText(http.StatusPartialContent))
+			},
+		},
 		{
 			desc:        "in the range",
 			errorPage:   &types.ErrorPage{Backend: "error", Query: "/test", Status: []string{"500-501", "503-599"}},