net/http/httputil: use response controller in reverse proxy

Previously, the reverse proxy is unable to detect
the support for hijack or flush if those things
are residing in the response writer in a wrapped
manner.

The reverse proxy now makes use of the new http
response controller as the means to discover
the underlying flusher and hijacker associated
with the response writer, allowing wrapped flusher
and hijacker become discoverable.

Change-Id: I53acbb12315c3897be068e8c00598ef42fc74649
Reviewed-on: https://go-review.googlesource.com/c/go/+/468755
Run-TryBot: Damien Neil <[email protected]>
Auto-Submit: Damien Neil <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

src/net/http/httputil/reverseproxy.go

@@ -524,9 +524,7 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 		// Force chunking if we saw a response trailer.
 		// This prevents net/http from calculating the length for short
 		// bodies and adding a Content-Length.
-		if fl, ok := rw.(http.Flusher); ok {
-			fl.Flush()
-		}
+		http.NewResponseController(rw).Flush()
 	}
 
 	if len(res.Trailer) == announcedTrailers {
@@ -601,29 +599,30 @@ func (p *ReverseProxy) flushInterval(res *http.Response) time.Duration {
 	return p.FlushInterval
 }
 
-func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader, flushInterval time.Duration) error {
+func (p *ReverseProxy) copyResponse(dst http.ResponseWriter, src io.Reader, flushInterval time.Duration) error {
+	var w io.Writer = dst
+
 	if flushInterval != 0 {
-		if wf, ok := dst.(writeFlusher); ok {
-			mlw := &maxLatencyWriter{
-				dst:     wf,
-				latency: flushInterval,
-			}
-			defer mlw.stop()
+		mlw := &maxLatencyWriter{
+			dst:     dst,
+			flush:   http.NewResponseController(dst).Flush,
+			latency: flushInterval,
+		}
+		defer mlw.stop()
 
-			// set up initial timer so headers get flushed even if body writes are delayed
-			mlw.flushPending = true
-			mlw.t = time.AfterFunc(flushInterval, mlw.delayedFlush)
+		// set up initial timer so headers get flushed even if body writes are delayed
+		mlw.flushPending = true
+		mlw.t = time.AfterFunc(flushInterval, mlw.delayedFlush)
 
-			dst = mlw
-		}
+		w = mlw
 	}
 
 	var buf []byte
 	if p.BufferPool != nil {
 		buf = p.BufferPool.Get()
 		defer p.BufferPool.Put(buf)
 	}
-	_, err := p.copyBuffer(dst, src, buf)
+	_, err := p.copyBuffer(w, src, buf)
 	return err
 }
 
@@ -668,13 +667,9 @@ func (p *ReverseProxy) logf(format string, args ...any) {
 	}
 }
 
-type writeFlusher interface {
-	io.Writer
-	http.Flusher
-}
-
 type maxLatencyWriter struct {
-	dst     writeFlusher
+	dst     io.Writer
+	flush   func() error
 	latency time.Duration // non-zero; negative means to flush immediately
 
 	mu           sync.Mutex // protects t, flushPending, and dst.Flush
@@ -687,7 +682,7 @@ func (m *maxLatencyWriter) Write(p []byte) (n int, err error) {
 	defer m.mu.Unlock()
 	n, err = m.dst.Write(p)
 	if m.latency < 0 {
-		m.dst.Flush()
+		m.flush()
 		return
 	}
 	if m.flushPending {
@@ -708,7 +703,7 @@ func (m *maxLatencyWriter) delayedFlush() {
 	if !m.flushPending { // if stop was called but AfterFunc already started this goroutine
 		return
 	}
-	m.dst.Flush()
+	m.flush()
 	m.flushPending = false
 }
 
@@ -739,17 +734,19 @@ func (p *ReverseProxy) handleUpgradeResponse(rw http.ResponseWriter, req *http.R
 		return
 	}
 
-	hj, ok := rw.(http.Hijacker)
-	if !ok {
-		p.getErrorHandler()(rw, req, fmt.Errorf("can't switch protocols using non-Hijacker ResponseWriter type %T", rw))
-		return
-	}
 	backConn, ok := res.Body.(io.ReadWriteCloser)
 	if !ok {
 		p.getErrorHandler()(rw, req, fmt.Errorf("internal error: 101 switching protocols response with non-writable body"))
 		return
 	}
 
+	rc := http.NewResponseController(rw)
+	conn, brw, hijackErr := rc.Hijack()
+	if errors.Is(hijackErr, http.ErrNotSupported) {
+		p.getErrorHandler()(rw, req, fmt.Errorf("can't switch protocols using non-Hijacker ResponseWriter type %T", rw))
+		return
+	}
+
 	backConnCloseCh := make(chan bool)
 	go func() {
 		// Ensure that the cancellation of a request closes the backend.
@@ -760,12 +757,10 @@ func (p *ReverseProxy) handleUpgradeResponse(rw http.ResponseWriter, req *http.R
 		}
 		backConn.Close()
 	}()
-
 	defer close(backConnCloseCh)
 
-	conn, brw, err := hj.Hijack()
-	if err != nil {
-		p.getErrorHandler()(rw, req, fmt.Errorf("Hijack failed on protocol switch: %v", err))
+	if hijackErr != nil {
+		p.getErrorHandler()(rw, req, fmt.Errorf("Hijack failed on protocol switch: %v", hijackErr))
 		return
 	}
 	defer conn.Close()

src/net/http/httputil/reverseproxy_test.go

@@ -478,6 +478,62 @@ func TestReverseProxyFlushInterval(t *testing.T) {
 	}
 }
 
+type mockFlusher struct {
+	http.ResponseWriter
+	flushed bool
+}
+
+func (m *mockFlusher) Flush() {
+	m.flushed = true
+}
+
+type wrappedRW struct {
+	http.ResponseWriter
+}
+
+func (w *wrappedRW) Unwrap() http.ResponseWriter {
+	return w.ResponseWriter
+}
+
+func TestReverseProxyResponseControllerFlushInterval(t *testing.T) {
+	const expected = "hi"
+	backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Write([]byte(expected))
+	}))
+	defer backend.Close()
+
+	backendURL, err := url.Parse(backend.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	mf := &mockFlusher{}
+	proxyHandler := NewSingleHostReverseProxy(backendURL)
+	proxyHandler.FlushInterval = -1 // flush immediately
+	proxyWithMiddleware := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		mf.ResponseWriter = w
+		w = &wrappedRW{mf}
+		proxyHandler.ServeHTTP(w, r)
+	})
+
+	frontend := httptest.NewServer(proxyWithMiddleware)
+	defer frontend.Close()
+
+	req, _ := http.NewRequest("GET", frontend.URL, nil)
+	req.Close = true
+	res, err := frontend.Client().Do(req)
+	if err != nil {
+		t.Fatalf("Get: %v", err)
+	}
+	defer res.Body.Close()
+	if bodyBytes, _ := io.ReadAll(res.Body); string(bodyBytes) != expected {
+		t.Errorf("got body %q; expected %q", bodyBytes, expected)
+	}
+	if !mf.flushed {
+		t.Errorf("response writer was not flushed")
+	}
+}
+
 func TestReverseProxyFlushIntervalHeaders(t *testing.T) {
 	const expected = "hi"
 	stopCh := make(chan struct{})