net/http/httputil: rewrite flushing code, disable on Server-Sent Events

* Rewrite the flushing code to not use a persistent goroutine, which
  also simplifies testing.
* Define the meaning of a negative flush interval. Its meaning doesn't
  change, but now it's locked in, and then we can use it to optimize
  the performance of the non-buffered case to avoid use of an AfterFunc.
* Support (internal-only) special casing of FlushInterval values per
  request/response.
* For now, treat Server-Sent Event responses as unbuffered. (or rather,
  immediately flushed from the buffer per-write)

Fixes #27816

Change-Id: Ie0f975c997daa3db539504137c741a96d7022665
Reviewed-on: https://go-review.googlesource.com/c/137335
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

src/net/http/httputil/reverseproxy.go

@@ -18,10 +18,6 @@ import (
 	"time"
 )
 
-// onExitFlushLoop is a callback set by tests to detect the state of the
-// flushLoop() goroutine.
-var onExitFlushLoop func()
-
 // ReverseProxy is an HTTP Handler that takes an incoming request and
 // sends it to another server, proxying the response back to the
 // client.
@@ -42,6 +38,12 @@ type ReverseProxy struct {
 	// to flush to the client while copying the
 	// response body.
 	// If zero, no periodic flushing is done.
+	// A negative value means to flush immediately
+	// after each write to the client.
+	// The FlushInterval is ignored when ReverseProxy
+	// recognizes a response as a streaming response;
+	// for such reponses, writes are flushed to the client
+	// immediately.
 	FlushInterval time.Duration
 
 	// ErrorLog specifies an optional logger for errors
@@ -271,7 +273,7 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 			fl.Flush()
 		}
 	}
-	err = p.copyResponse(rw, res.Body)
+	err = p.copyResponse(rw, res.Body, p.flushInterval(req, res))
 	if err != nil {
 		defer res.Body.Close()
 		// Since we're streaming the response, if we run into an error all we can do
@@ -332,15 +334,28 @@ func removeConnectionHeaders(h http.Header) {
 	}
 }
 
-func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader) error {
-	if p.FlushInterval != 0 {
+// flushInterval returns the p.FlushInterval value, conditionally
+// overriding its value for a specific request/response.
+func (p *ReverseProxy) flushInterval(req *http.Request, res *http.Response) time.Duration {
+	resCT := res.Header.Get("Content-Type")
+
+	// For Server-Sent Events responses, flush immediately.
+	// The MIME type is defined in https://www.w3.org/TR/eventsource/#text-event-stream
+	if resCT == "text/event-stream" {
+		return -1 // negative means immediately
+	}
+
+	// TODO: more specific cases? e.g. res.ContentLength == -1?
+	return p.FlushInterval
+}
+
+func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader, flushInterval time.Duration) error {
+	if flushInterval != 0 {
 		if wf, ok := dst.(writeFlusher); ok {
 			mlw := &maxLatencyWriter{
 				dst:     wf,
-				latency: p.FlushInterval,
-				done:    make(chan bool),
+				latency: flushInterval,
 			}
-			go mlw.flushLoop()
 			defer mlw.stop()
 			dst = mlw
 		}
@@ -403,34 +418,44 @@ type writeFlusher interface {
 
 type maxLatencyWriter struct {
 	dst     writeFlusher
-	latency time.Duration
+	latency time.Duration // non-zero; negative means to flush immediately
 
-	mu   sync.Mutex // protects Write + Flush
-	done chan bool
+	mu           sync.Mutex // protects t, flushPending, and dst.Flush
+	t            *time.Timer
+	flushPending bool
 }
 
-func (m *maxLatencyWriter) Write(p []byte) (int, error) {
+func (m *maxLatencyWriter) Write(p []byte) (n int, err error) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
-	return m.dst.Write(p)
-}
-
-func (m *maxLatencyWriter) flushLoop() {
-	t := time.NewTicker(m.latency)
-	defer t.Stop()
-	for {
-		select {
-		case <-m.done:
-			if onExitFlushLoop != nil {
-				onExitFlushLoop()
+	n, err = m.dst.Write(p)
+	if m.latency < 0 {
+		m.dst.Flush()
+		return
 	}
+	if m.flushPending {
 		return
-		case <-t.C:
-			m.mu.Lock()
-			m.dst.Flush()
-			m.mu.Unlock()
 	}
+	if m.t == nil {
+		m.t = time.AfterFunc(m.latency, m.delayedFlush)
+	} else {
+		m.t.Reset(m.latency)
 	}
+	m.flushPending = true
+	return
+}
+
+func (m *maxLatencyWriter) delayedFlush() {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	m.dst.Flush()
+	m.flushPending = false
 }
 
-func (m *maxLatencyWriter) stop() { m.done <- true }
+func (m *maxLatencyWriter) stop() {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if m.t != nil {
+		m.t.Stop()
+	}
+}

src/net/http/httputil/reverseproxy_test.go

@@ -297,10 +297,6 @@ func TestReverseProxyFlushInterval(t *testing.T) {
 	proxyHandler := NewSingleHostReverseProxy(backendURL)
 	proxyHandler.FlushInterval = time.Microsecond
 
-	done := make(chan bool)
-	onExitFlushLoop = func() { done <- true }
-	defer func() { onExitFlushLoop = nil }()
-
 	frontend := httptest.NewServer(proxyHandler)
 	defer frontend.Close()
 
@@ -314,13 +310,6 @@ func TestReverseProxyFlushInterval(t *testing.T) {
 	if bodyBytes, _ := ioutil.ReadAll(res.Body); string(bodyBytes) != expected {
 		t.Errorf("got body %q; expected %q", bodyBytes, expected)
 	}
-
-	select {
-	case <-done:
-		// OK
-	case <-time.After(5 * time.Second):
-		t.Error("maxLatencyWriter flushLoop() never exited")
-	}
 }
 
 func TestReverseProxyCancelation(t *testing.T) {
@@ -946,3 +935,48 @@ func TestReverseProxy_PanicBodyError(t *testing.T) {
 	req, _ := http.NewRequest("GET", "http://foo.tld/", nil)
 	rproxy.ServeHTTP(httptest.NewRecorder(), req)
 }
+
+func TestSelectFlushInterval(t *testing.T) {
+	tests := []struct {
+		name string
+		p    *ReverseProxy
+		req  *http.Request
+		res  *http.Response
+		want time.Duration
+	}{
+		{
+			name: "default",
+			res:  &http.Response{},
+			p:    &ReverseProxy{FlushInterval: 123},
+			want: 123,
+		},
+		{
+			name: "server-sent events overrides non-zero",
+			res: &http.Response{
+				Header: http.Header{
+					"Content-Type": {"text/event-stream"},
+				},
+			},
+			p:    &ReverseProxy{FlushInterval: 123},
+			want: -1,
+		},
+		{
+			name: "server-sent events overrides zero",
+			res: &http.Response{
+				Header: http.Header{
+					"Content-Type": {"text/event-stream"},
+				},
+			},
+			p:    &ReverseProxy{FlushInterval: 0},
+			want: -1,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := tt.p.flushInterval(tt.req, tt.res)
+			if got != tt.want {
+				t.Errorf("flushLatency = %v; want %v", got, tt.want)
+			}
+		})
+	}
+}