http: support for periodic flushing in ReverseProxy

Fixes #2012

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4662091

src/pkg/http/reverseproxy.go

@@ -10,7 +10,10 @@ import (
 	"io"
 	"log"
 	"net"
+	"os"
 	"strings"
+	"sync"
+	"time"
 )
 
 // ReverseProxy is an HTTP Handler that takes an incoming request and
@@ -26,6 +29,12 @@ type ReverseProxy struct {
 	// The Transport used to perform proxy requests.
 	// If nil, DefaultTransport is used.
 	Transport RoundTripper
+
+	// FlushInterval specifies the flush interval, in
+	// nanoseconds, to flush to the client while
+	// coping the response body.
+	// If zero, no periodic flushing is done.
+	FlushInterval int64
 }
 
 func singleJoiningSlash(a, b string) string {
@@ -95,6 +104,55 @@ func (p *ReverseProxy) ServeHTTP(rw ResponseWriter, req *Request) {
 	rw.WriteHeader(res.StatusCode)
 
 	if res.Body != nil {
-		io.Copy(rw, res.Body)
+		var dst io.Writer = rw
+		if p.FlushInterval != 0 {
+			if wf, ok := rw.(writeFlusher); ok {
+				dst = &maxLatencyWriter{dst: wf, latency: p.FlushInterval}
+			}
+		}
+		io.Copy(dst, res.Body)
+	}
+}
+
+type writeFlusher interface {
+	io.Writer
+	Flusher
+}
+
+type maxLatencyWriter struct {
+	dst     writeFlusher
+	latency int64 // nanos
+
+	lk   sync.Mutex // protects init of done, as well Write + Flush
+	done chan bool
+}
+
+func (m *maxLatencyWriter) Write(p []byte) (n int, err os.Error) {
+	m.lk.Lock()
+	defer m.lk.Unlock()
+	if m.done == nil {
+		m.done = make(chan bool)
+		go m.flushLoop()
+	}
+	n, err = m.dst.Write(p)
+	if err != nil {
+		m.done <- true
+	}
+	return
+}
+
+func (m *maxLatencyWriter) flushLoop() {
+	t := time.NewTicker(m.latency)
+	defer t.Stop()
+	for {
+		select {
+		case <-t.C:
+			m.lk.Lock()
+			m.dst.Flush()
+			m.lk.Unlock()
+		case <-m.done:
+			return
+		}
 	}
+	panic("unreached")
 }