net/http: update bundled http2

Updates http2 to x/net git rev b626cca for:

    http2: implement support for server push
    https://golang.org/cl/29439

    http2: reject stream self-dependencies
    https://golang.org/cl/31858

    http2: optimize server frame writes
    https://golang.org/cl/31495

    http2: interface to support pluggable schedulers
    https://golang.org/cl/25366
    (no user-visible behavior change or API surface)

    http2: add Server.IdleTimeout
    https://golang.org/cl/31727

    http2: make Server return conn protocol errors on bad idle stream frames
    https://golang.org/cl/31736

    http2: fix optimized write scheduling
    https://golang.org/cl/32217 (fix for CL 31495 above)

Change-Id: Ie894c72943d355115c8391573bf6b96dc1bd5894
Reviewed-on: https://go-review.googlesource.com/32215
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/h2_bundle.go

@@ -2161,6 +2161,18 @@ func (cc *http2ClientConn) Ping(ctx context.Context) error {
 
 func http2cloneTLSConfig(c *tls.Config) *tls.Config { return c.Clone() }
 
+var _ Pusher = (*http2responseWriter)(nil)
+
+// Push implements http.Pusher.
+func (w *http2responseWriter) Push(target string, opts *PushOptions) error {
+	internalOpts := http2pushOptions{}
+	if opts != nil {
+		internalOpts.Method = opts.Method
+		internalOpts.Header = opts.Header
+	}
+	return w.push(target, internalOpts)
+}
+
 var http2DebugGoroutines = os.Getenv("DEBUG_HTTP2_GOROUTINES") == "1"
 
 type http2goroutineLock uint64
@@ -2426,13 +2438,23 @@ var (
 
 type http2streamState int
 
+// HTTP/2 stream states.
+//
+// See http://tools.ietf.org/html/rfc7540#section-5.1.
+//
+// For simplicity, the server code merges "reserved (local)" into
+// "half-closed (remote)". This is one less state transition to track.
+// The only downside is that we send PUSH_PROMISEs slightly less
+// liberally than allowable. More discussion here:
+// https://lists.w3.org/Archives/Public/ietf-http-wg/2016JulSep/0599.html
+//
+// "reserved (remote)" is omitted since the client code does not
+// support server push.
 const (
 	http2stateIdle http2streamState = iota
 	http2stateOpen
 	http2stateHalfClosedLocal
 	http2stateHalfClosedRemote
-	http2stateResvLocal
-	http2stateResvRemote
 	http2stateClosed
 )
 
@@ -2441,8 +2463,6 @@ var http2stateName = [...]string{
 	http2stateOpen:             "Open",
 	http2stateHalfClosedLocal:  "HalfClosedLocal",
 	http2stateHalfClosedRemote: "HalfClosedRemote",
-	http2stateResvLocal:        "ResvLocal",
-	http2stateResvRemote:       "ResvRemote",
 	http2stateClosed:           "Closed",
 }
 
@@ -2603,13 +2623,27 @@ func http2newBufferedWriter(w io.Writer) *http2bufferedWriter {
 	return &http2bufferedWriter{w: w}
 }
 
+// bufWriterPoolBufferSize is the size of bufio.Writer's
+// buffers created using bufWriterPool.
+//
+// TODO: pick a less arbitrary value? this is a bit under
+// (3 x typical 1500 byte MTU) at least. Other than that,
+// not much thought went into it.
+const http2bufWriterPoolBufferSize = 4 << 10
+
 var http2bufWriterPool = sync.Pool{
 	New: func() interface{} {
-
-		return bufio.NewWriterSize(nil, 4<<10)
+		return bufio.NewWriterSize(nil, http2bufWriterPoolBufferSize)
 	},
 }
 
+func (w *http2bufferedWriter) Available() int {
+	if w.bw == nil {
+		return http2bufWriterPoolBufferSize
+	}
+	return w.bw.Available()
+}
+
 func (w *http2bufferedWriter) Write(p []byte) (n int, err error) {
 	if w.bw == nil {
 		bw := http2bufWriterPool.Get().(*bufio.Writer)
@@ -2912,6 +2946,15 @@ type http2Server struct {
 	// PermitProhibitedCipherSuites, if true, permits the use of
 	// cipher suites prohibited by the HTTP/2 spec.
 	PermitProhibitedCipherSuites bool
+
+	// IdleTimeout specifies how long until idle clients should be
+	// closed with a GOAWAY frame. PING frames are not considered
+	// activity for the purposes of IdleTimeout.
+	IdleTimeout time.Duration
+
+	// NewWriteScheduler constructs a write scheduler for a connection.
+	// If nil, a default scheduler is chosen.
+	NewWriteScheduler func() http2WriteScheduler
 }
 
 func (s *http2Server) maxReadFrameSize() uint32 {
@@ -3053,20 +3096,26 @@ func (s *http2Server) ServeConn(c net.Conn, opts *http2ServeConnOpts) {
 		handler:           opts.handler(),
 		streams:           make(map[uint32]*http2stream),
 		readFrameCh:       make(chan http2readFrameResult),
-		wantWriteFrameCh: make(chan http2frameWriteMsg, 8),
+		wantWriteFrameCh:  make(chan http2FrameWriteRequest, 8),
+		wantStartPushCh:   make(chan http2startPushRequest, 8),
 		wroteFrameCh:      make(chan http2frameWriteResult, 1),
 		bodyReadCh:        make(chan http2bodyReadMsg),
 		doneServing:       make(chan struct{}),
+		clientMaxStreams:  math.MaxUint32,
 		advMaxStreams:     s.maxConcurrentStreams(),
-		writeSched: http2writeScheduler{
-			maxFrameSize: http2initialMaxFrameSize,
-		},
 		initialWindowSize: http2initialWindowSize,
+		maxFrameSize:      http2initialMaxFrameSize,
 		headerTableSize:   http2initialHeaderTableSize,
 		serveG:            http2newGoroutineLock(),
 		pushEnabled:       true,
 	}
 
+	if s.NewWriteScheduler != nil {
+		sc.writeSched = s.NewWriteScheduler()
+	} else {
+		sc.writeSched = http2NewRandomWriteScheduler()
+	}
+
 	sc.flow.add(http2initialWindowSize)
 	sc.inflow.add(http2initialWindowSize)
 	sc.hpackEncoder = hpack.NewEncoder(&sc.headerWriteBuf)
@@ -3122,7 +3171,8 @@ type http2serverConn struct {
 	framer           *http2Framer
 	doneServing      chan struct{}               // closed when serverConn.serve ends
 	readFrameCh      chan http2readFrameResult   // written by serverConn.readFrames
-	wantWriteFrameCh chan http2frameWriteMsg    // from handlers -> serve
+	wantWriteFrameCh chan http2FrameWriteRequest // from handlers -> serve
+	wantStartPushCh  chan http2startPushRequest  // from handlers -> serve
 	wroteFrameCh     chan http2frameWriteResult  // from writeFrameAsync -> serve, tickles more frame writes
 	bodyReadCh       chan http2bodyReadMsg       // from handlers -> serve
 	testHookCh       chan func(int)              // code to run on the serve loop
@@ -3130,6 +3180,7 @@ type http2serverConn struct {
 	inflow           http2flow                   // conn-wide inbound flow control
 	tlsState         *tls.ConnectionState        // shared by all handlers, like net/http
 	remoteAddrStr    string
+	writeSched       http2WriteScheduler
 
 	// Everything following is owned by the serve loop; use serveG.check():
 	serveG                http2goroutineLock // used to verify funcs are on serve()
@@ -3139,21 +3190,27 @@ type http2serverConn struct {
 	unackedSettings       int    // how many SETTINGS have we sent without ACKs?
 	clientMaxStreams      uint32 // SETTINGS_MAX_CONCURRENT_STREAMS from client (our PUSH_PROMISE limit)
 	advMaxStreams         uint32 // our SETTINGS_MAX_CONCURRENT_STREAMS advertised the client
-	curOpenStreams        uint32 // client's number of open streams
-	maxStreamID           uint32 // max ever seen
+	curClientStreams      uint32 // number of open streams initiated by the client
+	curPushedStreams      uint32 // number of open streams initiated by server push
+	maxStreamID           uint32 // max ever seen from client
+	maxPushPromiseID      uint32 // ID of the last push promise, or 0 if there have been no pushes
 	streams               map[uint32]*http2stream
 	initialWindowSize     int32
+	maxFrameSize          int32
 	headerTableSize       uint32
 	peerMaxHeaderListSize uint32            // zero means unknown (default)
 	canonHeader           map[string]string // http2-lower-case -> Go-Canonical-Case
-	writingFrame          bool              // started write goroutine but haven't heard back on wroteFrameCh
+	writingFrame          bool              // started writing a frame (on serve goroutine or separate)
+	writingFrameAsync     bool              // started a frame on its own goroutine but haven't heard back on wroteFrameCh
 	needsFrameFlush       bool              // last frame write wasn't a flush
-	writeSched            http2writeScheduler
 	inGoAway              bool              // we've started to or sent GOAWAY
+	inFrameScheduleLoop   bool              // whether we're in the scheduleFrameWrite loop
 	needToSendGoAway      bool              // we need to schedule a GOAWAY frame write
 	goAwayCode            http2ErrCode
 	shutdownTimerCh       <-chan time.Time // nil until used
 	shutdownTimer         *time.Timer      // nil until used
+	idleTimer             *time.Timer      // nil if unused
+	idleTimerCh           <-chan time.Time // nil if unused
 
 	// Owned by the writeFrameAsync goroutine:
 	headerWriteBuf bytes.Buffer
@@ -3357,7 +3414,7 @@ func (sc *http2serverConn) readFrames() {
 
 // frameWriteResult is the message passed from writeFrameAsync to the serve goroutine.
 type http2frameWriteResult struct {
-	wm  http2frameWriteMsg // what was written (or attempted)
+	wr  http2FrameWriteRequest // what was written (or attempted)
 	err error                  // result of the writeFrame call
 }
 
@@ -3365,9 +3422,9 @@ type http2frameWriteResult struct {
 // and then reports when it's done.
 // At most one goroutine can be running writeFrameAsync at a time per
 // serverConn.
-func (sc *http2serverConn) writeFrameAsync(wm http2frameWriteMsg) {
-	err := wm.write.writeFrame(sc)
-	sc.wroteFrameCh <- http2frameWriteResult{wm, err}
+func (sc *http2serverConn) writeFrameAsync(wr http2FrameWriteRequest) {
+	err := wr.write.writeFrame(sc)
+	sc.wroteFrameCh <- http2frameWriteResult{wr, err}
 }
 
 func (sc *http2serverConn) closeAllStreamsOnConnClose() {
@@ -3411,7 +3468,7 @@ func (sc *http2serverConn) serve() {
 		sc.vlogf("http2: server connection from %v on %p", sc.conn.RemoteAddr(), sc.hs)
 	}
 
-	sc.writeFrame(http2frameWriteMsg{
+	sc.writeFrame(http2FrameWriteRequest{
 		write: http2writeSettings{
 			{http2SettingMaxFrameSize, sc.srv.maxReadFrameSize()},
 			{http2SettingMaxConcurrentStreams, sc.advMaxStreams},
@@ -3428,6 +3485,12 @@ func (sc *http2serverConn) serve() {
 	sc.setConnState(StateActive)
 	sc.setConnState(StateIdle)
 
+	if sc.srv.IdleTimeout != 0 {
+		sc.idleTimer = time.NewTimer(sc.srv.IdleTimeout)
+		defer sc.idleTimer.Stop()
+		sc.idleTimerCh = sc.idleTimer.C
+	}
+
 	go sc.readFrames()
 
 	settingsTimer := time.NewTimer(http2firstSettingsTimeout)
@@ -3435,8 +3498,10 @@ func (sc *http2serverConn) serve() {
 	for {
 		loopNum++
 		select {
-		case wm := <-sc.wantWriteFrameCh:
-			sc.writeFrame(wm)
+		case wr := <-sc.wantWriteFrameCh:
+			sc.writeFrame(wr)
+		case spr := <-sc.wantStartPushCh:
+			sc.startPush(spr)
 		case res := <-sc.wroteFrameCh:
 			sc.wroteFrame(res)
 		case res := <-sc.readFrameCh:
@@ -3456,6 +3521,9 @@ func (sc *http2serverConn) serve() {
 		case <-sc.shutdownTimerCh:
 			sc.vlogf("GOAWAY close timer fired; closing conn from %v", sc.conn.RemoteAddr())
 			return
+		case <-sc.idleTimerCh:
+			sc.vlogf("connection is idle")
+			sc.goAway(http2ErrCodeNo)
 		case fn := <-sc.testHookCh:
 			fn(loopNum)
 		}
@@ -3506,7 +3574,7 @@ func (sc *http2serverConn) writeDataFromHandler(stream *http2stream, data []byte
 	ch := http2errChanPool.Get().(chan error)
 	writeArg := http2writeDataPool.Get().(*http2writeData)
 	*writeArg = http2writeData{stream.id, data, endStream}
-	err := sc.writeFrameFromHandler(http2frameWriteMsg{
+	err := sc.writeFrameFromHandler(http2FrameWriteRequest{
 		write:  writeArg,
 		stream: stream,
 		done:   ch,
@@ -3536,17 +3604,17 @@ func (sc *http2serverConn) writeDataFromHandler(stream *http2stream, data []byte
 	return err
 }
 
-// writeFrameFromHandler sends wm to sc.wantWriteFrameCh, but aborts
+// writeFrameFromHandler sends wr to sc.wantWriteFrameCh, but aborts
 // if the connection has gone away.
 //
 // This must not be run from the serve goroutine itself, else it might
 // deadlock writing to sc.wantWriteFrameCh (which is only mildly
 // buffered and is read by serve itself). If you're on the serve
 // goroutine, call writeFrame instead.
-func (sc *http2serverConn) writeFrameFromHandler(wm http2frameWriteMsg) error {
+func (sc *http2serverConn) writeFrameFromHandler(wr http2FrameWriteRequest) error {
 	sc.serveG.checkNotOn()
 	select {
-	case sc.wantWriteFrameCh <- wm:
+	case sc.wantWriteFrameCh <- wr:
 		return nil
 	case <-sc.doneServing:
 
@@ -3562,36 +3630,36 @@ func (sc *http2serverConn) writeFrameFromHandler(wm http2frameWriteMsg) error {
 // make it onto the wire
 //
 // If you're not on the serve goroutine, use writeFrameFromHandler instead.
-func (sc *http2serverConn) writeFrame(wm http2frameWriteMsg) {
+func (sc *http2serverConn) writeFrame(wr http2FrameWriteRequest) {
 	sc.serveG.check()
 
 	var ignoreWrite bool
 
-	switch wm.write.(type) {
+	switch wr.write.(type) {
 	case *http2writeResHeaders:
-		wm.stream.wroteHeaders = true
+		wr.stream.wroteHeaders = true
 	case http2write100ContinueHeadersFrame:
-		if wm.stream.wroteHeaders {
+		if wr.stream.wroteHeaders {
 			ignoreWrite = true
 		}
 	}
 
 	if !ignoreWrite {
-		sc.writeSched.add(wm)
+		sc.writeSched.Push(wr)
 	}
 	sc.scheduleFrameWrite()
 }
 
-// startFrameWrite starts a goroutine to write wm (in a separate
+// startFrameWrite starts a goroutine to write wr (in a separate
 // goroutine since that might block on the network), and updates the
-// serve goroutine's state about the world, updated from info in wm.
-func (sc *http2serverConn) startFrameWrite(wm http2frameWriteMsg) {
+// serve goroutine's state about the world, updated from info in wr.
+func (sc *http2serverConn) startFrameWrite(wr http2FrameWriteRequest) {
 	sc.serveG.check()
 	if sc.writingFrame {
 		panic("internal error: can only be writing one frame at a time")
 	}
 
-	st := wm.stream
+	st := wr.stream
 	if st != nil {
 		switch st.state {
 		case http2stateHalfClosedLocal:
@@ -3602,13 +3670,31 @@ func (sc *http2serverConn) startFrameWrite(wm http2frameWriteMsg) {
 				sc.scheduleFrameWrite()
 				return
 			}
-			panic(fmt.Sprintf("internal error: attempt to send a write %v on a closed stream", wm))
+			panic(fmt.Sprintf("internal error: attempt to send a write %v on a closed stream", wr))
+		}
+	}
+	if wpp, ok := wr.write.(*http2writePushPromise); ok {
+		var err error
+		wpp.promisedID, err = wpp.allocatePromisedID()
+		if err != nil {
+			sc.writingFrameAsync = false
+			if wr.done != nil {
+				wr.done <- err
+			}
+			return
 		}
 	}
 
 	sc.writingFrame = true
 	sc.needsFrameFlush = true
-	go sc.writeFrameAsync(wm)
+	if wr.write.staysWithinBuffer(sc.bw.Available()) {
+		sc.writingFrameAsync = false
+		err := wr.write.writeFrame(sc)
+		sc.wroteFrame(http2frameWriteResult{wr, err})
+	} else {
+		sc.writingFrameAsync = true
+		go sc.writeFrameAsync(wr)
+	}
 }
 
 // errHandlerPanicked is the error given to any callers blocked in a read from
@@ -3624,24 +3710,25 @@ func (sc *http2serverConn) wroteFrame(res http2frameWriteResult) {
 		panic("internal error: expected to be already writing a frame")
 	}
 	sc.writingFrame = false
+	sc.writingFrameAsync = false
 
-	wm := res.wm
-	st := wm.stream
+	wr := res.wr
+	st := wr.stream
 
-	closeStream := http2endsStream(wm.write)
+	closeStream := http2endsStream(wr.write)
 
-	if _, ok := wm.write.(http2handlerPanicRST); ok {
+	if _, ok := wr.write.(http2handlerPanicRST); ok {
 		sc.closeStream(st, http2errHandlerPanicked)
 	}
 
-	if ch := wm.done; ch != nil {
+	if ch := wr.done; ch != nil {
 		select {
 		case ch <- res.err:
 		default:
-			panic(fmt.Sprintf("unbuffered done channel passed in for type %T", wm.write))
+			panic(fmt.Sprintf("unbuffered done channel passed in for type %T", wr.write))
 		}
 	}
-	wm.write = nil
+	wr.write = nil
 
 	if closeStream {
 		if st == nil {
@@ -3675,35 +3762,40 @@ func (sc *http2serverConn) wroteFrame(res http2frameWriteResult) {
 // flush the write buffer.
 func (sc *http2serverConn) scheduleFrameWrite() {
 	sc.serveG.check()
-	if sc.writingFrame {
+	if sc.writingFrame || sc.inFrameScheduleLoop {
 		return
 	}
+	sc.inFrameScheduleLoop = true
+	for !sc.writingFrameAsync {
 		if sc.needToSendGoAway {
 			sc.needToSendGoAway = false
-		sc.startFrameWrite(http2frameWriteMsg{
+			sc.startFrameWrite(http2FrameWriteRequest{
 				write: &http2writeGoAway{
 					maxStreamID: sc.maxStreamID,
 					code:        sc.goAwayCode,
 				},
 			})
-		return
+			continue
 		}
 		if sc.needToSendSettingsAck {
 			sc.needToSendSettingsAck = false
-		sc.startFrameWrite(http2frameWriteMsg{write: http2writeSettingsAck{}})
-		return
+			sc.startFrameWrite(http2FrameWriteRequest{write: http2writeSettingsAck{}})
+			continue
 		}
 		if !sc.inGoAway {
-		if wm, ok := sc.writeSched.take(); ok {
-			sc.startFrameWrite(wm)
-			return
+			if wr, ok := sc.writeSched.Pop(); ok {
+				sc.startFrameWrite(wr)
+				continue
 			}
 		}
 		if sc.needsFrameFlush {
-		sc.startFrameWrite(http2frameWriteMsg{write: http2flushFrameWriter{}})
+			sc.startFrameWrite(http2FrameWriteRequest{write: http2flushFrameWriter{}})
 			sc.needsFrameFlush = false
-		return
+			continue
+		}
+		break
 	}
+	sc.inFrameScheduleLoop = false
 }
 
 func (sc *http2serverConn) goAway(code http2ErrCode) {
@@ -3731,7 +3823,7 @@ func (sc *http2serverConn) shutDownIn(d time.Duration) {
 
 func (sc *http2serverConn) resetStream(se http2StreamError) {
 	sc.serveG.check()
-	sc.writeFrame(http2frameWriteMsg{write: se})
+	sc.writeFrame(http2FrameWriteRequest{write: se})
 	if st, ok := sc.streams[se.StreamID]; ok {
 		st.sentReset = true
 		sc.closeStream(st, se)
@@ -3830,15 +3922,25 @@ func (sc *http2serverConn) processPing(f *http2PingFrame) error {
 
 		return http2ConnectionError(http2ErrCodeProtocol)
 	}
-	sc.writeFrame(http2frameWriteMsg{write: http2writePingAck{f}})
+	if sc.inGoAway {
+		return nil
+	}
+	sc.writeFrame(http2FrameWriteRequest{write: http2writePingAck{f}})
 	return nil
 }
 
 func (sc *http2serverConn) processWindowUpdate(f *http2WindowUpdateFrame) error {
 	sc.serveG.check()
+	if sc.inGoAway {
+		return nil
+	}
 	switch {
 	case f.StreamID != 0:
-		st := sc.streams[f.StreamID]
+		state, st := sc.state(f.StreamID)
+		if state == http2stateIdle {
+
+			return http2ConnectionError(http2ErrCodeProtocol)
+		}
 		if st == nil {
 
 			return nil
@@ -3857,6 +3959,9 @@ func (sc *http2serverConn) processWindowUpdate(f *http2WindowUpdateFrame) error
 
 func (sc *http2serverConn) processResetStream(f *http2RSTStreamFrame) error {
 	sc.serveG.check()
+	if sc.inGoAway {
+		return nil
+	}
 
 	state, st := sc.state(f.StreamID)
 	if state == http2stateIdle {
@@ -3877,11 +3982,18 @@ func (sc *http2serverConn) closeStream(st *http2stream, err error) {
 		panic(fmt.Sprintf("invariant; can't close stream in state %v", st.state))
 	}
 	st.state = http2stateClosed
-	sc.curOpenStreams--
-	if sc.curOpenStreams == 0 {
+	if st.isPushed() {
+		sc.curPushedStreams--
+	} else {
+		sc.curClientStreams--
+	}
+	if sc.curClientStreams+sc.curPushedStreams == 0 {
 		sc.setConnState(StateIdle)
 	}
 	delete(sc.streams, st.id)
+	if len(sc.streams) == 0 && sc.srv.IdleTimeout != 0 {
+		sc.idleTimer.Reset(sc.srv.IdleTimeout)
+	}
 	if p := st.body; p != nil {
 
 		sc.sendWindowUpdate(nil, p.Len())
@@ -3889,7 +4001,7 @@ func (sc *http2serverConn) closeStream(st *http2stream, err error) {
 		p.CloseWithError(err)
 	}
 	st.cw.Close()
-	sc.writeSched.forgetStream(st.id)
+	sc.writeSched.CloseStream(st.id)
 }
 
 func (sc *http2serverConn) processSettings(f *http2SettingsFrame) error {
@@ -3902,6 +4014,9 @@ func (sc *http2serverConn) processSettings(f *http2SettingsFrame) error {
 		}
 		return nil
 	}
+	if sc.inGoAway {
+		return nil
+	}
 	if err := f.ForeachSetting(sc.processSetting); err != nil {
 		return err
 	}
@@ -3929,7 +4044,7 @@ func (sc *http2serverConn) processSetting(s http2Setting) error {
 	case http2SettingInitialWindowSize:
 		return sc.processSettingInitialWindowSize(s.Val)
 	case http2SettingMaxFrameSize:
-		sc.writeSched.maxFrameSize = s.Val
+		sc.maxFrameSize = int32(s.Val)
 	case http2SettingMaxHeaderListSize:
 		sc.peerMaxHeaderListSize = s.Val
 	default:
@@ -3958,11 +4073,18 @@ func (sc *http2serverConn) processSettingInitialWindowSize(val uint32) error {
 
 func (sc *http2serverConn) processData(f *http2DataFrame) error {
 	sc.serveG.check()
+	if sc.inGoAway {
+		return nil
+	}
 	data := f.Data()
 
 	id := f.Header().StreamID
-	st, ok := sc.streams[id]
-	if !ok || st.state != http2stateOpen || st.gotTrailerHeader {
+	state, st := sc.state(id)
+	if id == 0 || state == http2stateIdle {
+
+		return http2ConnectionError(http2ErrCodeProtocol)
+	}
+	if st == nil || state != http2stateOpen || st.gotTrailerHeader {
 
 		if sc.inflow.available() < int32(f.Length) {
 			return http2streamError(id, http2ErrCodeFlowControl)
@@ -4010,6 +4132,11 @@ func (sc *http2serverConn) processData(f *http2DataFrame) error {
 	return nil
 }
 
+// isPushed reports whether the stream is server-initiated.
+func (st *http2stream) isPushed() bool {
+	return st.id%2 == 0
+}
+
 // endStream closes a Request.Body's pipe. It is called when a DATA
 // frame says a request body is over (or after trailers).
 func (st *http2stream) endStream() {
@@ -4039,7 +4166,7 @@ func (st *http2stream) copyTrailersToHandlerRequest() {
 
 func (sc *http2serverConn) processHeaders(f *http2MetaHeadersFrame) error {
 	sc.serveG.check()
-	id := f.Header().StreamID
+	id := f.StreamID
 	if sc.inGoAway {
 
 		return nil
@@ -4049,8 +4176,7 @@ func (sc *http2serverConn) processHeaders(f *http2MetaHeadersFrame) error {
 		return http2ConnectionError(http2ErrCodeProtocol)
 	}
 
-	st := sc.streams[f.Header().StreamID]
-	if st != nil {
+	if st := sc.streams[f.StreamID]; st != nil {
 		return st.processTrailerHeaders(f)
 	}
 
@@ -4059,40 +4185,30 @@ func (sc *http2serverConn) processHeaders(f *http2MetaHeadersFrame) error {
 	}
 	sc.maxStreamID = id
 
-	ctx, cancelCtx := http2contextWithCancel(sc.baseCtx)
-	st = &http2stream{
-		sc:        sc,
-		id:        id,
-		state:     http2stateOpen,
-		ctx:       ctx,
-		cancelCtx: cancelCtx,
-	}
-	if f.StreamEnded() {
-		st.state = http2stateHalfClosedRemote
+	if sc.idleTimer != nil {
+		sc.idleTimer.Stop()
 	}
-	st.cw.Init()
 
-	st.flow.conn = &sc.flow
-	st.flow.add(sc.initialWindowSize)
-	st.inflow.conn = &sc.inflow
-	st.inflow.add(http2initialWindowSize)
+	if sc.curClientStreams+1 > sc.advMaxStreams {
+		if sc.unackedSettings == 0 {
 
-	sc.streams[id] = st
-	if f.HasPriority() {
-		http2adjustStreamPriority(sc.streams, st.id, f.Priority)
-	}
-	sc.curOpenStreams++
-	if sc.curOpenStreams == 1 {
-		sc.setConnState(StateActive)
+			return http2streamError(id, http2ErrCodeProtocol)
 		}
-	if sc.curOpenStreams > sc.advMaxStreams {
 
-		if sc.unackedSettings == 0 {
+		return http2streamError(id, http2ErrCodeRefusedStream)
+	}
 
-			return http2streamError(st.id, http2ErrCodeProtocol)
+	initialState := http2stateOpen
+	if f.StreamEnded() {
+		initialState = http2stateHalfClosedRemote
 	}
+	st := sc.newStream(id, 0, initialState)
 
-		return http2streamError(st.id, http2ErrCodeRefusedStream)
+	if f.HasPriority() {
+		if err := http2checkPriority(f.StreamID, f.Priority); err != nil {
+			return err
+		}
+		sc.writeSched.AdjustStream(st.id, f.Priority)
 	}
 
 	rw, req, err := sc.newWriterAndRequest(st, f)
@@ -4110,7 +4226,7 @@ func (sc *http2serverConn) processHeaders(f *http2MetaHeadersFrame) error {
 	if f.Truncated {
 
 		handler = http2handleHeaderListTooLong
-	} else if err := http2checkValidHTTP2Request(req); err != nil {
+	} else if err := http2checkValidHTTP2RequestHeaders(req.Header); err != nil {
 		handler = http2new400Handler(err)
 	}
 
@@ -4150,90 +4266,138 @@ func (st *http2stream) processTrailerHeaders(f *http2MetaHeadersFrame) error {
 	return nil
 }
 
-func (sc *http2serverConn) processPriority(f *http2PriorityFrame) error {
-	http2adjustStreamPriority(sc.streams, f.StreamID, f.http2PriorityParam)
+func http2checkPriority(streamID uint32, p http2PriorityParam) error {
+	if streamID == p.StreamDep {
+
+		return http2streamError(streamID, http2ErrCodeProtocol)
+	}
 	return nil
 }
 
-func http2adjustStreamPriority(streams map[uint32]*http2stream, streamID uint32, priority http2PriorityParam) {
-	st, ok := streams[streamID]
-	if !ok {
-
-		return
+func (sc *http2serverConn) processPriority(f *http2PriorityFrame) error {
+	if sc.inGoAway {
+		return nil
 	}
-	st.weight = priority.Weight
-	parent := streams[priority.StreamDep]
-	if parent == st {
-
-		return
+	if err := http2checkPriority(f.StreamID, f.http2PriorityParam); err != nil {
+		return err
 	}
+	sc.writeSched.AdjustStream(f.StreamID, f.http2PriorityParam)
+	return nil
+}
 
-	for piter := parent; piter != nil; piter = piter.parent {
-		if piter == st {
-			parent.parent = st.parent
-			break
-		}
+func (sc *http2serverConn) newStream(id, pusherID uint32, state http2streamState) *http2stream {
+	sc.serveG.check()
+	if id == 0 {
+		panic("internal error: cannot create stream with id 0")
 	}
-	st.parent = parent
-	if priority.Exclusive && (st.parent != nil || priority.StreamDep == 0) {
-		for _, openStream := range streams {
-			if openStream != st && openStream.parent == st.parent {
-				openStream.parent = st
+
+	ctx, cancelCtx := http2contextWithCancel(sc.baseCtx)
+	st := &http2stream{
+		sc:        sc,
+		id:        id,
+		state:     state,
+		ctx:       ctx,
+		cancelCtx: cancelCtx,
 	}
+	st.cw.Init()
+	st.flow.conn = &sc.flow
+	st.flow.add(sc.initialWindowSize)
+	st.inflow.conn = &sc.inflow
+	st.inflow.add(http2initialWindowSize)
+
+	sc.streams[id] = st
+	sc.writeSched.OpenStream(st.id, http2OpenStreamOptions{PusherID: pusherID})
+	if st.isPushed() {
+		sc.curPushedStreams++
+	} else {
+		sc.curClientStreams++
 	}
+	if sc.curClientStreams+sc.curPushedStreams == 1 {
+		sc.setConnState(StateActive)
 	}
+
+	return st
 }
 
 func (sc *http2serverConn) newWriterAndRequest(st *http2stream, f *http2MetaHeadersFrame) (*http2responseWriter, *Request, error) {
 	sc.serveG.check()
 
-	method := f.PseudoValue("method")
-	path := f.PseudoValue("path")
-	scheme := f.PseudoValue("scheme")
-	authority := f.PseudoValue("authority")
+	rp := http2requestParam{
+		method:    f.PseudoValue("method"),
+		scheme:    f.PseudoValue("scheme"),
+		authority: f.PseudoValue("authority"),
+		path:      f.PseudoValue("path"),
+	}
 
-	isConnect := method == "CONNECT"
+	isConnect := rp.method == "CONNECT"
 	if isConnect {
-		if path != "" || scheme != "" || authority == "" {
+		if rp.path != "" || rp.scheme != "" || rp.authority == "" {
 			return nil, nil, http2streamError(f.StreamID, http2ErrCodeProtocol)
 		}
-	} else if method == "" || path == "" ||
-		(scheme != "https" && scheme != "http") {
+	} else if rp.method == "" || rp.path == "" || (rp.scheme != "https" && rp.scheme != "http") {
 
 		return nil, nil, http2streamError(f.StreamID, http2ErrCodeProtocol)
 	}
 
 	bodyOpen := !f.StreamEnded()
-	if method == "HEAD" && bodyOpen {
+	if rp.method == "HEAD" && bodyOpen {
 
 		return nil, nil, http2streamError(f.StreamID, http2ErrCodeProtocol)
 	}
-	var tlsState *tls.ConnectionState // nil if not scheme https
 
-	if scheme == "https" {
-		tlsState = sc.tlsState
+	rp.header = make(Header)
+	for _, hf := range f.RegularFields() {
+		rp.header.Add(sc.canonicalHeader(hf.Name), hf.Value)
+	}
+	if rp.authority == "" {
+		rp.authority = rp.header.Get("Host")
 	}
 
-	header := make(Header)
-	for _, hf := range f.RegularFields() {
-		header.Add(sc.canonicalHeader(hf.Name), hf.Value)
+	rw, req, err := sc.newWriterAndRequestNoBody(st, rp)
+	if err != nil {
+		return nil, nil, err
+	}
+	if bodyOpen {
+		st.reqBuf = http2getRequestBodyBuf()
+		req.Body.(*http2requestBody).pipe = &http2pipe{
+			b: &http2fixedBuffer{buf: st.reqBuf},
+		}
+
+		if vv, ok := rp.header["Content-Length"]; ok {
+			req.ContentLength, _ = strconv.ParseInt(vv[0], 10, 64)
+		} else {
+			req.ContentLength = -1
 		}
+	}
+	return rw, req, nil
+}
+
+type http2requestParam struct {
+	method                  string
+	scheme, authority, path string
+	header                  Header
+}
+
+func (sc *http2serverConn) newWriterAndRequestNoBody(st *http2stream, rp http2requestParam) (*http2responseWriter, *Request, error) {
+	sc.serveG.check()
 
-	if authority == "" {
-		authority = header.Get("Host")
+	var tlsState *tls.ConnectionState // nil if not scheme https
+	if rp.scheme == "https" {
+		tlsState = sc.tlsState
 	}
-	needsContinue := header.Get("Expect") == "100-continue"
+
+	needsContinue := rp.header.Get("Expect") == "100-continue"
 	if needsContinue {
-		header.Del("Expect")
+		rp.header.Del("Expect")
 	}
 
-	if cookies := header["Cookie"]; len(cookies) > 1 {
-		header.Set("Cookie", strings.Join(cookies, "; "))
+	if cookies := rp.header["Cookie"]; len(cookies) > 1 {
+		rp.header.Set("Cookie", strings.Join(cookies, "; "))
 	}
 
 	// Setup Trailers
 	var trailer Header
-	for _, v := range header["Trailer"] {
+	for _, v := range rp.header["Trailer"] {
 		for _, key := range strings.Split(v, ",") {
 			key = CanonicalHeaderKey(strings.TrimSpace(key))
 			switch key {
@@ -4247,53 +4411,42 @@ func (sc *http2serverConn) newWriterAndRequest(st *http2stream, f *http2MetaHead
 			}
 		}
 	}
-	delete(header, "Trailer")
+	delete(rp.header, "Trailer")
 
-	body := &http2requestBody{
-		conn:          sc,
-		stream:        st,
-		needsContinue: needsContinue,
-	}
 	var url_ *url.URL
 	var requestURI string
-	if isConnect {
-		url_ = &url.URL{Host: authority}
-		requestURI = authority
+	if rp.method == "CONNECT" {
+		url_ = &url.URL{Host: rp.authority}
+		requestURI = rp.authority
 	} else {
 		var err error
-		url_, err = url.ParseRequestURI(path)
+		url_, err = url.ParseRequestURI(rp.path)
 		if err != nil {
-			return nil, nil, http2streamError(f.StreamID, http2ErrCodeProtocol)
+			return nil, nil, http2streamError(st.id, http2ErrCodeProtocol)
+		}
+		requestURI = rp.path
 	}
-		requestURI = path
+
+	body := &http2requestBody{
+		conn:          sc,
+		stream:        st,
+		needsContinue: needsContinue,
 	}
 	req := &Request{
-		Method:     method,
+		Method:     rp.method,
 		URL:        url_,
 		RemoteAddr: sc.remoteAddrStr,
-		Header:     header,
+		Header:     rp.header,
 		RequestURI: requestURI,
 		Proto:      "HTTP/2.0",
 		ProtoMajor: 2,
 		ProtoMinor: 0,
 		TLS:        tlsState,
-		Host:       authority,
+		Host:       rp.authority,
 		Body:       body,
 		Trailer:    trailer,
 	}
 	req = http2requestWithContext(req, st.ctx)
-	if bodyOpen {
-		st.reqBuf = http2getRequestBodyBuf()
-		body.pipe = &http2pipe{
-			b: &http2fixedBuffer{buf: st.reqBuf},
-		}
-
-		if vv, ok := header["Content-Length"]; ok {
-			req.ContentLength, _ = strconv.ParseInt(vv[0], 10, 64)
-		} else {
-			req.ContentLength = -1
-		}
-	}
 
 	rws := http2responseWriterStatePool.Get().(*http2responseWriterState)
 	bwSave := rws.bw
@@ -4338,7 +4491,7 @@ func (sc *http2serverConn) runHandler(rw *http2responseWriter, req *Request, han
 			const size = 64 << 10
 			buf := make([]byte, size)
 			buf = buf[:runtime.Stack(buf, false)]
-			sc.writeFrameFromHandler(http2frameWriteMsg{
+			sc.writeFrameFromHandler(http2FrameWriteRequest{
 				write:  http2handlerPanicRST{rw.rws.stream.id},
 				stream: rw.rws.stream,
 			})
@@ -4370,7 +4523,7 @@ func (sc *http2serverConn) writeHeaders(st *http2stream, headerData *http2writeR
 
 		errc = http2errChanPool.Get().(chan error)
 	}
-	if err := sc.writeFrameFromHandler(http2frameWriteMsg{
+	if err := sc.writeFrameFromHandler(http2FrameWriteRequest{
 		write:  headerData,
 		stream: st,
 		done:   errc,
@@ -4393,7 +4546,7 @@ func (sc *http2serverConn) writeHeaders(st *http2stream, headerData *http2writeR
 
 // called from handler goroutines.
 func (sc *http2serverConn) write100ContinueHeaders(st *http2stream) {
-	sc.writeFrameFromHandler(http2frameWriteMsg{
+	sc.writeFrameFromHandler(http2FrameWriteRequest{
 		write:  http2write100ContinueHeadersFrame{st.id},
 		stream: st,
 	})
@@ -4463,7 +4616,7 @@ func (sc *http2serverConn) sendWindowUpdate32(st *http2stream, n int32) {
 	if st != nil {
 		streamID = st.id
 	}
-	sc.writeFrame(http2frameWriteMsg{
+	sc.writeFrame(http2FrameWriteRequest{
 		write:  http2writeWindowUpdate{streamID: streamID, n: uint32(n)},
 		stream: st,
 	})
@@ -4844,67 +4997,229 @@ func (w *http2responseWriter) handlerDone() {
 	http2responseWriterStatePool.Put(rws)
 }
 
-// foreachHeaderElement splits v according to the "#rule" construction
-// in RFC 2616 section 2.1 and calls fn for each non-empty element.
-func http2foreachHeaderElement(v string, fn func(string)) {
-	v = textproto.TrimString(v)
-	if v == "" {
-		return
+// Push errors.
+var (
+	http2ErrRecursivePush    = errors.New("http2: recursive push not allowed")
+	http2ErrPushLimitReached = errors.New("http2: push would exceed peer's SETTINGS_MAX_CONCURRENT_STREAMS")
+)
+
+// pushOptions is the internal version of http.PushOptions, which we
+// cannot include here because it's only defined in Go 1.8 and later.
+type http2pushOptions struct {
+	Method string
+	Header Header
+}
+
+func (w *http2responseWriter) push(target string, opts http2pushOptions) error {
+	st := w.rws.stream
+	sc := st.sc
+	sc.serveG.checkNotOn()
+
+	if st.isPushed() {
+		return http2ErrRecursivePush
 	}
-	if !strings.Contains(v, ",") {
-		fn(v)
-		return
+
+	if opts.Method == "" {
+		opts.Method = "GET"
 	}
-	for _, f := range strings.Split(v, ",") {
-		if f = textproto.TrimString(f); f != "" {
-			fn(f)
+	if opts.Header == nil {
+		opts.Header = Header{}
 	}
+	wantScheme := "http"
+	if w.rws.req.TLS != nil {
+		wantScheme = "https"
 	}
-}
 
-// From http://httpwg.org/specs/rfc7540.html#rfc.section.8.1.2.2
-var http2connHeaders = []string{
-	"Connection",
-	"Keep-Alive",
-	"Proxy-Connection",
-	"Transfer-Encoding",
-	"Upgrade",
-}
+	u, err := url.Parse(target)
+	if err != nil {
+		return err
+	}
+	if u.Scheme == "" {
+		if !strings.HasPrefix(target, "/") {
+			return fmt.Errorf("target must be an absolute URL or an absolute path: %q", target)
+		}
+		u.Scheme = wantScheme
+		u.Host = w.rws.req.Host
+	} else {
+		if u.Scheme != wantScheme {
+			return fmt.Errorf("cannot push URL with scheme %q from request with scheme %q", u.Scheme, wantScheme)
+		}
+		if u.Host == "" {
+			return errors.New("URL must have a host")
+		}
+	}
+	for k := range opts.Header {
+		if strings.HasPrefix(k, ":") {
+			return fmt.Errorf("promised request headers cannot include psuedo header %q", k)
+		}
 
-// checkValidHTTP2Request checks whether req is a valid HTTP/2 request,
-// per RFC 7540 Section 8.1.2.2.
-// The returned error is reported to users.
-func http2checkValidHTTP2Request(req *Request) error {
-	for _, h := range http2connHeaders {
-		if _, ok := req.Header[h]; ok {
-			return fmt.Errorf("request header %q is not valid in HTTP/2", h)
+		switch strings.ToLower(k) {
+		case "content-length", "content-encoding", "trailer", "te", "expect", "host":
+			return fmt.Errorf("promised request headers cannot include %q", k)
 		}
 	}
-	te := req.Header["Te"]
-	if len(te) > 0 && (len(te) > 1 || (te[0] != "trailers" && te[0] != "")) {
-		return errors.New(`request header "TE" may only be "trailers" in HTTP/2`)
+	if err := http2checkValidHTTP2RequestHeaders(opts.Header); err != nil {
+		return err
 	}
-	return nil
-}
 
-func http2new400Handler(err error) HandlerFunc {
-	return func(w ResponseWriter, r *Request) {
-		Error(w, err.Error(), StatusBadRequest)
+	if opts.Method != "GET" && opts.Method != "HEAD" {
+		return fmt.Errorf("method %q must be GET or HEAD", opts.Method)
 	}
-}
 
-// ValidTrailerHeader reports whether name is a valid header field name to appear
-// in trailers.
-// See: http://tools.ietf.org/html/rfc7230#section-4.1.2
-func http2ValidTrailerHeader(name string) bool {
-	name = CanonicalHeaderKey(name)
-	if strings.HasPrefix(name, "If-") || http2badTrailer[name] {
-		return false
+	msg := http2startPushRequest{
+		parent: st,
+		method: opts.Method,
+		url:    u,
+		header: http2cloneHeader(opts.Header),
+		done:   http2errChanPool.Get().(chan error),
+	}
+
+	select {
+	case <-sc.doneServing:
+		return http2errClientDisconnected
+	case <-st.cw:
+		return http2errStreamClosed
+	case sc.wantStartPushCh <- msg:
+	}
+
+	select {
+	case <-sc.doneServing:
+		return http2errClientDisconnected
+	case <-st.cw:
+		return http2errStreamClosed
+	case err := <-msg.done:
+		http2errChanPool.Put(msg.done)
+		return err
 	}
-	return true
 }
 
-var http2badTrailer = map[string]bool{
+type http2startPushRequest struct {
+	parent *http2stream
+	method string
+	url    *url.URL
+	header Header
+	done   chan error
+}
+
+func (sc *http2serverConn) startPush(msg http2startPushRequest) {
+	sc.serveG.check()
+
+	if msg.parent.state != http2stateOpen && msg.parent.state != http2stateHalfClosedRemote {
+
+		msg.done <- http2errStreamClosed
+		return
+	}
+
+	if !sc.pushEnabled {
+		msg.done <- ErrNotSupported
+		return
+	}
+
+	allocatePromisedID := func() (uint32, error) {
+		sc.serveG.check()
+
+		if !sc.pushEnabled {
+			return 0, ErrNotSupported
+		}
+
+		if sc.curPushedStreams+1 > sc.clientMaxStreams {
+			return 0, http2ErrPushLimitReached
+		}
+
+		sc.maxPushPromiseID += 2
+		promisedID := sc.maxPushPromiseID
+
+		promised := sc.newStream(promisedID, msg.parent.id, http2stateHalfClosedRemote)
+		rw, req, err := sc.newWriterAndRequestNoBody(promised, http2requestParam{
+			method:    msg.method,
+			scheme:    msg.url.Scheme,
+			authority: msg.url.Host,
+			path:      msg.url.RequestURI(),
+			header:    msg.header,
+		})
+		if err != nil {
+
+			panic(fmt.Sprintf("newWriterAndRequestNoBody(%+v): %v", msg.url, err))
+		}
+
+		go sc.runHandler(rw, req, sc.handler.ServeHTTP)
+		return promisedID, nil
+	}
+
+	sc.writeFrame(http2FrameWriteRequest{
+		write: &http2writePushPromise{
+			streamID:           msg.parent.id,
+			method:             msg.method,
+			url:                msg.url,
+			h:                  msg.header,
+			allocatePromisedID: allocatePromisedID,
+		},
+		stream: msg.parent,
+		done:   msg.done,
+	})
+}
+
+// foreachHeaderElement splits v according to the "#rule" construction
+// in RFC 2616 section 2.1 and calls fn for each non-empty element.
+func http2foreachHeaderElement(v string, fn func(string)) {
+	v = textproto.TrimString(v)
+	if v == "" {
+		return
+	}
+	if !strings.Contains(v, ",") {
+		fn(v)
+		return
+	}
+	for _, f := range strings.Split(v, ",") {
+		if f = textproto.TrimString(f); f != "" {
+			fn(f)
+		}
+	}
+}
+
+// From http://httpwg.org/specs/rfc7540.html#rfc.section.8.1.2.2
+var http2connHeaders = []string{
+	"Connection",
+	"Keep-Alive",
+	"Proxy-Connection",
+	"Transfer-Encoding",
+	"Upgrade",
+}
+
+// checkValidHTTP2RequestHeaders checks whether h is a valid HTTP/2 request,
+// per RFC 7540 Section 8.1.2.2.
+// The returned error is reported to users.
+func http2checkValidHTTP2RequestHeaders(h Header) error {
+	for _, k := range http2connHeaders {
+		if _, ok := h[k]; ok {
+			return fmt.Errorf("request header %q is not valid in HTTP/2", k)
+		}
+	}
+	te := h["Te"]
+	if len(te) > 0 && (len(te) > 1 || (te[0] != "trailers" && te[0] != "")) {
+		return errors.New(`request header "TE" may only be "trailers" in HTTP/2`)
+	}
+	return nil
+}
+
+func http2new400Handler(err error) HandlerFunc {
+	return func(w ResponseWriter, r *Request) {
+		Error(w, err.Error(), StatusBadRequest)
+	}
+}
+
+// ValidTrailerHeader reports whether name is a valid header field name to appear
+// in trailers.
+// See: http://tools.ietf.org/html/rfc7230#section-4.1.2
+func http2ValidTrailerHeader(name string) bool {
+	name = CanonicalHeaderKey(name)
+	if strings.HasPrefix(name, "If-") || http2badTrailer[name] {
+		return false
+	}
+	return true
+}
+
+var http2badTrailer = map[string]bool{
 	"Authorization":       true,
 	"Cache-Control":       true,
 	"Connection":          true,
@@ -6852,6 +7167,11 @@ func http2isConnectionCloseRequest(req *Request) bool {
 // writeFramer is implemented by any type that is used to write frames.
 type http2writeFramer interface {
 	writeFrame(http2writeContext) error
+
+	// staysWithinBuffer reports whether this writer promises that
+	// it will only write less than or equal to size bytes, and it
+	// won't Flush the write context.
+	staysWithinBuffer(size int) bool
 }
 
 // writeContext is the interface needed by the various frame writer
@@ -6894,8 +7214,16 @@ func (http2flushFrameWriter) writeFrame(ctx http2writeContext) error {
 	return ctx.Flush()
 }
 
+func (http2flushFrameWriter) staysWithinBuffer(max int) bool { return false }
+
 type http2writeSettings []http2Setting
 
+func (s http2writeSettings) staysWithinBuffer(max int) bool {
+	const settingSize = 6 // uint16 + uint32
+	return http2frameHeaderLen+settingSize*len(s) <= max
+
+}
+
 func (s http2writeSettings) writeFrame(ctx http2writeContext) error {
 	return ctx.Framer().WriteSettings([]http2Setting(s)...)
 }
@@ -6915,6 +7243,8 @@ func (p *http2writeGoAway) writeFrame(ctx http2writeContext) error {
 	return err
 }
 
+func (*http2writeGoAway) staysWithinBuffer(max int) bool { return false }
+
 type http2writeData struct {
 	streamID  uint32
 	p         []byte
@@ -6929,6 +7259,10 @@ func (w *http2writeData) writeFrame(ctx http2writeContext) error {
 	return ctx.Framer().WriteData(w.streamID, w.endStream, w.p)
 }
 
+func (w *http2writeData) staysWithinBuffer(max int) bool {
+	return http2frameHeaderLen+len(w.p) <= max
+}
+
 // handlerPanicRST is the message sent from handler goroutines when
 // the handler panics.
 type http2handlerPanicRST struct {
@@ -6939,22 +7273,59 @@ func (hp http2handlerPanicRST) writeFrame(ctx http2writeContext) error {
 	return ctx.Framer().WriteRSTStream(hp.StreamID, http2ErrCodeInternal)
 }
 
+func (hp http2handlerPanicRST) staysWithinBuffer(max int) bool { return http2frameHeaderLen+4 <= max }
+
 func (se http2StreamError) writeFrame(ctx http2writeContext) error {
 	return ctx.Framer().WriteRSTStream(se.StreamID, se.Code)
 }
 
+func (se http2StreamError) staysWithinBuffer(max int) bool { return http2frameHeaderLen+4 <= max }
+
 type http2writePingAck struct{ pf *http2PingFrame }
 
 func (w http2writePingAck) writeFrame(ctx http2writeContext) error {
 	return ctx.Framer().WritePing(true, w.pf.Data)
 }
 
+func (w http2writePingAck) staysWithinBuffer(max int) bool {
+	return http2frameHeaderLen+len(w.pf.Data) <= max
+}
+
 type http2writeSettingsAck struct{}
 
 func (http2writeSettingsAck) writeFrame(ctx http2writeContext) error {
 	return ctx.Framer().WriteSettingsAck()
 }
 
+func (http2writeSettingsAck) staysWithinBuffer(max int) bool { return http2frameHeaderLen <= max }
+
+// splitHeaderBlock splits headerBlock into fragments so that each fragment fits
+// in a single frame, then calls fn for each fragment. firstFrag/lastFrag are true
+// for the first/last fragment, respectively.
+func http2splitHeaderBlock(ctx http2writeContext, headerBlock []byte, fn func(ctx http2writeContext, frag []byte, firstFrag, lastFrag bool) error) error {
+	// For now we're lazy and just pick the minimum MAX_FRAME_SIZE
+	// that all peers must support (16KB). Later we could care
+	// more and send larger frames if the peer advertised it, but
+	// there's little point. Most headers are small anyway (so we
+	// generally won't have CONTINUATION frames), and extra frames
+	// only waste 9 bytes anyway.
+	const maxFrameSize = 16384
+
+	first := true
+	for len(headerBlock) > 0 {
+		frag := headerBlock
+		if len(frag) > maxFrameSize {
+			frag = frag[:maxFrameSize]
+		}
+		headerBlock = headerBlock[len(frag):]
+		if err := fn(ctx, frag, first, len(headerBlock) == 0); err != nil {
+			return err
+		}
+		first = false
+	}
+	return nil
+}
+
 // writeResHeaders is a request to write a HEADERS and 0+ CONTINUATION frames
 // for HTTP response headers or trailers from a server handler.
 type http2writeResHeaders struct {
@@ -6976,6 +7347,11 @@ func http2encKV(enc *hpack.Encoder, k, v string) {
 	enc.WriteField(hpack.HeaderField{Name: k, Value: v})
 }
 
+func (w *http2writeResHeaders) staysWithinBuffer(max int) bool {
+
+	return false
+}
+
 func (w *http2writeResHeaders) writeFrame(ctx http2writeContext) error {
 	enc, buf := ctx.HeaderEncoder()
 	buf.Reset()
@@ -7001,39 +7377,69 @@ func (w *http2writeResHeaders) writeFrame(ctx http2writeContext) error {
 		panic("unexpected empty hpack")
 	}
 
-	// For now we're lazy and just pick the minimum MAX_FRAME_SIZE
-	// that all peers must support (16KB). Later we could care
-	// more and send larger frames if the peer advertised it, but
-	// there's little point. Most headers are small anyway (so we
-	// generally won't have CONTINUATION frames), and extra frames
-	// only waste 9 bytes anyway.
-	const maxFrameSize = 16384
+	return http2splitHeaderBlock(ctx, headerBlock, w.writeHeaderBlock)
+}
 
-	first := true
-	for len(headerBlock) > 0 {
-		frag := headerBlock
-		if len(frag) > maxFrameSize {
-			frag = frag[:maxFrameSize]
-		}
-		headerBlock = headerBlock[len(frag):]
-		endHeaders := len(headerBlock) == 0
-		var err error
-		if first {
-			first = false
-			err = ctx.Framer().WriteHeaders(http2HeadersFrameParam{
+func (w *http2writeResHeaders) writeHeaderBlock(ctx http2writeContext, frag []byte, firstFrag, lastFrag bool) error {
+	if firstFrag {
+		return ctx.Framer().WriteHeaders(http2HeadersFrameParam{
 			StreamID:      w.streamID,
 			BlockFragment: frag,
 			EndStream:     w.endStream,
-				EndHeaders:    endHeaders,
+			EndHeaders:    lastFrag,
 		})
 	} else {
-			err = ctx.Framer().WriteContinuation(w.streamID, endHeaders, frag)
+		return ctx.Framer().WriteContinuation(w.streamID, lastFrag, frag)
 	}
-		if err != nil {
-			return err
+}
+
+// writePushPromise is a request to write a PUSH_PROMISE and 0+ CONTINUATION frames.
+type http2writePushPromise struct {
+	streamID uint32   // pusher stream
+	method   string   // for :method
+	url      *url.URL // for :scheme, :authority, :path
+	h        Header
+
+	// Creates an ID for a pushed stream. This runs on serveG just before
+	// the frame is written. The returned ID is copied to promisedID.
+	allocatePromisedID func() (uint32, error)
+	promisedID         uint32
+}
+
+func (w *http2writePushPromise) staysWithinBuffer(max int) bool {
+
+	return false
+}
+
+func (w *http2writePushPromise) writeFrame(ctx http2writeContext) error {
+	enc, buf := ctx.HeaderEncoder()
+	buf.Reset()
+
+	http2encKV(enc, ":method", w.method)
+	http2encKV(enc, ":scheme", w.url.Scheme)
+	http2encKV(enc, ":authority", w.url.Host)
+	http2encKV(enc, ":path", w.url.RequestURI())
+	http2encodeHeaders(enc, w.h, nil)
+
+	headerBlock := buf.Bytes()
+	if len(headerBlock) == 0 {
+		panic("unexpected empty hpack")
 	}
+
+	return http2splitHeaderBlock(ctx, headerBlock, w.writeHeaderBlock)
+}
+
+func (w *http2writePushPromise) writeHeaderBlock(ctx http2writeContext, frag []byte, firstFrag, lastFrag bool) error {
+	if firstFrag {
+		return ctx.Framer().WritePushPromise(http2PushPromiseParam{
+			StreamID:      w.streamID,
+			PromiseID:     w.promisedID,
+			BlockFragment: frag,
+			EndHeaders:    lastFrag,
+		})
+	} else {
+		return ctx.Framer().WriteContinuation(w.streamID, lastFrag, frag)
 	}
-	return nil
 }
 
 type http2write100ContinueHeadersFrame struct {
@@ -7052,15 +7458,24 @@ func (w http2write100ContinueHeadersFrame) writeFrame(ctx http2writeContext) err
 	})
 }
 
+func (w http2write100ContinueHeadersFrame) staysWithinBuffer(max int) bool {
+
+	return 9+2*(len(":status")+len("100")) <= max
+}
+
 type http2writeWindowUpdate struct {
 	streamID uint32 // or 0 for conn-level
 	n        uint32
 }
 
+func (wu http2writeWindowUpdate) staysWithinBuffer(max int) bool { return http2frameHeaderLen+4 <= max }
+
 func (wu http2writeWindowUpdate) writeFrame(ctx http2writeContext) error {
 	return ctx.Framer().WriteWindowUpdate(wu.streamID, wu.n)
 }
 
+// encodeHeaders encodes an http.Header. If keys is not nil, then (k, h[k])
+// is encoded only only if k is in keys.
 func http2encodeHeaders(enc *hpack.Encoder, h Header, keys []string) {
 	if keys == nil {
 		sorter := http2sorterPool.Get().(*http2sorter)
@@ -7090,14 +7505,51 @@ func http2encodeHeaders(enc *hpack.Encoder, h Header, keys []string) {
 	}
 }
 
-// frameWriteMsg is a request to write a frame.
-type http2frameWriteMsg struct {
+// WriteScheduler is the interface implemented by HTTP/2 write schedulers.
+// Methods are never called concurrently.
+type http2WriteScheduler interface {
+	// OpenStream opens a new stream in the write scheduler.
+	// It is illegal to call this with streamID=0 or with a streamID that is
+	// already open -- the call may panic.
+	OpenStream(streamID uint32, options http2OpenStreamOptions)
+
+	// CloseStream closes a stream in the write scheduler. Any frames queued on
+	// this stream should be discarded. It is illegal to call this on a stream
+	// that is not open -- the call may panic.
+	CloseStream(streamID uint32)
+
+	// AdjustStream adjusts the priority of the given stream. This may be called
+	// on a stream that has not yet been opened or has been closed. Note that
+	// RFC 7540 allows PRIORITY frames to be sent on streams in any state. See:
+	// https://tools.ietf.org/html/rfc7540#section-5.1
+	AdjustStream(streamID uint32, priority http2PriorityParam)
+
+	// Push queues a frame in the scheduler.
+	Push(wr http2FrameWriteRequest)
+
+	// Pop dequeues the next frame to write. Returns false if no frames can
+	// be written. Frames with a given wr.StreamID() are Pop'd in the same
+	// order they are Push'd.
+	Pop() (wr http2FrameWriteRequest, ok bool)
+}
+
+// OpenStreamOptions specifies extra options for WriteScheduler.OpenStream.
+type http2OpenStreamOptions struct {
+	// PusherID is zero if the stream was initiated by the client. Otherwise,
+	// PusherID names the stream that pushed the newly opened stream.
+	PusherID uint32
+}
+
+// FrameWriteRequest is a request to write a frame.
+type http2FrameWriteRequest struct {
 	// write is the interface value that does the writing, once the
-	// writeScheduler (below) has decided to select this frame
-	// to write. The write functions are all defined in write.go.
+	// WriteScheduler has selected this frame to write. The write
+	// functions are all defined in write.go.
 	write http2writeFramer
 
-	stream *http2stream // used for prioritization. nil for non-stream frames.
+	// stream is the stream on which this frame will be written.
+	// nil for non-stream frames like PING and SETTINGS.
+	stream *http2stream
 
 	// done, if non-nil, must be a buffered channel with space for
 	// 1 message and is sent the return value from write (or an
@@ -7105,247 +7557,626 @@ type http2frameWriteMsg struct {
 	done chan error
 }
 
-// for debugging only:
-func (wm http2frameWriteMsg) String() string {
+// StreamID returns the id of the stream this frame will be written to.
+// 0 is used for non-stream frames such as PING and SETTINGS.
+func (wr http2FrameWriteRequest) StreamID() uint32 {
+	if wr.stream == nil {
+		return 0
+	}
+	return wr.stream.id
+}
+
+// DataSize returns the number of flow control bytes that must be consumed
+// to write this entire frame. This is 0 for non-DATA frames.
+func (wr http2FrameWriteRequest) DataSize() int {
+	if wd, ok := wr.write.(*http2writeData); ok {
+		return len(wd.p)
+	}
+	return 0
+}
+
+// Consume consumes min(n, available) bytes from this frame, where available
+// is the number of flow control bytes available on the stream. Consume returns
+// 0, 1, or 2 frames, where the integer return value gives the number of frames
+// returned.
+//
+// If flow control prevents consuming any bytes, this returns (_, _, 0). If
+// the entire frame was consumed, this returns (wr, _, 1). Otherwise, this
+// returns (consumed, rest, 2), where 'consumed' contains the consumed bytes and
+// 'rest' contains the remaining bytes. The consumed bytes are deducted from the
+// underlying stream's flow control budget.
+func (wr http2FrameWriteRequest) Consume(n int32) (http2FrameWriteRequest, http2FrameWriteRequest, int) {
+	var empty http2FrameWriteRequest
+
+	wd, ok := wr.write.(*http2writeData)
+	if !ok || len(wd.p) == 0 {
+		return wr, empty, 1
+	}
+
+	allowed := wr.stream.flow.available()
+	if n < allowed {
+		allowed = n
+	}
+	if wr.stream.sc.maxFrameSize < allowed {
+		allowed = wr.stream.sc.maxFrameSize
+	}
+	if allowed <= 0 {
+		return empty, empty, 0
+	}
+	if len(wd.p) > int(allowed) {
+		wr.stream.flow.take(allowed)
+		consumed := http2FrameWriteRequest{
+			stream: wr.stream,
+			write: &http2writeData{
+				streamID: wd.streamID,
+				p:        wd.p[:allowed],
+
+				endStream: false,
+			},
+
+			done: nil,
+		}
+		rest := http2FrameWriteRequest{
+			stream: wr.stream,
+			write: &http2writeData{
+				streamID:  wd.streamID,
+				p:         wd.p[allowed:],
+				endStream: wd.endStream,
+			},
+			done: wr.done,
+		}
+		return consumed, rest, 2
+	}
+
+	wr.stream.flow.take(int32(len(wd.p)))
+	return wr, empty, 1
+}
+
+// String is for debugging only.
+func (wr http2FrameWriteRequest) String() string {
 	var streamID uint32
-	if wm.stream != nil {
-		streamID = wm.stream.id
+	if wr.stream != nil {
+		streamID = wr.stream.id
 	}
 	var des string
-	if s, ok := wm.write.(fmt.Stringer); ok {
+	if s, ok := wr.write.(fmt.Stringer); ok {
 		des = s.String()
 	} else {
-		des = fmt.Sprintf("%T", wm.write)
+		des = fmt.Sprintf("%T", wr.write)
 	}
-	return fmt.Sprintf("[frameWriteMsg stream=%d, ch=%v, type: %v]", streamID, wm.done != nil, des)
+	return fmt.Sprintf("[FrameWriteRequest stream=%d, ch=%v, writer=%v]", streamID, wr.done != nil, des)
 }
 
-// writeScheduler tracks pending frames to write, priorities, and decides
-// the next one to use. It is not thread-safe.
-type http2writeScheduler struct {
-	// zero are frames not associated with a specific stream.
-	// They're sent before any stream-specific freams.
-	zero http2writeQueue
+// writeQueue is used by implementations of WriteScheduler.
+type http2writeQueue struct {
+	s []http2FrameWriteRequest
+}
 
-	// maxFrameSize is the maximum size of a DATA frame
-	// we'll write. Must be non-zero and between 16K-16M.
-	maxFrameSize uint32
+func (q *http2writeQueue) empty() bool { return len(q.s) == 0 }
 
-	// sq contains the stream-specific queues, keyed by stream ID.
-	// when a stream is idle, it's deleted from the map.
-	sq map[uint32]*http2writeQueue
+func (q *http2writeQueue) push(wr http2FrameWriteRequest) {
+	q.s = append(q.s, wr)
+}
+
+func (q *http2writeQueue) shift() http2FrameWriteRequest {
+	if len(q.s) == 0 {
+		panic("invalid use of queue")
+	}
+	wr := q.s[0]
 
-	// canSend is a slice of memory that's reused between frame
-	// scheduling decisions to hold the list of writeQueues (from sq)
-	// which have enough flow control data to send. After canSend is
-	// built, the best is selected.
-	canSend []*http2writeQueue
+	copy(q.s, q.s[1:])
+	q.s[len(q.s)-1] = http2FrameWriteRequest{}
+	q.s = q.s[:len(q.s)-1]
+	return wr
+}
 
-	// pool of empty queues for reuse.
-	queuePool []*http2writeQueue
+// consume consumes up to n bytes from q.s[0]. If the frame is
+// entirely consumed, it is removed from the queue. If the frame
+// is partially consumed, the frame is kept with the consumed
+// bytes removed. Returns true iff any bytes were consumed.
+func (q *http2writeQueue) consume(n int32) (http2FrameWriteRequest, bool) {
+	if len(q.s) == 0 {
+		return http2FrameWriteRequest{}, false
+	}
+	consumed, rest, numresult := q.s[0].Consume(n)
+	switch numresult {
+	case 0:
+		return http2FrameWriteRequest{}, false
+	case 1:
+		q.shift()
+	case 2:
+		q.s[0] = rest
+	}
+	return consumed, true
 }
 
-func (ws *http2writeScheduler) putEmptyQueue(q *http2writeQueue) {
-	if len(q.s) != 0 {
-		panic("queue must be empty")
+type http2writeQueuePool []*http2writeQueue
+
+// put inserts an unused writeQueue into the pool.
+func (p *http2writeQueuePool) put(q *http2writeQueue) {
+	for i := range q.s {
+		q.s[i] = http2FrameWriteRequest{}
 	}
-	ws.queuePool = append(ws.queuePool, q)
+	q.s = q.s[:0]
+	*p = append(*p, q)
 }
 
-func (ws *http2writeScheduler) getEmptyQueue() *http2writeQueue {
-	ln := len(ws.queuePool)
+// get returns an empty writeQueue.
+func (p *http2writeQueuePool) get() *http2writeQueue {
+	ln := len(*p)
 	if ln == 0 {
 		return new(http2writeQueue)
 	}
-	q := ws.queuePool[ln-1]
-	ws.queuePool = ws.queuePool[:ln-1]
+	x := ln - 1
+	q := (*p)[x]
+	(*p)[x] = nil
+	*p = (*p)[:x]
 	return q
 }
 
-func (ws *http2writeScheduler) empty() bool { return ws.zero.empty() && len(ws.sq) == 0 }
+// RFC 7540, Section 5.3.5: the default weight is 16.
+const http2priorityDefaultWeight = 15 // 16 = 15 + 1
 
-func (ws *http2writeScheduler) add(wm http2frameWriteMsg) {
-	st := wm.stream
-	if st == nil {
-		ws.zero.push(wm)
+// PriorityWriteSchedulerConfig configures a priorityWriteScheduler.
+type http2PriorityWriteSchedulerConfig struct {
+	// MaxClosedNodesInTree controls the maximum number of closed streams to
+	// retain in the priority tree. Setting this to zero saves a small amount
+	// of memory at the cost of performance.
+	//
+	// See RFC 7540, Section 5.3.4:
+	//   "It is possible for a stream to become closed while prioritization
+	//   information ... is in transit. ... This potentially creates suboptimal
+	//   prioritization, since the stream could be given a priority that is
+	//   different from what is intended. To avoid these problems, an endpoint
+	//   SHOULD retain stream prioritization state for a period after streams
+	//   become closed. The longer state is retained, the lower the chance that
+	//   streams are assigned incorrect or default priority values."
+	MaxClosedNodesInTree int
+
+	// MaxIdleNodesInTree controls the maximum number of idle streams to
+	// retain in the priority tree. Setting this to zero saves a small amount
+	// of memory at the cost of performance.
+	//
+	// See RFC 7540, Section 5.3.4:
+	//   Similarly, streams that are in the "idle" state can be assigned
+	//   priority or become a parent of other streams. This allows for the
+	//   creation of a grouping node in the dependency tree, which enables
+	//   more flexible expressions of priority. Idle streams begin with a
+	//   default priority (Section 5.3.5).
+	MaxIdleNodesInTree int
+
+	// ThrottleOutOfOrderWrites enables write throttling to help ensure that
+	// data is delivered in priority order. This works around a race where
+	// stream B depends on stream A and both streams are about to call Write
+	// to queue DATA frames. If B wins the race, a naive scheduler would eagerly
+	// write as much data from B as possible, but this is suboptimal because A
+	// is a higher-priority stream. With throttling enabled, we write a small
+	// amount of data from B to minimize the amount of bandwidth that B can
+	// steal from A.
+	ThrottleOutOfOrderWrites bool
+}
+
+// NewPriorityWriteScheduler constructs a WriteScheduler that schedules
+// frames by following HTTP/2 priorities as described in RFC 7340 Section 5.3.
+// If cfg is nil, default options are used.
+func http2NewPriorityWriteScheduler(cfg *http2PriorityWriteSchedulerConfig) http2WriteScheduler {
+	if cfg == nil {
+
+		cfg = &http2PriorityWriteSchedulerConfig{
+			MaxClosedNodesInTree:     10,
+			MaxIdleNodesInTree:       10,
+			ThrottleOutOfOrderWrites: false,
+		}
+	}
+
+	ws := &http2priorityWriteScheduler{
+		nodes:                make(map[uint32]*http2priorityNode),
+		maxClosedNodesInTree: cfg.MaxClosedNodesInTree,
+		maxIdleNodesInTree:   cfg.MaxIdleNodesInTree,
+		enableWriteThrottle:  cfg.ThrottleOutOfOrderWrites,
+	}
+	ws.nodes[0] = &ws.root
+	if cfg.ThrottleOutOfOrderWrites {
+		ws.writeThrottleLimit = 1024
 	} else {
-		ws.streamQueue(st.id).push(wm)
+		ws.writeThrottleLimit = math.MaxInt32
 	}
+	return ws
 }
 
-func (ws *http2writeScheduler) streamQueue(streamID uint32) *http2writeQueue {
-	if q, ok := ws.sq[streamID]; ok {
-		return q
+type http2priorityNodeState int
+
+const (
+	http2priorityNodeOpen http2priorityNodeState = iota
+	http2priorityNodeClosed
+	http2priorityNodeIdle
+)
+
+// priorityNode is a node in an HTTP/2 priority tree.
+// Each node is associated with a single stream ID.
+// See RFC 7540, Section 5.3.
+type http2priorityNode struct {
+	q            http2writeQueue        // queue of pending frames to write
+	id           uint32                 // id of the stream, or 0 for the root of the tree
+	weight       uint8                  // the actual weight is weight+1, so the value is in [1,256]
+	state        http2priorityNodeState // open | closed | idle
+	bytes        int64                  // number of bytes written by this node, or 0 if closed
+	subtreeBytes int64                  // sum(node.bytes) of all nodes in this subtree
+
+	// These links form the priority tree.
+	parent     *http2priorityNode
+	kids       *http2priorityNode // start of the kids list
+	prev, next *http2priorityNode // doubly-linked list of siblings
+}
+
+func (n *http2priorityNode) setParent(parent *http2priorityNode) {
+	if n == parent {
+		panic("setParent to self")
+	}
+	if n.parent == parent {
+		return
+	}
+
+	if parent := n.parent; parent != nil {
+		if n.prev == nil {
+			parent.kids = n.next
+		} else {
+			n.prev.next = n.next
 		}
-	if ws.sq == nil {
-		ws.sq = make(map[uint32]*http2writeQueue)
+		if n.next != nil {
+			n.next.prev = n.prev
+		}
+	}
+
+	n.parent = parent
+	if parent == nil {
+		n.next = nil
+		n.prev = nil
+	} else {
+		n.next = parent.kids
+		n.prev = nil
+		if n.next != nil {
+			n.next.prev = n
+		}
+		parent.kids = n
 	}
-	q := ws.getEmptyQueue()
-	ws.sq[streamID] = q
-	return q
 }
 
-// take returns the most important frame to write and removes it from the scheduler.
-// It is illegal to call this if the scheduler is empty or if there are no connection-level
-// flow control bytes available.
-func (ws *http2writeScheduler) take() (wm http2frameWriteMsg, ok bool) {
-	if ws.maxFrameSize == 0 {
-		panic("internal error: ws.maxFrameSize not initialized or invalid")
+func (n *http2priorityNode) addBytes(b int64) {
+	n.bytes += b
+	for ; n != nil; n = n.parent {
+		n.subtreeBytes += b
 	}
+}
 
-	if !ws.zero.empty() {
-		return ws.zero.shift(), true
+// walkReadyInOrder iterates over the tree in priority order, calling f for each node
+// with a non-empty write queue. When f returns true, this funcion returns true and the
+// walk halts. tmp is used as scratch space for sorting.
+//
+// f(n, openParent) takes two arguments: the node to visit, n, and a bool that is true
+// if any ancestor p of n is still open (ignoring the root node).
+func (n *http2priorityNode) walkReadyInOrder(openParent bool, tmp *[]*http2priorityNode, f func(*http2priorityNode, bool) bool) bool {
+	if !n.q.empty() && f(n, openParent) {
+		return true
 	}
-	if len(ws.sq) == 0 {
-		return
+	if n.kids == nil {
+		return false
+	}
+
+	if n.id != 0 {
+		openParent = openParent || (n.state == http2priorityNodeOpen)
 	}
 
-	for id, q := range ws.sq {
-		if q.firstIsNoCost() {
-			return ws.takeFrom(id, q)
+	w := n.kids.weight
+	needSort := false
+	for k := n.kids.next; k != nil; k = k.next {
+		if k.weight != w {
+			needSort = true
+			break
+		}
+	}
+	if !needSort {
+		for k := n.kids; k != nil; k = k.next {
+			if k.walkReadyInOrder(openParent, tmp, f) {
+				return true
 			}
 		}
+		return false
+	}
 
-	if len(ws.canSend) != 0 {
-		panic("should be empty")
+	*tmp = (*tmp)[:0]
+	for n.kids != nil {
+		*tmp = append(*tmp, n.kids)
+		n.kids.setParent(nil)
 	}
-	for _, q := range ws.sq {
-		if n := ws.streamWritableBytes(q); n > 0 {
-			ws.canSend = append(ws.canSend, q)
+	sort.Sort(http2sortPriorityNodeSiblings(*tmp))
+	for i := len(*tmp) - 1; i >= 0; i-- {
+		(*tmp)[i].setParent(n)
 	}
+	for k := n.kids; k != nil; k = k.next {
+		if k.walkReadyInOrder(openParent, tmp, f) {
+			return true
 		}
-	if len(ws.canSend) == 0 {
-		return
 	}
-	defer ws.zeroCanSend()
+	return false
+}
 
-	q := ws.canSend[0]
+type http2sortPriorityNodeSiblings []*http2priorityNode
 
-	return ws.takeFrom(q.streamID(), q)
-}
+func (z http2sortPriorityNodeSiblings) Len() int { return len(z) }
+
+func (z http2sortPriorityNodeSiblings) Swap(i, k int) { z[i], z[k] = z[k], z[i] }
+
+func (z http2sortPriorityNodeSiblings) Less(i, k int) bool {
 
-// zeroCanSend is defered from take.
-func (ws *http2writeScheduler) zeroCanSend() {
-	for i := range ws.canSend {
-		ws.canSend[i] = nil
+	wi, bi := float64(z[i].weight+1), float64(z[i].subtreeBytes)
+	wk, bk := float64(z[k].weight+1), float64(z[k].subtreeBytes)
+	if bi == 0 && bk == 0 {
+		return wi >= wk
+	}
+	if bk == 0 {
+		return false
 	}
-	ws.canSend = ws.canSend[:0]
+	return bi/bk <= wi/wk
 }
 
-// streamWritableBytes returns the number of DATA bytes we could write
-// from the given queue's stream, if this stream/queue were
-// selected. It is an error to call this if q's head isn't a
-// *writeData.
-func (ws *http2writeScheduler) streamWritableBytes(q *http2writeQueue) int32 {
-	wm := q.head()
-	ret := wm.stream.flow.available()
-	if ret == 0 {
-		return 0
+type http2priorityWriteScheduler struct {
+	// root is the root of the priority tree, where root.id = 0.
+	// The root queues control frames that are not associated with any stream.
+	root http2priorityNode
+
+	// nodes maps stream ids to priority tree nodes.
+	nodes map[uint32]*http2priorityNode
+
+	// maxID is the maximum stream id in nodes.
+	maxID uint32
+
+	// lists of nodes that have been closed or are idle, but are kept in
+	// the tree for improved prioritization. When the lengths exceed either
+	// maxClosedNodesInTree or maxIdleNodesInTree, old nodes are discarded.
+	closedNodes, idleNodes []*http2priorityNode
+
+	// From the config.
+	maxClosedNodesInTree int
+	maxIdleNodesInTree   int
+	writeThrottleLimit   int32
+	enableWriteThrottle  bool
+
+	// tmp is scratch space for priorityNode.walkReadyInOrder to reduce allocations.
+	tmp []*http2priorityNode
+
+	// pool of empty queues for reuse.
+	queuePool http2writeQueuePool
+}
+
+func (ws *http2priorityWriteScheduler) OpenStream(streamID uint32, options http2OpenStreamOptions) {
+
+	if curr := ws.nodes[streamID]; curr != nil {
+		if curr.state != http2priorityNodeIdle {
+			panic(fmt.Sprintf("stream %d already opened", streamID))
+		}
+		curr.state = http2priorityNodeOpen
+		return
 	}
-	if int32(ws.maxFrameSize) < ret {
-		ret = int32(ws.maxFrameSize)
+
+	parent := ws.nodes[options.PusherID]
+	if parent == nil {
+		parent = &ws.root
 	}
-	if ret == 0 {
-		panic("internal error: ws.maxFrameSize not initialized or invalid")
+	n := &http2priorityNode{
+		q:      *ws.queuePool.get(),
+		id:     streamID,
+		weight: http2priorityDefaultWeight,
+		state:  http2priorityNodeOpen,
 	}
-	wd := wm.write.(*http2writeData)
-	if len(wd.p) < int(ret) {
-		ret = int32(len(wd.p))
+	n.setParent(parent)
+	ws.nodes[streamID] = n
+	if streamID > ws.maxID {
+		ws.maxID = streamID
 	}
-	return ret
 }
 
-func (ws *http2writeScheduler) takeFrom(id uint32, q *http2writeQueue) (wm http2frameWriteMsg, ok bool) {
-	wm = q.head()
+func (ws *http2priorityWriteScheduler) CloseStream(streamID uint32) {
+	if streamID == 0 {
+		panic("violation of WriteScheduler interface: cannot close stream 0")
+	}
+	if ws.nodes[streamID] == nil {
+		panic(fmt.Sprintf("violation of WriteScheduler interface: unknown stream %d", streamID))
+	}
+	if ws.nodes[streamID].state != http2priorityNodeOpen {
+		panic(fmt.Sprintf("violation of WriteScheduler interface: stream %d already closed", streamID))
+	}
 
-	if wd, ok := wm.write.(*http2writeData); ok && len(wd.p) > 0 {
-		allowed := wm.stream.flow.available()
-		if allowed == 0 {
+	n := ws.nodes[streamID]
+	n.state = http2priorityNodeClosed
+	n.addBytes(-n.bytes)
 
-			return http2frameWriteMsg{}, false
+	q := n.q
+	ws.queuePool.put(&q)
+	n.q.s = nil
+	if ws.maxClosedNodesInTree > 0 {
+		ws.addClosedOrIdleNode(&ws.closedNodes, ws.maxClosedNodesInTree, n)
+	} else {
+		ws.removeNode(n)
 	}
-		if int32(ws.maxFrameSize) < allowed {
-			allowed = int32(ws.maxFrameSize)
+}
+
+func (ws *http2priorityWriteScheduler) AdjustStream(streamID uint32, priority http2PriorityParam) {
+	if streamID == 0 {
+		panic("adjustPriority on root")
 	}
 
-		if len(wd.p) > int(allowed) {
-			wm.stream.flow.take(allowed)
-			chunk := wd.p[:allowed]
-			wd.p = wd.p[allowed:]
+	n := ws.nodes[streamID]
+	if n == nil {
+		if streamID <= ws.maxID || ws.maxIdleNodesInTree == 0 {
+			return
+		}
+		ws.maxID = streamID
+		n = &http2priorityNode{
+			q:      *ws.queuePool.get(),
+			id:     streamID,
+			weight: http2priorityDefaultWeight,
+			state:  http2priorityNodeIdle,
+		}
+		n.setParent(&ws.root)
+		ws.nodes[streamID] = n
+		ws.addClosedOrIdleNode(&ws.idleNodes, ws.maxIdleNodesInTree, n)
+	}
 
-			return http2frameWriteMsg{
-				stream: wm.stream,
-				write: &http2writeData{
-					streamID: wd.streamID,
-					p:        chunk,
+	parent := ws.nodes[priority.StreamDep]
+	if parent == nil {
+		n.setParent(&ws.root)
+		n.weight = http2priorityDefaultWeight
+		return
+	}
 
-					endStream: false,
-				},
+	if n == parent {
+		return
+	}
 
-				done: nil,
-			}, true
+	for x := parent.parent; x != nil; x = x.parent {
+		if x == n {
+			parent.setParent(n.parent)
+			break
 		}
-		wm.stream.flow.take(int32(len(wd.p)))
 	}
 
-	q.shift()
-	if q.empty() {
-		ws.putEmptyQueue(q)
-		delete(ws.sq, id)
+	if priority.Exclusive {
+		k := parent.kids
+		for k != nil {
+			next := k.next
+			if k != n {
+				k.setParent(n)
 			}
-	return wm, true
+			k = next
+		}
+	}
+
+	n.setParent(parent)
+	n.weight = priority.Weight
 }
 
-func (ws *http2writeScheduler) forgetStream(id uint32) {
-	q, ok := ws.sq[id]
+func (ws *http2priorityWriteScheduler) Push(wr http2FrameWriteRequest) {
+	var n *http2priorityNode
+	if id := wr.StreamID(); id == 0 {
+		n = &ws.root
+	} else {
+		n = ws.nodes[id]
+		if n == nil {
+			panic("add on non-open stream")
+		}
+	}
+	n.q.push(wr)
+}
+
+func (ws *http2priorityWriteScheduler) Pop() (wr http2FrameWriteRequest, ok bool) {
+	ws.root.walkReadyInOrder(false, &ws.tmp, func(n *http2priorityNode, openParent bool) bool {
+		limit := int32(math.MaxInt32)
+		if openParent {
+			limit = ws.writeThrottleLimit
+		}
+		wr, ok = n.q.consume(limit)
 		if !ok {
+			return false
+		}
+		n.addBytes(int64(wr.DataSize()))
+
+		if openParent {
+			ws.writeThrottleLimit += 1024
+			if ws.writeThrottleLimit < 0 {
+				ws.writeThrottleLimit = math.MaxInt32
+			}
+		} else if ws.enableWriteThrottle {
+			ws.writeThrottleLimit = 1024
+		}
+		return true
+	})
+	return wr, ok
+}
+
+func (ws *http2priorityWriteScheduler) addClosedOrIdleNode(list *[]*http2priorityNode, maxSize int, n *http2priorityNode) {
+	if maxSize == 0 {
 		return
 	}
-	delete(ws.sq, id)
+	if len(*list) == maxSize {
 
-	for i := range q.s {
-		q.s[i] = http2frameWriteMsg{}
+		ws.removeNode((*list)[0])
+		x := (*list)[1:]
+		copy(*list, x)
+		*list = (*list)[:len(x)]
 	}
-	q.s = q.s[:0]
-	ws.putEmptyQueue(q)
+	*list = append(*list, n)
 }
 
-type http2writeQueue struct {
-	s []http2frameWriteMsg
+func (ws *http2priorityWriteScheduler) removeNode(n *http2priorityNode) {
+	for k := n.kids; k != nil; k = k.next {
+		k.setParent(n.parent)
+	}
+	n.setParent(nil)
+	delete(ws.nodes, n.id)
 }
 
-// streamID returns the stream ID for a non-empty stream-specific queue.
-func (q *http2writeQueue) streamID() uint32 { return q.s[0].stream.id }
+// NewRandomWriteScheduler constructs a WriteScheduler that ignores HTTP/2
+// priorities. Control frames like SETTINGS and PING are written before DATA
+// frames, but if no control frames are queued and multiple streams have queued
+// HEADERS or DATA frames, Pop selects a ready stream arbitrarily.
+func http2NewRandomWriteScheduler() http2WriteScheduler {
+	return &http2randomWriteScheduler{sq: make(map[uint32]*http2writeQueue)}
+}
 
-func (q *http2writeQueue) empty() bool { return len(q.s) == 0 }
+type http2randomWriteScheduler struct {
+	// zero are frames not associated with a specific stream.
+	zero http2writeQueue
+
+	// sq contains the stream-specific queues, keyed by stream ID.
+	// When a stream is idle or closed, it's deleted from the map.
+	sq map[uint32]*http2writeQueue
 
-func (q *http2writeQueue) push(wm http2frameWriteMsg) {
-	q.s = append(q.s, wm)
+	// pool of empty queues for reuse.
+	queuePool http2writeQueuePool
 }
 
-// head returns the next item that would be removed by shift.
-func (q *http2writeQueue) head() http2frameWriteMsg {
-	if len(q.s) == 0 {
-		panic("invalid use of queue")
-	}
-	return q.s[0]
+func (ws *http2randomWriteScheduler) OpenStream(streamID uint32, options http2OpenStreamOptions) {
+
 }
 
-func (q *http2writeQueue) shift() http2frameWriteMsg {
-	if len(q.s) == 0 {
-		panic("invalid use of queue")
+func (ws *http2randomWriteScheduler) CloseStream(streamID uint32) {
+	q, ok := ws.sq[streamID]
+	if !ok {
+		return
 	}
-	wm := q.s[0]
+	delete(ws.sq, streamID)
+	ws.queuePool.put(q)
+}
+
+func (ws *http2randomWriteScheduler) AdjustStream(streamID uint32, priority http2PriorityParam) {
 
-	copy(q.s, q.s[1:])
-	q.s[len(q.s)-1] = http2frameWriteMsg{}
-	q.s = q.s[:len(q.s)-1]
-	return wm
 }
 
-func (q *http2writeQueue) firstIsNoCost() bool {
-	if df, ok := q.s[0].write.(*http2writeData); ok {
-		return len(df.p) == 0
+func (ws *http2randomWriteScheduler) Push(wr http2FrameWriteRequest) {
+	id := wr.StreamID()
+	if id == 0 {
+		ws.zero.push(wr)
+		return
 	}
-	return true
+	q, ok := ws.sq[id]
+	if !ok {
+		q = ws.queuePool.get()
+		ws.sq[id] = q
+	}
+	q.push(wr)
+}
+
+func (ws *http2randomWriteScheduler) Pop() (http2FrameWriteRequest, bool) {
+
+	if !ws.zero.empty() {
+		return ws.zero.shift(), true
+	}
+
+	for _, q := range ws.sq {
+		if wr, ok := q.consume(math.MaxInt32); ok {
+			return wr, true
+		}
+	}
+	return http2FrameWriteRequest{}, false
 }