http2_client.go

/*
 *
 * Copyright 2014, Google Inc.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *     * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *     * Redistributions in binary form must reproduce the above
 * copyright notice, this list of conditions and the following disclaimer
 * in the documentation and/or other materials provided with the
 * distribution.
 *     * Neither the name of Google Inc. nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 */

package transport

import (
	"bytes"
	"errors"
	"io"
	"math"
	"net"
	"strings"
	"sync"
	"time"

	"golang.org/x/net/context"
	"golang.org/x/net/http2"
	"golang.org/x/net/http2/hpack"
	"google.golang.org/grpc/codes"
	"google.golang.org/grpc/credentials"
	"google.golang.org/grpc/grpclog"
	"google.golang.org/grpc/metadata"
	"google.golang.org/grpc/peer"
)

// http2Client implements the ClientTransport interface with HTTP2.
type http2Client struct {
	target    string // server name/addr
	userAgent string
	conn      net.Conn             // underlying communication channel
	authInfo  credentials.AuthInfo // auth info about the connection
	nextID    uint32               // the next stream ID to be used

	// writableChan synchronizes write access to the transport.
	// A writer acquires the write lock by sending a value on writableChan
	// and releases it by receiving from writableChan.
	writableChan chan int
	// shutdownChan is closed when Close is called.
	// Blocking operations should select on shutdownChan to avoid
	// blocking forever after Close.
	// TODO(zhaoq): Maybe have a channel context?
	shutdownChan chan struct{}
	// errorChan is closed to notify the I/O error to the caller.
	errorChan chan struct{}

	framer *framer
	hBuf   *bytes.Buffer  // the buffer for HPACK encoding
	hEnc   *hpack.Encoder // HPACK encoder

	// controlBuf delivers all the control related tasks (e.g., window
	// updates, reset streams, and various settings) to the controller.
	controlBuf *recvBuffer
	fc         *inFlow
	// sendQuotaPool provides flow control to outbound message.
	sendQuotaPool *quotaPool
	// streamsQuota limits the max number of concurrent streams.
	streamsQuota *quotaPool

	// The scheme used: https if TLS is on, http otherwise.
	scheme string

	authCreds []credentials.Credentials

	mu            sync.Mutex     // guard the following variables
	state         transportState // the state of underlying connection
	activeStreams map[uint32]*Stream
	// The max number of concurrent streams
	maxStreams int
	// the per-stream outbound flow control window size set by the peer.
	streamSendQuota uint32
}

// newHTTP2Client constructs a connected ClientTransport to addr based on HTTP2
// and starts to receive messages on it. Non-nil error returns if construction
// fails.
func newHTTP2Client(addr string, opts *ConnectOptions) (_ ClientTransport, err error) {
	if opts.Dialer == nil {
		// Set the default Dialer.
		opts.Dialer = func(addr string, timeout time.Duration) (net.Conn, error) {
			return net.DialTimeout("tcp", addr, timeout)
		}
	}
	scheme := "http"
	startT := time.Now()
	timeout := opts.Timeout
	conn, connErr := opts.Dialer(addr, timeout)
	if connErr != nil {
		return nil, ConnectionErrorf("transport: %v", connErr)
	}
	var authInfo credentials.AuthInfo
	for _, c := range opts.AuthOptions {
		if ccreds, ok := c.(credentials.TransportAuthenticator); ok {
			scheme = "https"
			// TODO(zhaoq): Now the first TransportAuthenticator is used if there are
			// multiple ones provided. Revisit this if it is not appropriate. Probably
			// place the ClientTransport construction into a separate function to make
			// things clear.
			if timeout > 0 {
				timeout -= time.Since(startT)
			}
			conn, authInfo, connErr = ccreds.ClientHandshake(addr, conn, timeout)
			break
		}
	}
	if connErr != nil {
		return nil, ConnectionErrorf("transport: %v", connErr)
	}
	defer func() {
		if err != nil {
			conn.Close()
		}
	}()
	// Send connection preface to server.
	n, err := conn.Write(clientPreface)
	if err != nil {
		return nil, ConnectionErrorf("transport: %v", err)
	}
	if n != len(clientPreface) {
		return nil, ConnectionErrorf("transport: preface mismatch, wrote %d bytes; want %d", n, len(clientPreface))
	}
	framer := newFramer(conn)
	if initialWindowSize != defaultWindowSize {
		err = framer.writeSettings(true, http2.Setting{http2.SettingInitialWindowSize, uint32(initialWindowSize)})
	} else {
		err = framer.writeSettings(true)
	}
	if err != nil {
		return nil, ConnectionErrorf("transport: %v", err)
	}
	// Adjust the connection flow control window if needed.
	if delta := uint32(initialConnWindowSize - defaultWindowSize); delta > 0 {
		if err := framer.writeWindowUpdate(true, 0, delta); err != nil {
			return nil, ConnectionErrorf("transport: %v", err)
		}
	}
	ua := primaryUA
	if opts.UserAgent != "" {
		ua = opts.UserAgent + " " + ua
	}
	var buf bytes.Buffer
	t := &http2Client{
		target:    addr,
		userAgent: ua,
		conn:      conn,
		authInfo:  authInfo,
		// The client initiated stream id is odd starting from 1.
		nextID:          1,
		writableChan:    make(chan int, 1),
		shutdownChan:    make(chan struct{}),
		errorChan:       make(chan struct{}),
		framer:          framer,
		hBuf:            &buf,
		hEnc:            hpack.NewEncoder(&buf),
		controlBuf:      newRecvBuffer(),
		fc:              &inFlow{limit: initialConnWindowSize},
		sendQuotaPool:   newQuotaPool(defaultWindowSize),
		scheme:          scheme,
		state:           reachable,
		activeStreams:   make(map[uint32]*Stream),
		authCreds:       opts.AuthOptions,
		maxStreams:      math.MaxInt32,
		streamSendQuota: defaultWindowSize,
	}
	go t.controller()
	t.writableChan <- 0
	// Start the reader goroutine for incoming message. The threading model
	// on receiving is that each transport has a dedicated goroutine which
	// reads HTTP2 frame from network. Then it dispatches the frame to the
	// corresponding stream entity.
	go t.reader()
	return t, nil
}

func (t *http2Client) newStream(ctx context.Context, callHdr *CallHdr) *Stream {
	fc := &inFlow{
		limit: initialWindowSize,
		conn:  t.fc,
	}
	// TODO(zhaoq): Handle uint32 overflow of Stream.id.
	s := &Stream{
		id:            t.nextID,
		method:        callHdr.Method,
		sendCompress:  callHdr.SendCompress,
		buf:           newRecvBuffer(),
		fc:            fc,
		sendQuotaPool: newQuotaPool(int(t.streamSendQuota)),
		headerChan:    make(chan struct{}),
	}
	t.nextID += 2
	s.windowHandler = func(n int) {
		t.updateWindow(s, uint32(n))
	}
	// Make a stream be able to cancel the pending operations by itself.
	s.ctx, s.cancel = context.WithCancel(ctx)
	s.dec = &recvBufferReader{
		ctx:  s.ctx,
		recv: s.buf,
	}
	return s
}

// NewStream creates a stream and register it into the transport as "active"
// streams.
func (t *http2Client) NewStream(ctx context.Context, callHdr *CallHdr) (_ *Stream, err error) {
	// Record the timeout value on the context.
	var timeout time.Duration
	if dl, ok := ctx.Deadline(); ok {
		timeout = dl.Sub(time.Now())
		if timeout <= 0 {
			return nil, ContextErr(context.DeadlineExceeded)
		}
	}
	pr := &peer.Peer{
		Addr: t.conn.RemoteAddr(),
	}
	// Attach Auth info if there is any.
	if t.authInfo != nil {
		pr.AuthInfo = t.authInfo
	}
	ctx = peer.NewContext(ctx, pr)
	authData := make(map[string]string)
	for _, c := range t.authCreds {
		// Construct URI required to get auth request metadata.
		var port string
		if pos := strings.LastIndex(t.target, ":"); pos != -1 {
			// Omit port if it is the default one.
			if t.target[pos+1:] != "443" {
				port = ":" + t.target[pos+1:]
			}
		}
		pos := strings.LastIndex(callHdr.Method, "/")
		if pos == -1 {
			return nil, StreamErrorf(codes.InvalidArgument, "transport: malformed method name: %q", callHdr.Method)
		}
		audience := "https://" + callHdr.Host + port + callHdr.Method[:pos]
		data, err := c.GetRequestMetadata(ctx, audience)
		if err != nil {
			return nil, StreamErrorf(codes.InvalidArgument, "transport: %v", err)
		}
		for k, v := range data {
			authData[k] = v
		}
	}
	t.mu.Lock()
	if t.state != reachable {
		t.mu.Unlock()
		return nil, ErrConnClosing
	}
	checkStreamsQuota := t.streamsQuota != nil
	t.mu.Unlock()
	if checkStreamsQuota {
		sq, err := wait(ctx, t.shutdownChan, t.streamsQuota.acquire())
		if err != nil {
			return nil, err
		}
		// Returns the quota balance back.
		if sq > 1 {
			t.streamsQuota.add(sq - 1)
		}
	}
	if _, err := wait(ctx, t.shutdownChan, t.writableChan); err != nil {
		// t.streamsQuota will be updated when t.CloseStream is invoked.
		return nil, err
	}
	t.mu.Lock()
	if t.state != reachable {
		t.mu.Unlock()
		return nil, ErrConnClosing
	}
	s := t.newStream(ctx, callHdr)
	t.activeStreams[s.id] = s

	// This stream is not counted when applySetings(...) initialize t.streamsQuota.
	// Reset t.streamsQuota to the right value.
	var reset bool
	if !checkStreamsQuota && t.streamsQuota != nil {
		reset = true
	}
	t.mu.Unlock()
	if reset {
		t.streamsQuota.reset(-1)
	}

	// HPACK encodes various headers. Note that once WriteField(...) is
	// called, the corresponding headers/continuation frame has to be sent
	// because hpack.Encoder is stateful.
	t.hBuf.Reset()
	t.hEnc.WriteField(hpack.HeaderField{Name: ":method", Value: "POST"})
	t.hEnc.WriteField(hpack.HeaderField{Name: ":scheme", Value: t.scheme})
	t.hEnc.WriteField(hpack.HeaderField{Name: ":path", Value: callHdr.Method})
	t.hEnc.WriteField(hpack.HeaderField{Name: ":authority", Value: callHdr.Host})
	t.hEnc.WriteField(hpack.HeaderField{Name: "content-type", Value: "application/grpc"})
	t.hEnc.WriteField(hpack.HeaderField{Name: "user-agent", Value: t.userAgent})
	t.hEnc.WriteField(hpack.HeaderField{Name: "te", Value: "trailers"})

	if callHdr.SendCompress != "" {
		t.hEnc.WriteField(hpack.HeaderField{Name: "grpc-encoding", Value: callHdr.SendCompress})
	}
	if timeout > 0 {
		t.hEnc.WriteField(hpack.HeaderField{Name: "grpc-timeout", Value: timeoutEncode(timeout)})
	}
	for k, v := range authData {
		t.hEnc.WriteField(hpack.HeaderField{Name: k, Value: v})
	}
	var (
		hasMD      bool
		endHeaders bool
	)
	if md, ok := metadata.FromContext(ctx); ok {
		hasMD = true
		for k, v := range md {
			for _, entry := range v {
				t.hEnc.WriteField(hpack.HeaderField{Name: k, Value: entry})
			}
		}
	}
	first := true
	// Sends the headers in a single batch even when they span multiple frames.
	for !endHeaders {
		size := t.hBuf.Len()
		if size > http2MaxFrameLen {
			size = http2MaxFrameLen
		} else {
			endHeaders = true
		}
		var flush bool
		if endHeaders && (hasMD || callHdr.Flush) {
			flush = true
		}
		if first {
			// Sends a HeadersFrame to server to start a new stream.
			p := http2.HeadersFrameParam{
				StreamID:      s.id,
				BlockFragment: t.hBuf.Next(size),
				EndStream:     false,
				EndHeaders:    endHeaders,
			}
			// Do a force flush for the buffered frames iff it is the last headers frame
			// and there is header metadata to be sent. Otherwise, there is flushing until
			// the corresponding data frame is written.
			err = t.framer.writeHeaders(flush, p)
			first = false
		} else {
			// Sends Continuation frames for the leftover headers.
			err = t.framer.writeContinuation(flush, s.id, endHeaders, t.hBuf.Next(size))
		}
		if err != nil {
			t.notifyError(err)
			return nil, ConnectionErrorf("transport: %v", err)
		}
	}
	t.writableChan <- 0
	return s, nil
}

// CloseStream clears the footprint of a stream when the stream is not needed any more.
// This must not be executed in reader's goroutine.
func (t *http2Client) CloseStream(s *Stream, err error) {
	var updateStreams bool
	t.mu.Lock()
	if t.streamsQuota != nil {
		updateStreams = true
	}
	delete(t.activeStreams, s.id)
	t.mu.Unlock()
	if updateStreams {
		t.streamsQuota.add(1)
	}
	// In case stream sending and receiving are invoked in separate
	// goroutines (e.g., bi-directional streaming), the caller needs
	// to call cancel on the stream to interrupt the blocking on
	// other goroutines.
	s.cancel()
	s.mu.Lock()
	if q := s.fc.restoreConn(); q > 0 {
		t.controlBuf.put(&windowUpdate{0, q})
	}
	if s.state == streamDone {
		s.mu.Unlock()
		return
	}
	if !s.headerDone {
		close(s.headerChan)
		s.headerDone = true
	}
	s.state = streamDone
	s.mu.Unlock()
	if _, ok := err.(StreamError); ok {
		t.controlBuf.put(&resetStream{s.id, http2.ErrCodeCancel})
	}
}

// Close kicks off the shutdown process of the transport. This should be called
// only once on a transport. Once it is called, the transport should not be
// accessed any more.
func (t *http2Client) Close() (err error) {
	t.mu.Lock()
	if t.state == closing {
		t.mu.Unlock()
		return errors.New("transport: Close() was already called")
	}
	t.state = closing
	t.mu.Unlock()
	close(t.shutdownChan)
	err = t.conn.Close()
	t.mu.Lock()
	streams := t.activeStreams
	t.activeStreams = nil
	t.mu.Unlock()
	// Notify all active streams.
	for _, s := range streams {
		s.mu.Lock()
		if !s.headerDone {
			close(s.headerChan)
			s.headerDone = true
		}
		s.mu.Unlock()
		s.write(recvMsg{err: ErrConnClosing})
	}
	return
}

// Write formats the data into HTTP2 data frame(s) and sends it out. The caller
// should proceed only if Write returns nil.
// TODO(zhaoq): opts.Delay is ignored in this implementation. Support it later
// if it improves the performance.
func (t *http2Client) Write(s *Stream, data []byte, opts *Options) error {
	r := bytes.NewBuffer(data)
	for {
		var p []byte
		if r.Len() > 0 {
			size := http2MaxFrameLen
			s.sendQuotaPool.add(0)
			// Wait until the stream has some quota to send the data.
			sq, err := wait(s.ctx, t.shutdownChan, s.sendQuotaPool.acquire())
			if err != nil {
				return err
			}
			t.sendQuotaPool.add(0)
			// Wait until the transport has some quota to send the data.
			tq, err := wait(s.ctx, t.shutdownChan, t.sendQuotaPool.acquire())
			if err != nil {
				if _, ok := err.(StreamError); ok {
					t.sendQuotaPool.cancel()
				}
				return err
			}
			if sq < size {
				size = sq
			}
			if tq < size {
				size = tq
			}
			p = r.Next(size)
			ps := len(p)
			if ps < sq {
				// Overbooked stream quota. Return it back.
				s.sendQuotaPool.add(sq - ps)
			}
			if ps < tq {
				// Overbooked transport quota. Return it back.
				t.sendQuotaPool.add(tq - ps)
			}
		}
		var (
			endStream  bool
			forceFlush bool
		)
		if opts.Last && r.Len() == 0 {
			endStream = true
		}
		// Indicate there is a writer who is about to write a data frame.
		t.framer.adjustNumWriters(1)
		// Got some quota. Try to acquire writing privilege on the transport.
		if _, err := wait(s.ctx, t.shutdownChan, t.writableChan); err != nil {
			if t.framer.adjustNumWriters(-1) == 0 {
				// This writer is the last one in this batch and has the
				// responsibility to flush the buffered frames. It queues
				// a flush request to controlBuf instead of flushing directly
				// in order to avoid the race with other writing or flushing.
				t.controlBuf.put(&flushIO{})
			}
			return err
		}
		if r.Len() == 0 && t.framer.adjustNumWriters(0) == 1 {
			// Do a force flush iff this is last frame for the entire gRPC message
			// and the caller is the only writer at this moment.
			forceFlush = true
		}
		// If WriteData fails, all the pending streams will be handled
		// by http2Client.Close(). No explicit CloseStream() needs to be
		// invoked.
		if err := t.framer.writeData(forceFlush, s.id, endStream, p); err != nil {
			t.notifyError(err)
			return ConnectionErrorf("transport: %v", err)
		}
		if t.framer.adjustNumWriters(-1) == 0 {
			t.framer.flushWrite()
		}
		t.writableChan <- 0
		if r.Len() == 0 {
			break
		}
	}
	if !opts.Last {
		return nil
	}
	s.mu.Lock()
	if s.state != streamDone {
		if s.state == streamReadDone {
			s.state = streamDone
		} else {
			s.state = streamWriteDone
		}
	}
	s.mu.Unlock()
	return nil
}

func (t *http2Client) getStream(f http2.Frame) (*Stream, bool) {
	t.mu.Lock()
	defer t.mu.Unlock()
	if t.activeStreams == nil {
		// The transport is closing.
		return nil, false
	}
	if s, ok := t.activeStreams[f.Header().StreamID]; ok {
		return s, true
	}
	return nil, false
}

// updateWindow adjusts the inbound quota for the stream and the transport.
// Window updates will deliver to the controller for sending when
// the cumulative quota exceeds the corresponding threshold.
func (t *http2Client) updateWindow(s *Stream, n uint32) {
	swu, cwu := s.fc.onRead(n)
	if swu > 0 {
		t.controlBuf.put(&windowUpdate{s.id, swu})
	}
	if cwu > 0 {
		t.controlBuf.put(&windowUpdate{0, cwu})
	}
}

func (t *http2Client) handleData(f *http2.DataFrame) {
	// Select the right stream to dispatch.
	s, ok := t.getStream(f)
	if !ok {
		return
	}
	size := len(f.Data())
	if size > 0 {
		if err := s.fc.onData(uint32(size)); err != nil {
			if _, ok := err.(ConnectionError); ok {
				t.notifyError(err)
				return
			}
			s.mu.Lock()
			if s.state == streamDone {
				s.mu.Unlock()
				return
			}
			s.state = streamDone
			s.statusCode = codes.Internal
			s.statusDesc = err.Error()
			s.mu.Unlock()
			s.write(recvMsg{err: io.EOF})
			t.controlBuf.put(&resetStream{s.id, http2.ErrCodeFlowControl})
			return
		}
		// TODO(bradfitz, zhaoq): A copy is required here because there is no
		// guarantee f.Data() is consumed before the arrival of next frame.
		// Can this copy be eliminated?
		data := make([]byte, size)
		copy(data, f.Data())
		s.write(recvMsg{data: data})
	}
	// The server has closed the stream without sending trailers.  Record that
	// the read direction is closed, and set the status appropriately.
	if f.FrameHeader.Flags.Has(http2.FlagDataEndStream) {
		s.mu.Lock()
		if s.state == streamWriteDone {
			s.state = streamDone
		} else {
			s.state = streamReadDone
		}
		s.statusCode = codes.Internal
		s.statusDesc = "server closed the stream without sending trailers"
		s.mu.Unlock()
		s.write(recvMsg{err: io.EOF})
	}
}

func (t *http2Client) handleRSTStream(f *http2.RSTStreamFrame) {
	s, ok := t.getStream(f)
	if !ok {
		return
	}
	s.mu.Lock()
	if s.state == streamDone {
		s.mu.Unlock()
		return
	}
	s.state = streamDone
	if !s.headerDone {
		close(s.headerChan)
		s.headerDone = true
	}
	s.statusCode, ok = http2RSTErrConvTab[http2.ErrCode(f.ErrCode)]
	if !ok {
		grpclog.Println("transport: http2Client.handleRSTStream found no mapped gRPC status for the received http2 error ", f.ErrCode)
	}
	s.mu.Unlock()
	s.write(recvMsg{err: io.EOF})
}

func (t *http2Client) handleSettings(f *http2.SettingsFrame) {
	if f.IsAck() {
		return
	}
	var ss []http2.Setting
	f.ForeachSetting(func(s http2.Setting) error {
		ss = append(ss, s)
		return nil
	})
	// The settings will be applied once the ack is sent.
	t.controlBuf.put(&settings{ack: true, ss: ss})
}

func (t *http2Client) handlePing(f *http2.PingFrame) {
	pingAck := &ping{ack: true}
	copy(pingAck.data[:], f.Data[:])
	t.controlBuf.put(pingAck)
}

func (t *http2Client) handleGoAway(f *http2.GoAwayFrame) {
	// TODO(zhaoq): GoAwayFrame handler to be implemented
}

func (t *http2Client) handleWindowUpdate(f *http2.WindowUpdateFrame) {
	id := f.Header().StreamID
	incr := f.Increment
	if id == 0 {
		t.sendQuotaPool.add(int(incr))
		return
	}
	if s, ok := t.getStream(f); ok {
		s.sendQuotaPool.add(int(incr))
	}
}

// operateHeader takes action on the decoded headers. It returns the current
// stream if there are remaining headers on the wire (in the following
// Continuation frame).
func (t *http2Client) operateHeaders(hDec *hpackDecoder, s *Stream, frame headerFrame, endStream bool) (pendingStream *Stream) {
	defer func() {
		if pendingStream == nil {
			hDec.state = decodeState{}
		}
	}()
	endHeaders, err := hDec.decodeClientHTTP2Headers(frame)
	if s == nil {
		// s has been closed.
		return nil
	}
	if err != nil {
		s.write(recvMsg{err: err})
		// Something wrong. Stops reading even when there is remaining.
		return nil
	}
	if !endHeaders {
		return s
	}
	s.mu.Lock()
	if !endStream {
		s.recvCompress = hDec.state.encoding
	}
	if !s.headerDone {
		if !endStream && len(hDec.state.mdata) > 0 {
			s.header = hDec.state.mdata
		}
		close(s.headerChan)
		s.headerDone = true
	}
	if !endStream || s.state == streamDone {
		s.mu.Unlock()
		return nil
	}

	if len(hDec.state.mdata) > 0 {
		s.trailer = hDec.state.mdata
	}
	s.state = streamDone
	s.statusCode = hDec.state.statusCode
	s.statusDesc = hDec.state.statusDesc
	s.mu.Unlock()

	s.write(recvMsg{err: io.EOF})
	return nil
}

// reader runs as a separate goroutine in charge of reading data from network
// connection.
//
// TODO(zhaoq): currently one reader per transport. Investigate whether this is
// optimal.
// TODO(zhaoq): Check the validity of the incoming frame sequence.
func (t *http2Client) reader() {
	// Check the validity of server preface.
	frame, err := t.framer.readFrame()
	if err != nil {
		t.notifyError(err)
		return
	}
	sf, ok := frame.(*http2.SettingsFrame)
	if !ok {
		t.notifyError(err)
		return
	}
	t.handleSettings(sf)

	hDec := newHPACKDecoder()
	var curStream *Stream
	// loop to keep reading incoming messages on this transport.
	for {
		frame, err := t.framer.readFrame()
		if err != nil {
			t.notifyError(err)
			return
		}
		switch frame := frame.(type) {
		case *http2.HeadersFrame:
			// operateHeaders has to be invoked regardless the value of curStream
			// because the HPACK decoder needs to be updated using the received
			// headers.
			curStream, _ = t.getStream(frame)
			endStream := frame.Header().Flags.Has(http2.FlagHeadersEndStream)
			curStream = t.operateHeaders(hDec, curStream, frame, endStream)
		case *http2.ContinuationFrame:
			curStream = t.operateHeaders(hDec, curStream, frame, frame.HeadersEnded())
		case *http2.DataFrame:
			t.handleData(frame)
		case *http2.RSTStreamFrame:
			t.handleRSTStream(frame)
		case *http2.SettingsFrame:
			t.handleSettings(frame)
		case *http2.PingFrame:
			t.handlePing(frame)
		case *http2.GoAwayFrame:
			t.handleGoAway(frame)
		case *http2.WindowUpdateFrame:
			t.handleWindowUpdate(frame)
		default:
			grpclog.Printf("transport: http2Client.reader got unhandled frame type %v.", frame)
		}
	}
}

func (t *http2Client) applySettings(ss []http2.Setting) {
	for _, s := range ss {
		switch s.ID {
		case http2.SettingMaxConcurrentStreams:
			// TODO(zhaoq): This is a hack to avoid significant refactoring of the
			// code to deal with the unrealistic int32 overflow. Probably will try
			// to find a better way to handle this later.
			if s.Val > math.MaxInt32 {
				s.Val = math.MaxInt32
			}
			t.mu.Lock()
			reset := t.streamsQuota != nil
			if !reset {
				t.streamsQuota = newQuotaPool(int(s.Val) - len(t.activeStreams))
			}
			ms := t.maxStreams
			t.maxStreams = int(s.Val)
			t.mu.Unlock()
			if reset {
				t.streamsQuota.reset(int(s.Val) - ms)
			}
		case http2.SettingInitialWindowSize:
			t.mu.Lock()
			for _, stream := range t.activeStreams {
				// Adjust the sending quota for each stream.
				stream.sendQuotaPool.reset(int(s.Val - t.streamSendQuota))
			}
			t.streamSendQuota = s.Val
			t.mu.Unlock()
		}
	}
}

// controller running in a separate goroutine takes charge of sending control
// frames (e.g., window update, reset stream, setting, etc.) to the server.
func (t *http2Client) controller() {
	for {
		select {
		case i := <-t.controlBuf.get():
			t.controlBuf.load()
			select {
			case <-t.writableChan:
				switch i := i.(type) {
				case *windowUpdate:
					t.framer.writeWindowUpdate(true, i.streamID, i.increment)
				case *settings:
					if i.ack {
						t.framer.writeSettingsAck(true)
						t.applySettings(i.ss)
					} else {
						t.framer.writeSettings(true, i.ss...)
					}
				case *resetStream:
					t.framer.writeRSTStream(true, i.streamID, i.code)
				case *flushIO:
					t.framer.flushWrite()
				case *ping:
					t.framer.writePing(true, i.ack, i.data)
				default:
					grpclog.Printf("transport: http2Client.controller got unexpected item type %v\n", i)
				}
				t.writableChan <- 0
				continue
			case <-t.shutdownChan:
				return
			}
		case <-t.shutdownChan:
			return
		}
	}
}

func (t *http2Client) Error() <-chan struct{} {
	return t.errorChan
}

func (t *http2Client) notifyError(err error) {
	t.mu.Lock()
	defer t.mu.Unlock()
	// make sure t.errorChan is closed only once.
	if t.state == reachable {
		t.state = unreachable
		close(t.errorChan)
		grpclog.Printf("transport: http2Client.notifyError got notified that the client transport was broken %v.", err)
	}
}