diff --git a/vendor/google.golang.org/grpc/internal/transport/controlbuf.go b/vendor/google.golang.org/grpc/internal/transport/controlbuf.go
new file mode 100644
index 0000000..b8e0aa4
--- /dev/null
+++ b/vendor/google.golang.org/grpc/internal/transport/controlbuf.go
@@ -0,0 +1,930 @@
+/*
+ *
+ * Copyright 2014 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package transport
+
+import (
+	"bytes"
+	"fmt"
+	"runtime"
+	"sync"
+	"sync/atomic"
+
+	"golang.org/x/net/http2"
+	"golang.org/x/net/http2/hpack"
+)
+
+var updateHeaderTblSize = func(e *hpack.Encoder, v uint32) {
+	e.SetMaxDynamicTableSizeLimit(v)
+}
+
+type itemNode struct {
+	it   interface{}
+	next *itemNode
+}
+
+type itemList struct {
+	head *itemNode
+	tail *itemNode
+}
+
+func (il *itemList) enqueue(i interface{}) {
+	n := &itemNode{it: i}
+	if il.tail == nil {
+		il.head, il.tail = n, n
+		return
+	}
+	il.tail.next = n
+	il.tail = n
+}
+
+// peek returns the first item in the list without removing it from the
+// list.
+func (il *itemList) peek() interface{} {
+	return il.head.it
+}
+
+func (il *itemList) dequeue() interface{} {
+	if il.head == nil {
+		return nil
+	}
+	i := il.head.it
+	il.head = il.head.next
+	if il.head == nil {
+		il.tail = nil
+	}
+	return i
+}
+
+func (il *itemList) dequeueAll() *itemNode {
+	h := il.head
+	il.head, il.tail = nil, nil
+	return h
+}
+
+func (il *itemList) isEmpty() bool {
+	return il.head == nil
+}
+
+// The following defines various control items which could flow through
+// the control buffer of transport. They represent different aspects of
+// control tasks, e.g., flow control, settings, streaming resetting, etc.
+
+// maxQueuedTransportResponseFrames is the most queued "transport response"
+// frames we will buffer before preventing new reads from occurring on the
+// transport.  These are control frames sent in response to client requests,
+// such as RST_STREAM due to bad headers or settings acks.
+const maxQueuedTransportResponseFrames = 50
+
+type cbItem interface {
+	isTransportResponseFrame() bool
+}
+
+// registerStream is used to register an incoming stream with loopy writer.
+type registerStream struct {
+	streamID uint32
+	wq       *writeQuota
+}
+
+func (*registerStream) isTransportResponseFrame() bool { return false }
+
+// headerFrame is also used to register stream on the client-side.
+type headerFrame struct {
+	streamID   uint32
+	hf         []hpack.HeaderField
+	endStream  bool                       // Valid on server side.
+	initStream func(uint32) (bool, error) // Used only on the client side.
+	onWrite    func()
+	wq         *writeQuota    // write quota for the stream created.
+	cleanup    *cleanupStream // Valid on the server side.
+	onOrphaned func(error)    // Valid on client-side
+}
+
+func (h *headerFrame) isTransportResponseFrame() bool {
+	return h.cleanup != nil && h.cleanup.rst // Results in a RST_STREAM
+}
+
+type cleanupStream struct {
+	streamID uint32
+	rst      bool
+	rstCode  http2.ErrCode
+	onWrite  func()
+}
+
+func (c *cleanupStream) isTransportResponseFrame() bool { return c.rst } // Results in a RST_STREAM
+
+type dataFrame struct {
+	streamID  uint32
+	endStream bool
+	h         []byte
+	d         []byte
+	// onEachWrite is called every time
+	// a part of d is written out.
+	onEachWrite func()
+}
+
+func (*dataFrame) isTransportResponseFrame() bool { return false }
+
+type incomingWindowUpdate struct {
+	streamID  uint32
+	increment uint32
+}
+
+func (*incomingWindowUpdate) isTransportResponseFrame() bool { return false }
+
+type outgoingWindowUpdate struct {
+	streamID  uint32
+	increment uint32
+}
+
+func (*outgoingWindowUpdate) isTransportResponseFrame() bool {
+	return false // window updates are throttled by thresholds
+}
+
+type incomingSettings struct {
+	ss []http2.Setting
+}
+
+func (*incomingSettings) isTransportResponseFrame() bool { return true } // Results in a settings ACK
+
+type outgoingSettings struct {
+	ss []http2.Setting
+}
+
+func (*outgoingSettings) isTransportResponseFrame() bool { return false }
+
+type incomingGoAway struct {
+}
+
+func (*incomingGoAway) isTransportResponseFrame() bool { return false }
+
+type goAway struct {
+	code      http2.ErrCode
+	debugData []byte
+	headsUp   bool
+	closeConn bool
+}
+
+func (*goAway) isTransportResponseFrame() bool { return false }
+
+type ping struct {
+	ack  bool
+	data [8]byte
+}
+
+func (*ping) isTransportResponseFrame() bool { return true }
+
+type outFlowControlSizeRequest struct {
+	resp chan uint32
+}
+
+func (*outFlowControlSizeRequest) isTransportResponseFrame() bool { return false }
+
+type outStreamState int
+
+const (
+	active outStreamState = iota
+	empty
+	waitingOnStreamQuota
+)
+
+type outStream struct {
+	id               uint32
+	state            outStreamState
+	itl              *itemList
+	bytesOutStanding int
+	wq               *writeQuota
+
+	next *outStream
+	prev *outStream
+}
+
+func (s *outStream) deleteSelf() {
+	if s.prev != nil {
+		s.prev.next = s.next
+	}
+	if s.next != nil {
+		s.next.prev = s.prev
+	}
+	s.next, s.prev = nil, nil
+}
+
+type outStreamList struct {
+	// Following are sentinel objects that mark the
+	// beginning and end of the list. They do not
+	// contain any item lists. All valid objects are
+	// inserted in between them.
+	// This is needed so that an outStream object can
+	// deleteSelf() in O(1) time without knowing which
+	// list it belongs to.
+	head *outStream
+	tail *outStream
+}
+
+func newOutStreamList() *outStreamList {
+	head, tail := new(outStream), new(outStream)
+	head.next = tail
+	tail.prev = head
+	return &outStreamList{
+		head: head,
+		tail: tail,
+	}
+}
+
+func (l *outStreamList) enqueue(s *outStream) {
+	e := l.tail.prev
+	e.next = s
+	s.prev = e
+	s.next = l.tail
+	l.tail.prev = s
+}
+
+// remove from the beginning of the list.
+func (l *outStreamList) dequeue() *outStream {
+	b := l.head.next
+	if b == l.tail {
+		return nil
+	}
+	b.deleteSelf()
+	return b
+}
+
+// controlBuffer is a way to pass information to loopy.
+// Information is passed as specific struct types called control frames.
+// A control frame not only represents data, messages or headers to be sent out
+// but can also be used to instruct loopy to update its internal state.
+// It shouldn't be confused with an HTTP2 frame, although some of the control frames
+// like dataFrame and headerFrame do go out on wire as HTTP2 frames.
+type controlBuffer struct {
+	ch              chan struct{}
+	done            <-chan struct{}
+	mu              sync.Mutex
+	consumerWaiting bool
+	list            *itemList
+	err             error
+
+	// transportResponseFrames counts the number of queued items that represent
+	// the response of an action initiated by the peer.  trfChan is created
+	// when transportResponseFrames >= maxQueuedTransportResponseFrames and is
+	// closed and nilled when transportResponseFrames drops below the
+	// threshold.  Both fields are protected by mu.
+	transportResponseFrames int
+	trfChan                 atomic.Value // *chan struct{}
+}
+
+func newControlBuffer(done <-chan struct{}) *controlBuffer {
+	return &controlBuffer{
+		ch:   make(chan struct{}, 1),
+		list: &itemList{},
+		done: done,
+	}
+}
+
+// throttle blocks if there are too many incomingSettings/cleanupStreams in the
+// controlbuf.
+func (c *controlBuffer) throttle() {
+	ch, _ := c.trfChan.Load().(*chan struct{})
+	if ch != nil {
+		select {
+		case <-*ch:
+		case <-c.done:
+		}
+	}
+}
+
+func (c *controlBuffer) put(it cbItem) error {
+	_, err := c.executeAndPut(nil, it)
+	return err
+}
+
+func (c *controlBuffer) executeAndPut(f func(it interface{}) bool, it cbItem) (bool, error) {
+	var wakeUp bool
+	c.mu.Lock()
+	if c.err != nil {
+		c.mu.Unlock()
+		return false, c.err
+	}
+	if f != nil {
+		if !f(it) { // f wasn't successful
+			c.mu.Unlock()
+			return false, nil
+		}
+	}
+	if c.consumerWaiting {
+		wakeUp = true
+		c.consumerWaiting = false
+	}
+	c.list.enqueue(it)
+	if it.isTransportResponseFrame() {
+		c.transportResponseFrames++
+		if c.transportResponseFrames == maxQueuedTransportResponseFrames {
+			// We are adding the frame that puts us over the threshold; create
+			// a throttling channel.
+			ch := make(chan struct{})
+			c.trfChan.Store(&ch)
+		}
+	}
+	c.mu.Unlock()
+	if wakeUp {
+		select {
+		case c.ch <- struct{}{}:
+		default:
+		}
+	}
+	return true, nil
+}
+
+// Note argument f should never be nil.
+func (c *controlBuffer) execute(f func(it interface{}) bool, it interface{}) (bool, error) {
+	c.mu.Lock()
+	if c.err != nil {
+		c.mu.Unlock()
+		return false, c.err
+	}
+	if !f(it) { // f wasn't successful
+		c.mu.Unlock()
+		return false, nil
+	}
+	c.mu.Unlock()
+	return true, nil
+}
+
+func (c *controlBuffer) get(block bool) (interface{}, error) {
+	for {
+		c.mu.Lock()
+		if c.err != nil {
+			c.mu.Unlock()
+			return nil, c.err
+		}
+		if !c.list.isEmpty() {
+			h := c.list.dequeue().(cbItem)
+			if h.isTransportResponseFrame() {
+				if c.transportResponseFrames == maxQueuedTransportResponseFrames {
+					// We are removing the frame that put us over the
+					// threshold; close and clear the throttling channel.
+					ch := c.trfChan.Load().(*chan struct{})
+					close(*ch)
+					c.trfChan.Store((*chan struct{})(nil))
+				}
+				c.transportResponseFrames--
+			}
+			c.mu.Unlock()
+			return h, nil
+		}
+		if !block {
+			c.mu.Unlock()
+			return nil, nil
+		}
+		c.consumerWaiting = true
+		c.mu.Unlock()
+		select {
+		case <-c.ch:
+		case <-c.done:
+			c.finish()
+			return nil, ErrConnClosing
+		}
+	}
+}
+
+func (c *controlBuffer) finish() {
+	c.mu.Lock()
+	if c.err != nil {
+		c.mu.Unlock()
+		return
+	}
+	c.err = ErrConnClosing
+	// There may be headers for streams in the control buffer.
+	// These streams need to be cleaned out since the transport
+	// is still not aware of these yet.
+	for head := c.list.dequeueAll(); head != nil; head = head.next {
+		hdr, ok := head.it.(*headerFrame)
+		if !ok {
+			continue
+		}
+		if hdr.onOrphaned != nil { // It will be nil on the server-side.
+			hdr.onOrphaned(ErrConnClosing)
+		}
+	}
+	c.mu.Unlock()
+}
+
+type side int
+
+const (
+	clientSide side = iota
+	serverSide
+)
+
+// Loopy receives frames from the control buffer.
+// Each frame is handled individually; most of the work done by loopy goes
+// into handling data frames. Loopy maintains a queue of active streams, and each
+// stream maintains a queue of data frames; as loopy receives data frames
+// it gets added to the queue of the relevant stream.
+// Loopy goes over this list of active streams by processing one node every iteration,
+// thereby closely resemebling to a round-robin scheduling over all streams. While
+// processing a stream, loopy writes out data bytes from this stream capped by the min
+// of http2MaxFrameLen, connection-level flow control and stream-level flow control.
+type loopyWriter struct {
+	side      side
+	cbuf      *controlBuffer
+	sendQuota uint32
+	oiws      uint32 // outbound initial window size.
+	// estdStreams is map of all established streams that are not cleaned-up yet.
+	// On client-side, this is all streams whose headers were sent out.
+	// On server-side, this is all streams whose headers were received.
+	estdStreams map[uint32]*outStream // Established streams.
+	// activeStreams is a linked-list of all streams that have data to send and some
+	// stream-level flow control quota.
+	// Each of these streams internally have a list of data items(and perhaps trailers
+	// on the server-side) to be sent out.
+	activeStreams *outStreamList
+	framer        *framer
+	hBuf          *bytes.Buffer  // The buffer for HPACK encoding.
+	hEnc          *hpack.Encoder // HPACK encoder.
+	bdpEst        *bdpEstimator
+	draining      bool
+
+	// Side-specific handlers
+	ssGoAwayHandler func(*goAway) (bool, error)
+}
+
+func newLoopyWriter(s side, fr *framer, cbuf *controlBuffer, bdpEst *bdpEstimator) *loopyWriter {
+	var buf bytes.Buffer
+	l := &loopyWriter{
+		side:          s,
+		cbuf:          cbuf,
+		sendQuota:     defaultWindowSize,
+		oiws:          defaultWindowSize,
+		estdStreams:   make(map[uint32]*outStream),
+		activeStreams: newOutStreamList(),
+		framer:        fr,
+		hBuf:          &buf,
+		hEnc:          hpack.NewEncoder(&buf),
+		bdpEst:        bdpEst,
+	}
+	return l
+}
+
+const minBatchSize = 1000
+
+// run should be run in a separate goroutine.
+// It reads control frames from controlBuf and processes them by:
+// 1. Updating loopy's internal state, or/and
+// 2. Writing out HTTP2 frames on the wire.
+//
+// Loopy keeps all active streams with data to send in a linked-list.
+// All streams in the activeStreams linked-list must have both:
+// 1. Data to send, and
+// 2. Stream level flow control quota available.
+//
+// In each iteration of run loop, other than processing the incoming control
+// frame, loopy calls processData, which processes one node from the activeStreams linked-list.
+// This results in writing of HTTP2 frames into an underlying write buffer.
+// When there's no more control frames to read from controlBuf, loopy flushes the write buffer.
+// As an optimization, to increase the batch size for each flush, loopy yields the processor, once
+// if the batch size is too low to give stream goroutines a chance to fill it up.
+func (l *loopyWriter) run() (err error) {
+	defer func() {
+		if err == ErrConnClosing {
+			// Don't log ErrConnClosing as error since it happens
+			// 1. When the connection is closed by some other known issue.
+			// 2. User closed the connection.
+			// 3. A graceful close of connection.
+			infof("transport: loopyWriter.run returning. %v", err)
+			err = nil
+		}
+	}()
+	for {
+		it, err := l.cbuf.get(true)
+		if err != nil {
+			return err
+		}
+		if err = l.handle(it); err != nil {
+			return err
+		}
+		if _, err = l.processData(); err != nil {
+			return err
+		}
+		gosched := true
+	hasdata:
+		for {
+			it, err := l.cbuf.get(false)
+			if err != nil {
+				return err
+			}
+			if it != nil {
+				if err = l.handle(it); err != nil {
+					return err
+				}
+				if _, err = l.processData(); err != nil {
+					return err
+				}
+				continue hasdata
+			}
+			isEmpty, err := l.processData()
+			if err != nil {
+				return err
+			}
+			if !isEmpty {
+				continue hasdata
+			}
+			if gosched {
+				gosched = false
+				if l.framer.writer.offset < minBatchSize {
+					runtime.Gosched()
+					continue hasdata
+				}
+			}
+			l.framer.writer.Flush()
+			break hasdata
+
+		}
+	}
+}
+
+func (l *loopyWriter) outgoingWindowUpdateHandler(w *outgoingWindowUpdate) error {
+	return l.framer.fr.WriteWindowUpdate(w.streamID, w.increment)
+}
+
+func (l *loopyWriter) incomingWindowUpdateHandler(w *incomingWindowUpdate) error {
+	// Otherwise update the quota.
+	if w.streamID == 0 {
+		l.sendQuota += w.increment
+		return nil
+	}
+	// Find the stream and update it.
+	if str, ok := l.estdStreams[w.streamID]; ok {
+		str.bytesOutStanding -= int(w.increment)
+		if strQuota := int(l.oiws) - str.bytesOutStanding; strQuota > 0 && str.state == waitingOnStreamQuota {
+			str.state = active
+			l.activeStreams.enqueue(str)
+			return nil
+		}
+	}
+	return nil
+}
+
+func (l *loopyWriter) outgoingSettingsHandler(s *outgoingSettings) error {
+	return l.framer.fr.WriteSettings(s.ss...)
+}
+
+func (l *loopyWriter) incomingSettingsHandler(s *incomingSettings) error {
+	if err := l.applySettings(s.ss); err != nil {
+		return err
+	}
+	return l.framer.fr.WriteSettingsAck()
+}
+
+func (l *loopyWriter) registerStreamHandler(h *registerStream) error {
+	str := &outStream{
+		id:    h.streamID,
+		state: empty,
+		itl:   &itemList{},
+		wq:    h.wq,
+	}
+	l.estdStreams[h.streamID] = str
+	return nil
+}
+
+func (l *loopyWriter) headerHandler(h *headerFrame) error {
+	if l.side == serverSide {
+		str, ok := l.estdStreams[h.streamID]
+		if !ok {
+			warningf("transport: loopy doesn't recognize the stream: %d", h.streamID)
+			return nil
+		}
+		// Case 1.A: Server is responding back with headers.
+		if !h.endStream {
+			return l.writeHeader(h.streamID, h.endStream, h.hf, h.onWrite)
+		}
+		// else:  Case 1.B: Server wants to close stream.
+
+		if str.state != empty { // either active or waiting on stream quota.
+			// add it str's list of items.
+			str.itl.enqueue(h)
+			return nil
+		}
+		if err := l.writeHeader(h.streamID, h.endStream, h.hf, h.onWrite); err != nil {
+			return err
+		}
+		return l.cleanupStreamHandler(h.cleanup)
+	}
+	// Case 2: Client wants to originate stream.
+	str := &outStream{
+		id:    h.streamID,
+		state: empty,
+		itl:   &itemList{},
+		wq:    h.wq,
+	}
+	str.itl.enqueue(h)
+	return l.originateStream(str)
+}
+
+func (l *loopyWriter) originateStream(str *outStream) error {
+	hdr := str.itl.dequeue().(*headerFrame)
+	sendPing, err := hdr.initStream(str.id)
+	if err != nil {
+		if err == ErrConnClosing {
+			return err
+		}
+		// Other errors(errStreamDrain) need not close transport.
+		return nil
+	}
+	if err = l.writeHeader(str.id, hdr.endStream, hdr.hf, hdr.onWrite); err != nil {
+		return err
+	}
+	l.estdStreams[str.id] = str
+	if sendPing {
+		return l.pingHandler(&ping{data: [8]byte{}})
+	}
+	return nil
+}
+
+func (l *loopyWriter) writeHeader(streamID uint32, endStream bool, hf []hpack.HeaderField, onWrite func()) error {
+	if onWrite != nil {
+		onWrite()
+	}
+	l.hBuf.Reset()
+	for _, f := range hf {
+		if err := l.hEnc.WriteField(f); err != nil {
+			warningf("transport: loopyWriter.writeHeader encountered error while encoding headers:", err)
+		}
+	}
+	var (
+		err               error
+		endHeaders, first bool
+	)
+	first = true
+	for !endHeaders {
+		size := l.hBuf.Len()
+		if size > http2MaxFrameLen {
+			size = http2MaxFrameLen
+		} else {
+			endHeaders = true
+		}
+		if first {
+			first = false
+			err = l.framer.fr.WriteHeaders(http2.HeadersFrameParam{
+				StreamID:      streamID,
+				BlockFragment: l.hBuf.Next(size),
+				EndStream:     endStream,
+				EndHeaders:    endHeaders,
+			})
+		} else {
+			err = l.framer.fr.WriteContinuation(
+				streamID,
+				endHeaders,
+				l.hBuf.Next(size),
+			)
+		}
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (l *loopyWriter) preprocessData(df *dataFrame) error {
+	str, ok := l.estdStreams[df.streamID]
+	if !ok {
+		return nil
+	}
+	// If we got data for a stream it means that
+	// stream was originated and the headers were sent out.
+	str.itl.enqueue(df)
+	if str.state == empty {
+		str.state = active
+		l.activeStreams.enqueue(str)
+	}
+	return nil
+}
+
+func (l *loopyWriter) pingHandler(p *ping) error {
+	if !p.ack {
+		l.bdpEst.timesnap(p.data)
+	}
+	return l.framer.fr.WritePing(p.ack, p.data)
+
+}
+
+func (l *loopyWriter) outFlowControlSizeRequestHandler(o *outFlowControlSizeRequest) error {
+	o.resp <- l.sendQuota
+	return nil
+}
+
+func (l *loopyWriter) cleanupStreamHandler(c *cleanupStream) error {
+	c.onWrite()
+	if str, ok := l.estdStreams[c.streamID]; ok {
+		// On the server side it could be a trailers-only response or
+		// a RST_STREAM before stream initialization thus the stream might
+		// not be established yet.
+		delete(l.estdStreams, c.streamID)
+		str.deleteSelf()
+	}
+	if c.rst { // If RST_STREAM needs to be sent.
+		if err := l.framer.fr.WriteRSTStream(c.streamID, c.rstCode); err != nil {
+			return err
+		}
+	}
+	if l.side == clientSide && l.draining && len(l.estdStreams) == 0 {
+		return ErrConnClosing
+	}
+	return nil
+}
+
+func (l *loopyWriter) incomingGoAwayHandler(*incomingGoAway) error {
+	if l.side == clientSide {
+		l.draining = true
+		if len(l.estdStreams) == 0 {
+			return ErrConnClosing
+		}
+	}
+	return nil
+}
+
+func (l *loopyWriter) goAwayHandler(g *goAway) error {
+	// Handling of outgoing GoAway is very specific to side.
+	if l.ssGoAwayHandler != nil {
+		draining, err := l.ssGoAwayHandler(g)
+		if err != nil {
+			return err
+		}
+		l.draining = draining
+	}
+	return nil
+}
+
+func (l *loopyWriter) handle(i interface{}) error {
+	switch i := i.(type) {
+	case *incomingWindowUpdate:
+		return l.incomingWindowUpdateHandler(i)
+	case *outgoingWindowUpdate:
+		return l.outgoingWindowUpdateHandler(i)
+	case *incomingSettings:
+		return l.incomingSettingsHandler(i)
+	case *outgoingSettings:
+		return l.outgoingSettingsHandler(i)
+	case *headerFrame:
+		return l.headerHandler(i)
+	case *registerStream:
+		return l.registerStreamHandler(i)
+	case *cleanupStream:
+		return l.cleanupStreamHandler(i)
+	case *incomingGoAway:
+		return l.incomingGoAwayHandler(i)
+	case *dataFrame:
+		return l.preprocessData(i)
+	case *ping:
+		return l.pingHandler(i)
+	case *goAway:
+		return l.goAwayHandler(i)
+	case *outFlowControlSizeRequest:
+		return l.outFlowControlSizeRequestHandler(i)
+	default:
+		return fmt.Errorf("transport: unknown control message type %T", i)
+	}
+}
+
+func (l *loopyWriter) applySettings(ss []http2.Setting) error {
+	for _, s := range ss {
+		switch s.ID {
+		case http2.SettingInitialWindowSize:
+			o := l.oiws
+			l.oiws = s.Val
+			if o < l.oiws {
+				// If the new limit is greater make all depleted streams active.
+				for _, stream := range l.estdStreams {
+					if stream.state == waitingOnStreamQuota {
+						stream.state = active
+						l.activeStreams.enqueue(stream)
+					}
+				}
+			}
+		case http2.SettingHeaderTableSize:
+			updateHeaderTblSize(l.hEnc, s.Val)
+		}
+	}
+	return nil
+}
+
+// processData removes the first stream from active streams, writes out at most 16KB
+// of its data and then puts it at the end of activeStreams if there's still more data
+// to be sent and stream has some stream-level flow control.
+func (l *loopyWriter) processData() (bool, error) {
+	if l.sendQuota == 0 {
+		return true, nil
+	}
+	str := l.activeStreams.dequeue() // Remove the first stream.
+	if str == nil {
+		return true, nil
+	}
+	dataItem := str.itl.peek().(*dataFrame) // Peek at the first data item this stream.
+	// A data item is represented by a dataFrame, since it later translates into
+	// multiple HTTP2 data frames.
+	// Every dataFrame has two buffers; h that keeps grpc-message header and d that is acutal data.
+	// As an optimization to keep wire traffic low, data from d is copied to h to make as big as the
+	// maximum possilbe HTTP2 frame size.
+
+	if len(dataItem.h) == 0 && len(dataItem.d) == 0 { // Empty data frame
+		// Client sends out empty data frame with endStream = true
+		if err := l.framer.fr.WriteData(dataItem.streamID, dataItem.endStream, nil); err != nil {
+			return false, err
+		}
+		str.itl.dequeue() // remove the empty data item from stream
+		if str.itl.isEmpty() {
+			str.state = empty
+		} else if trailer, ok := str.itl.peek().(*headerFrame); ok { // the next item is trailers.
+			if err := l.writeHeader(trailer.streamID, trailer.endStream, trailer.hf, trailer.onWrite); err != nil {
+				return false, err
+			}
+			if err := l.cleanupStreamHandler(trailer.cleanup); err != nil {
+				return false, nil
+			}
+		} else {
+			l.activeStreams.enqueue(str)
+		}
+		return false, nil
+	}
+	var (
+		idx int
+		buf []byte
+	)
+	if len(dataItem.h) != 0 { // data header has not been written out yet.
+		buf = dataItem.h
+	} else {
+		idx = 1
+		buf = dataItem.d
+	}
+	size := http2MaxFrameLen
+	if len(buf) < size {
+		size = len(buf)
+	}
+	if strQuota := int(l.oiws) - str.bytesOutStanding; strQuota <= 0 { // stream-level flow control.
+		str.state = waitingOnStreamQuota
+		return false, nil
+	} else if strQuota < size {
+		size = strQuota
+	}
+
+	if l.sendQuota < uint32(size) { // connection-level flow control.
+		size = int(l.sendQuota)
+	}
+	// Now that outgoing flow controls are checked we can replenish str's write quota
+	str.wq.replenish(size)
+	var endStream bool
+	// If this is the last data message on this stream and all of it can be written in this iteration.
+	if dataItem.endStream && size == len(buf) {
+		// buf contains either data or it contains header but data is empty.
+		if idx == 1 || len(dataItem.d) == 0 {
+			endStream = true
+		}
+	}
+	if dataItem.onEachWrite != nil {
+		dataItem.onEachWrite()
+	}
+	if err := l.framer.fr.WriteData(dataItem.streamID, endStream, buf[:size]); err != nil {
+		return false, err
+	}
+	buf = buf[size:]
+	str.bytesOutStanding += size
+	l.sendQuota -= uint32(size)
+	if idx == 0 {
+		dataItem.h = buf
+	} else {
+		dataItem.d = buf
+	}
+
+	if len(dataItem.h) == 0 && len(dataItem.d) == 0 { // All the data from that message was written out.
+		str.itl.dequeue()
+	}
+	if str.itl.isEmpty() {
+		str.state = empty
+	} else if trailer, ok := str.itl.peek().(*headerFrame); ok { // The next item is trailers.
+		if err := l.writeHeader(trailer.streamID, trailer.endStream, trailer.hf, trailer.onWrite); err != nil {
+			return false, err
+		}
+		if err := l.cleanupStreamHandler(trailer.cleanup); err != nil {
+			return false, err
+		}
+	} else if int(l.oiws)-str.bytesOutStanding <= 0 { // Ran out of stream quota.
+		str.state = waitingOnStreamQuota
+	} else { // Otherwise add it back to the list of active streams.
+		l.activeStreams.enqueue(str)
+	}
+	return false, nil
+}