func (t *http2Server) keepalive() {
	p := &ping{}
	var pingSent bool
	maxIdle := time.NewTimer(t.kp.MaxConnectionIdle)
	maxAge := time.NewTimer(t.kp.MaxConnectionAge)
	keepalive := time.NewTimer(t.kp.Time)
	defer func() {
		if !maxIdle.Stop() {
			<-maxIdle.C
		}
		if !maxAge.Stop() {
			<-maxAge.C
		}
		if !keepalive.Stop() {
			<-keepalive.C
		}
	}()
	for {
		select {
		case <-maxIdle.C:
			t.mu.Lock()
			idle := t.idle
			if idle.IsZero() {  
				t.mu.Unlock()
				maxIdle.Reset(t.kp.MaxConnectionIdle)
				continue
			}
			val := t.kp.MaxConnectionIdle - time.Since(idle)
			t.mu.Unlock()
			if val <= 0 {
				t.drain(http2.ErrCodeNo, []byte{})
				maxIdle.Reset(infinity)
				return
			}
			maxIdle.Reset(val)
		case <-maxAge.C:
			t.drain(http2.ErrCodeNo, []byte{})
			maxAge.Reset(t.kp.MaxConnectionAgeGrace)
			select {
			case <-maxAge.C:
				t.Close()
				maxAge.Reset(infinity)
			case <-t.ctx.Done():
			}
			return
		case <-keepalive.C:
			if atomic.CompareAndSwapUint32(&t.activity, 1, 0) {
				pingSent = false
				keepalive.Reset(t.kp.Time)
				continue
			}
			if pingSent {
				t.Close()
				keepalive.Reset(infinity)
				return
			}
			pingSent = true
			t.controlBuf.put(p)
			keepalive.Reset(t.kp.Timeout)
		case <-t.ctx.Done():
			return
		}
	}
}
keepalive running in a separate goroutine does the following:
1. Gracefully closes an idle connection after a duration of keepalive.MaxConnectionIdle.
2. Gracefully closes any connection after a duration of keepalive.MaxConnectionAge.
3. Forcibly closes a connection after an additive period of keepalive.MaxConnectionAgeGrace over keepalive.MaxConnectionAge.
4. Makes sure a connection is alive by sending pings with a frequency of keepalive.Time and closes a non-responsive connection
after an additional duration of keepalive.Timeout.
package transport
import (
	"bytes"
	"errors"
	"fmt"
	"io"
	"math"
	"math/rand"
	"net"
	"strconv"
	"sync"
	"sync/atomic"
	"time"
	"github.com/golang/protobuf/proto"
	"golang.org/x/net/context"
	"golang.org/x/net/http2"
	"golang.org/x/net/http2/hpack"
	"google.golang.org/grpc/channelz"
	"google.golang.org/grpc/codes"
	"google.golang.org/grpc/credentials"
	"google.golang.org/grpc/keepalive"
	"google.golang.org/grpc/metadata"
	"google.golang.org/grpc/peer"
	"google.golang.org/grpc/stats"
	"google.golang.org/grpc/status"
	"google.golang.org/grpc/tap"
)
var ErrIllegalHeaderWrite = errors.New("transport: the stream is done or WriteHeader was already called")
type http2Server struct {
	ctx         context.Context
	ctxDone     <-chan struct{}  
	cancel      context.CancelFunc
	conn        net.Conn
	loopy       *loopyWriter
	readerDone  chan struct{}  
	writerDone  chan struct{}  
	remoteAddr  net.Addr
	localAddr   net.Addr
	maxStreamID uint32                
	authInfo    credentials.AuthInfo  
	inTapHandle tap.ServerInHandle
	framer      *framer
	maxStreams uint32
	controlBuf *controlBuffer
	fc         *trInFlow
	stats      stats.Handler
	activity uint32  
	kp keepalive.ServerParameters
	kep keepalive.EnforcementPolicy
	lastPingAt time.Time
	pingStrikes uint8
	resetPingStrikes  uint32  
	initialWindowSize int32
	bdpEst            *bdpEstimator
	channelzID int64  
	mu sync.Mutex  
	drainChan     chan struct{}
	state         transportState
	activeStreams map[uint32]*Stream
	idle time.Time
}
func newHTTP2Server(conn net.Conn, config *ServerConfig) (_ ServerTransport, err error) {
	writeBufSize := defaultWriteBufSize
	if config.WriteBufferSize > 0 {
		writeBufSize = config.WriteBufferSize
	}
	readBufSize := defaultReadBufSize
	if config.ReadBufferSize > 0 {
		readBufSize = config.ReadBufferSize
	}
	framer := newFramer(conn, writeBufSize, readBufSize)
	var isettings []http2.Setting
	maxStreams := config.MaxStreams
	if maxStreams == 0 {
		maxStreams = math.MaxUint32
	} else {
		isettings = append(isettings, http2.Setting{
			ID:  http2.SettingMaxConcurrentStreams,
			Val: maxStreams,
		})
	}
	dynamicWindow := true
	iwz := int32(initialWindowSize)
	if config.InitialWindowSize >= defaultWindowSize {
		iwz = config.InitialWindowSize
		dynamicWindow = false
	}
	icwz := int32(initialWindowSize)
	if config.InitialConnWindowSize >= defaultWindowSize {
		icwz = config.InitialConnWindowSize
		dynamicWindow = false
	}
	if iwz != defaultWindowSize {
		isettings = append(isettings, http2.Setting{
			ID:  http2.SettingInitialWindowSize,
			Val: uint32(iwz)})
	}
	if err := framer.fr.WriteSettings(isettings...); err != nil {
		return nil, connectionErrorf(false, err, "transport: %v", err)
	}
	if delta := uint32(icwz - defaultWindowSize); delta > 0 {
		if err := framer.fr.WriteWindowUpdate(0, delta); err != nil {
			return nil, connectionErrorf(false, err, "transport: %v", err)
		}
	}
	kp := config.KeepaliveParams
	if kp.MaxConnectionIdle == 0 {
		kp.MaxConnectionIdle = defaultMaxConnectionIdle
	}
	if kp.MaxConnectionAge == 0 {
		kp.MaxConnectionAge = defaultMaxConnectionAge
	}
	kp.MaxConnectionAge += getJitter(kp.MaxConnectionAge)
	if kp.MaxConnectionAgeGrace == 0 {
		kp.MaxConnectionAgeGrace = defaultMaxConnectionAgeGrace
	}
	if kp.Time == 0 {
		kp.Time = defaultServerKeepaliveTime
	}
	if kp.Timeout == 0 {
		kp.Timeout = defaultServerKeepaliveTimeout
	}
	kep := config.KeepalivePolicy
	if kep.MinTime == 0 {
		kep.MinTime = defaultKeepalivePolicyMinTime
	}
	ctx, cancel := context.WithCancel(context.Background())
	t := &http2Server{
		ctx:               ctx,
		cancel:            cancel,
		ctxDone:           ctx.Done(),
		conn:              conn,
		remoteAddr:        conn.RemoteAddr(),
		localAddr:         conn.LocalAddr(),
		authInfo:          config.AuthInfo,
		framer:            framer,
		readerDone:        make(chan struct{}),
		writerDone:        make(chan struct{}),
		maxStreams:        maxStreams,
		inTapHandle:       config.InTapHandle,
		fc:                &trInFlow{limit: uint32(icwz)},
		state:             reachable,
		activeStreams:     make(map[uint32]*Stream),
		stats:             config.StatsHandler,
		kp:                kp,
		idle:              time.Now(),
		kep:               kep,
		initialWindowSize: iwz,
	}
	t.controlBuf = newControlBuffer(t.ctxDone)
	if dynamicWindow {
		t.bdpEst = &bdpEstimator{
			bdp:               initialWindowSize,
			updateFlowControl: t.updateFlowControl,
		}
	}
	if t.stats != nil {
		t.ctx = t.stats.TagConn(t.ctx, &stats.ConnTagInfo{
			RemoteAddr: t.remoteAddr,
			LocalAddr:  t.localAddr,
		})
		connBegin := &stats.ConnBegin{}
		t.stats.HandleConn(t.ctx, connBegin)
	}
	if channelz.IsOn() {
		t.channelzID = channelz.RegisterNormalSocket(t, config.ChannelzParentID, "")
	}
	t.framer.writer.Flush()
	defer func() {
		if err != nil {
			t.Close()
		}
	}()
	preface := make([]byte, len(clientPreface))
	if _, err := io.ReadFull(t.conn, preface); err != nil {
		return nil, connectionErrorf(false, err, "transport: http2Server.HandleStreams failed to receive the preface from client: %v", err)
	}
	if !bytes.Equal(preface, clientPreface) {
		return nil, connectionErrorf(false, nil, "transport: http2Server.HandleStreams received bogus greeting from client: %q", preface)
	}
	frame, err := t.framer.fr.ReadFrame()
	if err == io.EOF || err == io.ErrUnexpectedEOF {
		return nil, err
	}
	if err != nil {
		return nil, connectionErrorf(false, err, "transport: http2Server.HandleStreams failed to read initial settings frame: %v", err)
	}
	atomic.StoreUint32(&t.activity, 1)
	sf, ok := frame.(*http2.SettingsFrame)
	if !ok {
		return nil, connectionErrorf(false, nil, "transport: http2Server.HandleStreams saw invalid preface type %T from client", frame)
	}
	t.handleSettings(sf)
	go func() {
		t.loopy = newLoopyWriter(serverSide, t.framer, t.controlBuf, t.bdpEst)
		t.loopy.ssGoAwayHandler = t.outgoingGoAwayHandler
		t.loopy.run()
		t.conn.Close()
		close(t.writerDone)
	}()
	go t.keepalive()
	return t, nil
}
func (t *http2Server) operateHeaders(frame *http2.MetaHeadersFrame, handle func(*Stream), traceCtx func(context.Context, string) context.Context) (close bool) {
	streamID := frame.Header().StreamID
	var state decodeState
	for _, hf := range frame.Fields {
		if err := state.processHeaderField(hf); err != nil {
			if se, ok := err.(StreamError); ok {
				t.controlBuf.put(&cleanupStream{
					streamID: streamID,
					rst:      true,
					rstCode:  statusCodeConvTab[se.Code],
					onWrite:  func() {},
				})
			}
			return
		}
	}
	buf := newRecvBuffer()
	s := &Stream{
		id:             streamID,
		st:             t,
		buf:            buf,
		fc:             &inFlow{limit: uint32(t.initialWindowSize)},
		recvCompress:   state.encoding,
		method:         state.method,
		contentSubtype: state.contentSubtype,
	}
	if frame.StreamEnded() {
		s.state = streamReadDone
	}
	if state.timeoutSet {
		s.ctx, s.cancel = context.WithTimeout(t.ctx, state.timeout)
	} else {
		s.ctx, s.cancel = context.WithCancel(t.ctx)
	}
	pr := &peer.Peer{
		Addr: t.remoteAddr,
	}
	if t.authInfo != nil {
		pr.AuthInfo = t.authInfo
	}
	s.ctx = peer.NewContext(s.ctx, pr)
	if len(state.mdata) > 0 {
		s.ctx = metadata.NewIncomingContext(s.ctx, state.mdata)
	}
	if state.statsTags != nil {
		s.ctx = stats.SetIncomingTags(s.ctx, state.statsTags)
	}
	if state.statsTrace != nil {
		s.ctx = stats.SetIncomingTrace(s.ctx, state.statsTrace)
	}
	if t.inTapHandle != nil {
		var err error
		info := &tap.Info{
			FullMethodName: state.method,
		}
		s.ctx, err = t.inTapHandle(s.ctx, info)
		if err != nil {
			warningf("transport: http2Server.operateHeaders got an error from InTapHandle: %v", err)
			t.controlBuf.put(&cleanupStream{
				streamID: s.id,
				rst:      true,
				rstCode:  http2.ErrCodeRefusedStream,
				onWrite:  func() {},
			})
			return
		}
	}
	t.mu.Lock()
	if t.state != reachable {
		t.mu.Unlock()
		return
	}
	if uint32(len(t.activeStreams)) >= t.maxStreams {
		t.mu.Unlock()
		t.controlBuf.put(&cleanupStream{
			streamID: streamID,
			rst:      true,
			rstCode:  http2.ErrCodeRefusedStream,
			onWrite:  func() {},
		})
		return
	}
	if streamID%2 != 1 || streamID <= t.maxStreamID {
		t.mu.Unlock()
		errorf("transport: http2Server.HandleStreams received an illegal stream id: %v", streamID)
		return true
	}
	t.maxStreamID = streamID
	t.activeStreams[streamID] = s
	if len(t.activeStreams) == 1 {
		t.idle = time.Time{}
	}
	t.mu.Unlock()
	s.requestRead = func(n int) {
		t.adjustWindow(s, uint32(n))
	}
	s.ctx = traceCtx(s.ctx, s.method)
	if t.stats != nil {
		s.ctx = t.stats.TagRPC(s.ctx, &stats.RPCTagInfo{FullMethodName: s.method})
		inHeader := &stats.InHeader{
			FullMethod:  s.method,
			RemoteAddr:  t.remoteAddr,
			LocalAddr:   t.localAddr,
			Compression: s.recvCompress,
			WireLength:  int(frame.Header().Length),
		}
		t.stats.HandleRPC(s.ctx, inHeader)
	}
	s.ctxDone = s.ctx.Done()
	s.wq = newWriteQuota(defaultWriteQuota, s.ctxDone)
	s.trReader = &transportReader{
		reader: &recvBufferReader{
			ctx:     s.ctx,
			ctxDone: s.ctxDone,
			recv:    s.buf,
		},
		windowHandler: func(n int) {
			t.updateWindow(s, uint32(n))
		},
	}
	handle(s)
	return
}
func (t *http2Server) HandleStreams(handle func(*Stream), traceCtx func(context.Context, string) context.Context) {
	defer close(t.readerDone)
	for {
		frame, err := t.framer.fr.ReadFrame()
		atomic.StoreUint32(&t.activity, 1)
		if err != nil {
			if se, ok := err.(http2.StreamError); ok {
				warningf("transport: http2Server.HandleStreams encountered http2.StreamError: %v", se)
				t.mu.Lock()
				s := t.activeStreams[se.StreamID]
				t.mu.Unlock()
				if s != nil {
					t.closeStream(s, true, se.Code, nil)
				} else {
					t.controlBuf.put(&cleanupStream{
						streamID: se.StreamID,
						rst:      true,
						rstCode:  se.Code,
						onWrite:  func() {},
					})
				}
				continue
			}
			if err == io.EOF || err == io.ErrUnexpectedEOF {
				t.Close()
				return
			}
			warningf("transport: http2Server.HandleStreams failed to read frame: %v", err)
			t.Close()
			return
		}
		switch frame := frame.(type) {
		case *http2.MetaHeadersFrame:
			if t.operateHeaders(frame, handle, traceCtx) {
				t.Close()
				break
			}
		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.WindowUpdateFrame:
			t.handleWindowUpdate(frame)
		case *http2.GoAwayFrame:
		default:
			errorf("transport: http2Server.HandleStreams found unhandled frame type %v.", frame)
		}
	}
}
func (t *http2Server) getStream(f http2.Frame) (*Stream, bool) {
	t.mu.Lock()
	defer t.mu.Unlock()
	if t.activeStreams == nil {
		return nil, false
	}
	s, ok := t.activeStreams[f.Header().StreamID]
	if !ok {
		return nil, false
	}
	return s, true
}
func (t *http2Server) adjustWindow(s *Stream, n uint32) {
	if w := s.fc.maybeAdjust(n); w > 0 {
		t.controlBuf.put(&outgoingWindowUpdate{streamID: s.id, increment: w})
	}
}
func (t *http2Server) updateWindow(s *Stream, n uint32) {
	if w := s.fc.onRead(n); w > 0 {
		t.controlBuf.put(&outgoingWindowUpdate{streamID: s.id,
			increment: w,
		})
	}
}
func (t *http2Server) updateFlowControl(n uint32) {
	t.mu.Lock()
	for _, s := range t.activeStreams {
		s.fc.newLimit(n)
	}
	t.initialWindowSize = int32(n)
	t.mu.Unlock()
	t.controlBuf.put(&outgoingWindowUpdate{
		streamID:  0,
		increment: t.fc.newLimit(n),
	})
	t.controlBuf.put(&outgoingSettings{
		ss: []http2.Setting{
			{
				ID:  http2.SettingInitialWindowSize,
				Val: uint32(n),
			},
		},
	})
}
func (t *http2Server) handleData(f *http2.DataFrame) {
	size := f.Header().Length
	var sendBDPPing bool
	if t.bdpEst != nil {
		sendBDPPing = t.bdpEst.add(uint32(size))
	}
	if w := t.fc.onData(uint32(size)); w > 0 {
		t.controlBuf.put(&outgoingWindowUpdate{
			streamID:  0,
			increment: w,
		})
	}
	if sendBDPPing {
		if w := t.fc.reset(); w > 0 {
			t.controlBuf.put(&outgoingWindowUpdate{
				streamID:  0,
				increment: w,
			})
		}
		t.controlBuf.put(bdpPing)
	}
	s, ok := t.getStream(f)
	if !ok {
		return
	}
	if size > 0 {
		if err := s.fc.onData(uint32(size)); err != nil {
			t.closeStream(s, true, http2.ErrCodeFlowControl, nil)
			return
		}
		if f.Header().Flags.Has(http2.FlagDataPadded) {
			if w := s.fc.onRead(uint32(size) - uint32(len(f.Data()))); w > 0 {
				t.controlBuf.put(&outgoingWindowUpdate{s.id, w})
			}
		}
		if len(f.Data()) > 0 {
			data := make([]byte, len(f.Data()))
			copy(data, f.Data())
			s.write(recvMsg{data: data})
		}
	}
	if f.Header().Flags.Has(http2.FlagDataEndStream) {
		s.compareAndSwapState(streamActive, streamReadDone)
		s.write(recvMsg{err: io.EOF})
	}
}
func (t *http2Server) handleRSTStream(f *http2.RSTStreamFrame) {
	s, ok := t.getStream(f)
	if !ok {
		return
	}
	t.closeStream(s, false, 0, nil)
}
func (t *http2Server) 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
	})
	t.controlBuf.put(&incomingSettings{
		ss: ss,
	})
}
const (
	maxPingStrikes     = 2
	defaultPingTimeout = 2 * time.Hour
)
func (t *http2Server) handlePing(f *http2.PingFrame) {
	if f.IsAck() {
		if f.Data == goAwayPing.data && t.drainChan != nil {
			close(t.drainChan)
			return
		}
		if t.bdpEst != nil {
			t.bdpEst.calculate(f.Data)
		}
		return
	}
	pingAck := &ping{ack: true}
	copy(pingAck.data[:], f.Data[:])
	t.controlBuf.put(pingAck)
	now := time.Now()
	defer func() {
		t.lastPingAt = now
	}()
	if atomic.CompareAndSwapUint32(&t.resetPingStrikes, 1, 0) {
		t.pingStrikes = 0
		return
	}
	t.mu.Lock()
	ns := len(t.activeStreams)
	t.mu.Unlock()
	if ns < 1 && !t.kep.PermitWithoutStream {
		if t.lastPingAt.Add(defaultPingTimeout).After(now) {
			t.pingStrikes++
		}
	} else {
		if t.lastPingAt.Add(t.kep.MinTime).After(now) {
			t.pingStrikes++
		}
	}
	if t.pingStrikes > maxPingStrikes {
		errorf("transport: Got too many pings from the client, closing the connection.")
		t.controlBuf.put(&goAway{code: http2.ErrCodeEnhanceYourCalm, debugData: []byte("too_many_pings"), closeConn: true})
	}
}
func (t *http2Server) handleWindowUpdate(f *http2.WindowUpdateFrame) {
	t.controlBuf.put(&incomingWindowUpdate{
		streamID:  f.Header().StreamID,
		increment: f.Increment,
	})
}
func (t *http2Server) WriteHeader(s *Stream, md metadata.MD) error {
	if s.headerOk || s.getState() == streamDone {
		return ErrIllegalHeaderWrite
	}
	s.headerOk = true
	if md.Len() > 0 {
		if s.header.Len() > 0 {
			s.header = metadata.Join(s.header, md)
		} else {
			s.header = md
		}
	}
	md = s.header
	headerFields := make([]hpack.HeaderField, 0, 2)  
	headerFields = append(headerFields, hpack.HeaderField{Name: ":status", Value: "200"})
	headerFields = append(headerFields, hpack.HeaderField{Name: "content-type", Value: contentType(s.contentSubtype)})
	if s.sendCompress != "" {
		headerFields = append(headerFields, hpack.HeaderField{Name: "grpc-encoding", Value: s.sendCompress})
	}
	for k, vv := range md {
		if isReservedHeader(k) {
			continue
		}
		for _, v := range vv {
			headerFields = append(headerFields, hpack.HeaderField{Name: k, Value: encodeMetadataHeader(k, v)})
		}
	}
	t.controlBuf.put(&headerFrame{
		streamID:  s.id,
		hf:        headerFields,
		endStream: false,
		onWrite: func() {
			atomic.StoreUint32(&t.resetPingStrikes, 1)
		},
		wq: s.wq,
	})
	if t.stats != nil {
		outHeader := &stats.OutHeader{}
		t.stats.HandleRPC(s.Context(), outHeader)
	}
	return nil
}
func (t *http2Server) WriteStatus(s *Stream, st *status.Status) error {
	if !s.headerOk && s.header.Len() > 0 {
		if err := t.WriteHeader(s, nil); err != nil {
			return err
		}
	} else {
		if s.getState() == streamDone {
			return nil
		}
	}
	headerFields := make([]hpack.HeaderField, 0, 2)  
	if !s.headerOk {
		headerFields = append(headerFields, hpack.HeaderField{Name: ":status", Value: "200"})
		headerFields = append(headerFields, hpack.HeaderField{Name: "content-type", Value: contentType(s.contentSubtype)})
	}
	headerFields = append(headerFields, hpack.HeaderField{Name: "grpc-status", Value: strconv.Itoa(int(st.Code()))})
	headerFields = append(headerFields, hpack.HeaderField{Name: "grpc-message", Value: encodeGrpcMessage(st.Message())})
	if p := st.Proto(); p != nil && len(p.Details) > 0 {
		stBytes, err := proto.Marshal(p)
		if err != nil {
			panic(err)
		}
		headerFields = append(headerFields, hpack.HeaderField{Name: "grpc-status-details-bin", Value: encodeBinHeader(stBytes)})
	}
	for k, vv := range s.trailer {
		if isReservedHeader(k) {
			continue
		}
		for _, v := range vv {
			headerFields = append(headerFields, hpack.HeaderField{Name: k, Value: encodeMetadataHeader(k, v)})
		}
	}
	trailer := &headerFrame{
		streamID:  s.id,
		hf:        headerFields,
		endStream: true,
		onWrite: func() {
			atomic.StoreUint32(&t.resetPingStrikes, 1)
		},
	}
	t.closeStream(s, false, 0, trailer)
	if t.stats != nil {
		t.stats.HandleRPC(s.Context(), &stats.OutTrailer{})
	}
	return nil
}
func (t *http2Server) Write(s *Stream, hdr []byte, data []byte, opts *Options) error {
	if !s.headerOk {  
		if err := t.WriteHeader(s, nil); err != nil {
			return streamErrorf(codes.Internal, "transport: %v", err)
		}
	} else {
		if s.getState() == streamDone {
			s.cancel()
			select {
			case <-t.ctx.Done():
				return ErrConnClosing
			default:
			}
			return ContextErr(s.ctx.Err())
		}
	}
	emptyLen := http2MaxFrameLen - len(hdr)
	if emptyLen > len(data) {
		emptyLen = len(data)
	}
	hdr = append(hdr, data[:emptyLen]...)
	data = data[emptyLen:]
	df := &dataFrame{
		streamID: s.id,
		h:        hdr,
		d:        data,
		onEachWrite: func() {
			atomic.StoreUint32(&t.resetPingStrikes, 1)
		},
	}
	if err := s.wq.get(int32(len(hdr) + len(data))); err != nil {
		select {
		case <-t.ctx.Done():
			return ErrConnClosing
		default:
		}
		return ContextErr(s.ctx.Err())
	}
	return t.controlBuf.put(df)
}