go gRPC 客户端内存暴涨原因分析
创建一个 gRPC 客户端连接，会创建的几个协程：
1）transport.loopyWriter.run 往服务端发送数据协程，流控时会阻塞，结果是数据堆积，内存上涨
2）transport.http2Client.reader 接收服务端数据协程，并会调用 t.controlBuf.throttle() 执行流控
现象描述：
客户端到服务端单个连接，压测时内存快速增长，直到 OOM 挂掉。在 OOM 之前停止压测，内存会逐渐下降。客户端到服务端改为两个连接时，压测时未出现内存快速增长。
问题原因：
每一个 gRPC 连接均有一个独立的队列，挂在该连接的所有 streams 共享，请求相当于生产，往服务端发送请求相当于消费，当生产速度大于消费速度时，就会出现内存持续上长。该队列没有长度限制，所以会持续上长。快速上涨的原因是协程 transport.loopyWriter).run 没有被调度运行，队列消费停止，导致队列只增不减。停止压测后，协程 transport.loopyWriter).run 会恢复执行。
当不再消费时，可观察到大量如下协程：
grpc/internal/transport.(*Stream).waitOnHeader (0x90c8d5)
runtime/netpoll.go:220 internal/poll.runtime_pollWait (0x46bdd5)
使用 netstat 命令可观察到发送队列大量堆积。
解决方案：
控制生产速度，即控制单个 gRPC 客户端连接发送的请求数量。此外，还可以启用客户端的 keepalive 关闭连接。
后话
go gRPC 如果提供取 controlBuffer 的队列 list 的大小接口，可使得更为简单和友好。
相关源码：
- http2Client
// 源码所在文件：google.golang.org/grpc/http2_client.go
// http2Client 实现了接口 ClientTransport
// http2Client implements the ClientTransport interface with HTTP2.
type http2Client struct {
conn net.Conn // underlying communication channel
loopy *loopyWriter // 生产和消费关联的队列在这里面，所在文件：controlbuf.go
// controlBuf delivers all the control related tasks (e.g., window
// updates, reset streams, and various settings) to the controller.
controlBuf *controlBuffer // 所在文件：controlbuf.go
maxConcurrentStreams uint32
streamQuota int64
streamsQuotaAvailable chan struct{}
waitingStreams uint32
initialWindowSize int32
}
type controlBuffer struct {
list *itemList // 队列
}
type loopyWriter struct {
// 关联上 controlBuffer，
// 消费 controlBuffer 中的队列 list，
// 生产由 http2Client 通过 controlBuffer 进行。
cbuf *controlBuffer
}
- 一个 gRPC 客户端连接被创建时，即会创建一个 run 协程，run 协程为队列的消费者
// 源码所在文件：internal/transport/http2_client.go
// 所在包名：transport
// 打断点方法：
// (dlv) b transport.newHTTP2Client
// 被调用：协程 grpc.addrConn.resetTransport
func newHTTP2Client(connectCtx, ctx context.Context, addr resolver.Address, opts ConnectOptions, onPrefaceReceipt func(), onGoAway func(GoAwayReason), onClose func()) (_ *http2Client, err error) {
// 建立连接，注意不同于 grpc.Dial，
// grpc.Dial 实际不包含连接，对于 block 调用也只是等待连接状态为 Ready 。
// transport.dial 的实现调用了 net.Dialer.DialContext，
// 而 net.Dialer.DialContext 是更底层 Go 自带包的组成部分，不是 gRPC 的组成部分。
// net.Dialer.DialContext 的实现支持：TCP、UDP、Unix等：。
conn, err := dial(connectCtx, opts.Dialer, addr.Addr)
t.controlBuf = newControlBuffer(t.ctxDone) // 含发送队列的初始化
if t.keepaliveEnabled {
t.kpDormancyCond = sync.NewCond(&t.mu)
go t.keepalive() // 保活协程
}
// Start the reader goroutine for incoming message. 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()
// Send connection preface to server.
n, err := t.conn.Write(clientPreface)
go func() {
t.loopy = newLoopyWriter(clientSide, t.framer, t.controlBuf, t.bdpEst)
err := t.loopy.run()
}
}
0 0x00000000008f305b in google.golang.org/grpc/internal/transport.newHTTP2Client
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/internal/transport/http2_client.go:166
1 0x00000000009285a8 in google.golang.org/grpc/internal/transport.NewClientTransport
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/internal/transport/transport.go:577
2 0x00000000009285a8 in google.golang.org/grpc.(*addrConn).createTransport
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/clientconn.go:1297
3 0x0000000000927e48 in google.golang.org/grpc.(*addrConn).tryAllAddrs
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/clientconn.go:1227
// 下列的 grpc.addrConn.resetTransport 是一个协程
4 0x000000000092737f in google.golang.org/grpc.(*addrConn).resetTransport
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/clientconn.go:1142
5 0x0000000000471821 in runtime.goexit
at /usr/local/go/src/runtime/asm_amd64.s:1374
// 源码所在文件：grpc/clientconn.go
// 所在包名：grpc
// 被调用：grpc.addrConn.getReadyTransport
func (ac *addrConn) connect() error {
// Start a goroutine connecting to the server asynchronously.
go ac.resetTransport()
}
// 传统类型的 RPC 调用从 grpc.ClientConn.Invoke 开始：
// XXX.pb.go // 编译 .proto 生成的文件
// -> main.helloServiceClient.Hello
// -> grpc.ClientConn.Invoke // 在 call.go 中，如果是 stream RPC，则从调用 grpc.ClientConn.NewStream 开始
// -> grpc.invoke // 在 call.go 中
// -> grpc.newClientStream // 在 stream.go 中
// -> grpc.clientStream.newAttemptLocked // 在 stream.go 中
// -> grpc.ClientConn.getTransport // 在 clientconn.go 中
// -> grpc.pickerWrapper.pick // 在 picker_wrapper.go 中
// -> grpc.addrConn.getReadyTransport
// -> grpc.addrConn.connect // 创建协程 resetTransport
// -> grpc.addrConn.resetTransport // ***是一个协程***
// -> grpc.addrConn.tryAllAddrs
// -> grpc.addrConn.createTransport // 在clientconn.go 中
// -> transport.NewClientTransport // 在 transport.go 中
// -> transport.newHTTP2Client
// -> transport.dial
// -> net.Dialer.DialContext // net 为 Go 自带包，不是 gRPC 包
// -> net.sysDialer.dialSerial
// -> net.sysDialer.dialSingle
// -> net.sysDialer.dialTCP/dialUDP/dialUnix/dialIP
// -> net.sysDialer.doDialTCP // 以 dialTCP 为例
// -> net.internetSocket // 从这开始，和 C 语言的使用类似了，只不过包装了不同平台的
// -> net.socket
// -> net.sysSocket
//
// stream 类型的 RPC 从 NewStream 开始：
// grpc.newClientStream 除被 grpc.invoke 调用外，还会被 stream.go 中的 grpc.ClientConn.NewStream 直接调用
// XXX.pb.go // 编译 .proto 生成的文件
// -> grpc.ClientConn.NewStream // 在 stream.go 中
// -> grpc.newClientStream // 在 stream.go 中
// -> 从这开始同上述流程
// 源码所在文件：grpc/clientconn.go
// 所在包名：grpc
// 被调用：调用源头为 grpc.ClientConn.NewStream，其实是 grpc.newClientStream 。
// getReadyTransport returns the transport if ac's state is READY.
// Otherwise it returns nil, false.
// If ac's state is IDLE, it will trigger ac to connect.
func (ac *addrConn) getReadyTransport() (transport.ClientTransport, bool) {
// Trigger idle ac to connect.
if idle {
ac.connect()
}
}
- 消费协程 run 相关源代码摘要
// 源码所在文件：internal/transport/controlbuf.go
// 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 {
// cbuf 维护了队列（list *itemList），
// 如果不加控制，就会导致内存大涨。
cbuf *controlBuffer
sendQuota uint32
// 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
}
// 源码所在文件：internal/transport/controlbuf.go
func (l *loopyWriter) run() (err error) {
// 通过 get 间接调用 dequeue 和 dequeueAll
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
}
}
}
func (c *controlBuffer) get(block bool) (interface{}, error) {
for {
c.mu.Lock() // 队列操作需要加锁保护
......
// 消费队列（出队）
h := c.list.dequeue().(cbItem)
......
if !block {
c.mu.Unlock()
return nil, nil
}
// 阻塞
c.consumerWaiting = true
c.mu.Unlock()
select {
case <-c.ch: // 对应 executeAndPut 中唤醒的：c.ch <- struct{}
case <-c.done:
c.finish() // 清空队列
return nil, ErrConnClosing // indicates that the transport is closing
}
}
}
func (c *controlBuffer) finish() {
......
// 清空队列
for head := c.list.dequeueAll(); head != nil; head = head.next {
......
}
- 特别说明
每一次 gRPC 调用，客户端均会创建一个新的 Stream，
该特性使得同一 gRPC 连接可以同时处理多个调用。请求的发送并不是同步的，而是基于队列的异步发送。
每一个 gRPC 客户端连接均有一个自己的队列，gRPC 并没有直接限定队列大小，所以如果不加任何限制则会内存暴涨，直到 OOM 发生。
- 生产者：发起调用的客户端*
message HelloReq { // 请求
string text = 1;
}
message HelloRes { // 响应
string text = 1;
}
service HelloService {
rpc Hello(HelloReq) returns (HelloRes) {}
}
grpcClient := grpc.Dial(endpoint, opts)
helloClient := NewHelloServiceClient(grpcClient)
// Hello 调用为生产源头
res, err := helloClient.Hello(ctx, &req)
// Hello 的实现，为 protoc 编译生成的代码
func (c *helloServiceClient) Hello(ctx context.Context, in *HelloReq, opts ...grpc.CallOption) (*HelloRes, error) {
out := new(HelloRes)
err := c.cc.Invoke(ctx, "/main.HelloService/Hello", in, out, opts...)
if err != nil {
return nil, err
}
return out, nil
}
// 源码所在文件：google.golang.org/grpc/call.go
func (cc *ClientConn) Invoke(ctx context.Context, method string, args, reply interface{}, opts ...CallOption) error {
// allow interceptor to see all applicable call options, which means those
// configured as defaults from dial option as well as per-call options
opts = combine(cc.dopts.callOptions, opts)
if cc.dopts.unaryInt != nil {
return cc.dopts.unaryInt(ctx, method, args, reply, cc, invoke, opts...)
}
// 转调用私有的 invoke 函数
return invoke(ctx, method, args, reply, cc, opts...)
}
// 源码所在文件：google.golang.org/grpc/call.go
func invoke(ctx context.Context, method string, req, reply interface{}, cc *ClientConn, opts ...CallOption) error {
// newClientStream 间接往队列中生产消息
// pprof 显示 newClientStream 调用的 withRetry 占用内存大头
cs, err := newClientStream(ctx, unaryStreamDesc, cc, method, opts...)
if err != nil {
return err
}
if err := cs.SendMsg(req); err != nil {
return err
}
return cs.RecvMsg(reply)
}
// 源码所在文件：google.golang.org/grpc/stream.go
// 设置断点：(dlv) b clientStream.SendMsg
func (cs *clientStream) SendMsg(m interface{}) (err error) {
}
// 源码所在文件：google.golang.org/grpc/stream.go
// pprof 显示 newClientStream 消耗太多内存，而这又发生在其调用的 withRetry 中
func newClientStream(ctx context.Context, desc *StreamDesc, cc *ClientConn, method string, opts ...CallOption) (_ ClientStream, err error) {
// 问题出在 newStream 分配的内存越来越多，
// 但并非严格的泄漏，只是不断积累，但压力下来后会缓慢释放。
// newStream 的实现是调用 NewStream：
// cs.callHdr.PreviousAttempts = cs.numRetries
// s, err := a.t.NewStream(cs.ctx, cs.callHdr)
// cs.attempt.s = s
// 这里 a 的类型为 csAttempt：
// implements a single transport stream attempt within a clientStream
op := func(a *csAttempt) error { return a.newStream() }
// 内存问题所在：withRetry，进一步内存发生在非直接调用的：NewStream
if err := cs.withRetry(op, func() { cs.bufferForRetryLocked(0, op) }); err != nil {
cs.finish(err)
return nil, err
}
}
// 源码所在文件：google.golang.org/grpc/stream.go
func (cs *clientStream) withRetry(op func(a *csAttempt) error, onSuccess func()) error {
for { // 循环，内存上涨，这儿并没有循环
// 调用 newStream，
// 这里间接往队列中生产消息。
err := op(a)
}
}
// 源码所在文件：google.golang.org/grpc/stream.go
func (a *csAttempt) newStream() error {
cs := a.cs
cs.callHdr.PreviousAttempts = cs.numRetries
// 下列的 t 类型为：transport.ClientTransport，
// 但注意 transport.ClientTransport 是一个 interface，并不是 struct。
// 而 http2Client 是一个针对 ClientTransport 接口的实现。
s, err := a.t.NewStream(cs.ctx, cs.callHdr)
}
// 源码所在文件：google.golang.org/grpc/internal/transport/http2_client.go
// NewStream creates a stream and registers it into the transport as "active"
// streams.
func (t *http2Client) NewStream(ctx context.Context, callHdr *CallHdr) (_ *Stream, err error) {
// 内存上涨，是因为队列中存了大量的 headerFields 和 s
headerFields, err := t.createHeaderFields(ctx, callHdr)
s := t.newStream(ctx, callHdr)
// hdr 聚合了 headerFields 和 s
hdr := &headerFrame{
hf: headerFields,
initStream: func(id uint32) error {
t.activeStreams[id] = s
},
wq: s.wq,
}
for {
// 调用 executeAndPut 入队（生产）
// 内存上涨，是因为队列中存了大量的 hdr 。
success, err := t.controlBuf.executeAndPut(func(it interface{}) bool {
}, hdr)
}
}
// 源码所在文件：google.golang.org/grpc/internal/transport/controlbuf.go
func (c *controlBuffer) executeAndPut(f func(it interface{}) bool, it cbItem) (bool, error) {
// 入队操作（生产）
// 当入队快于出队（消费）时，就会出现内存上涨。
c.list.enqueue(it)
if it.isTransportResponseFrame() { // 调用接口 cbItem 定义的方法
// counts the number of queued items that represent the response of an action initiated by the peer
// 变量 transportResponseFrames 记录了队列大小
c.transportResponseFrames++
}
}
// 源码所在文件：google.golang.org/grpc/internal/transport/controlbuf.go
func (c *controlBuffer) put(it cbItem) error {
// 入队操作（生产）
_, err := c.executeAndPut(nil, it)
return err
}
type cbItem interface {
isTransportResponseFrame() bool
}
func (*registerStream) isTransportResponseFrame() bool { return false }
func (h *headerFrame) isTransportResponseFrame() bool {
return h.cleanup != nil && h.cleanup.rst // Results in a RST_STREAM
}
func (c *cleanupStream) isTransportResponseFrame() bool { return c.rst } // Results in a RST_STREAM
func (*dataFrame) isTransportResponseFrame() bool { return false }
func (*incomingWindowUpdate) isTransportResponseFrame() bool { return false }
func (*outgoingWindowUpdate) isTransportResponseFrame() bool {
return false // window updates are throttled by thresholds
}
func (*incomingSettings) isTransportResponseFrame() bool { return true } // Results in a settings ACK
func (*outgoingSettings) isTransportResponseFrame() bool { return false }
func (*incomingGoAway) isTransportResponseFrame() bool { return false }
func (*goAway) isTransportResponseFrame() bool { return false }
func (*ping) isTransportResponseFrame() bool { return true }
func (*outFlowControlSizeRequest) isTransportResponseFrame() bool { return false }
- **协程 transport.loopyWriter.run **
0 0x000000000043bfa5 in runtime.gopark
at /usr/local/go/src/runtime/proc.go:307
1 0x000000000044c10f in runtime.selectgo
at /usr/local/go/src/runtime/select.go:338
一个连接只有一个 run 协程
2 0x00000000008eca2f in google.golang.org/grpc/internal/transport.(*controlBuffer).get
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/internal/transport/controlbuf.go:417
3 0x00000000008ed76e in google.golang.org/grpc/internal/transport.(*loopyWriter).run
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/internal/transport/controlbuf.go:544
4 0x000000000090f13b in google.golang.org/grpc/internal/transport.newHTTP2Client.func3
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/internal/transport/http2_client.go:356
5 0x0000000000471821 in runtime.goexit
at /usr/local/go/src/runtime/asm_amd64.s:1374
- 协程 transport.controlBuffer.throttle
- 协程 transport.http2Client.reader
0 0x000000000043bfa5 in runtime.gopark
at /usr/local/go/src/runtime/proc.go:307
1 0x000000000044c10f in runtime.selectgo
at /usr/local/go/src/runtime/select.go:338
2 0x00000000008ec335 in google.golang.org/grpc/internal/transport.(*controlBuffer).throttle
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/internal/transport/controlbuf.go:319
文件 http2_client.go 函数 transport.newHTTP2Client 会创建协程 reader
3 0x00000000008fdadd in google.golang.org/grpc/internal/transport.(*http2Client).reader
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/internal/transport/http2_client.go:1293
4 0x0000000000471821 in runtime.goexit
at /usr/local/go/src/runtime/asm_amd64.s:1374
- 协程 transport.Stream.waitOnHeader
// 大量 waitOnHeader 协程
func (s *Stream) waitOnHeader() {
if s.headerChan == nil {
// On the server headerChan is always nil since a stream originates
// only after having received headers.
return
}
select {
case <-s.ctx.Done():
// Close the stream to prevent headers/trailers from changing after
// this function returns.
s.ct.CloseStream(s, ContextErr(s.ctx.Err()))
// headerChan could possibly not be closed yet if closeStream raced
// with operateHeaders; wait until it is closed explicitly here.
<-s.headerChan
case <-s.headerChan:
}
}
// 实为调用协程
0 0x000000000043bfa5 in runtime.gopark
at /usr/local/go/src/runtime/proc.go:307
1 0x000000000044c10f in runtime.selectgo
at /usr/local/go/src/runtime/select.go:338
2 0x000000000090c8d5 in google.golang.org/grpc/internal/transport.(*Stream).waitOnHeader
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/internal/transport/transport.go:321
3 0x0000000000942805 in google.golang.org/grpc/internal/transport.(*Stream).RecvCompress
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/internal/transport/transport.go:336
4 0x0000000000942805 in google.golang.org/grpc.(*csAttempt).recvMsg
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/stream.go:894
5 0x000000000094ad06 in google.golang.org/grpc.(*clientStream).RecvMsg.func1
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/stream.go:759
6 0x000000000094057c in google.golang.org/grpc.(*clientStream).withRetry
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/stream.go:617
7 0x0000000000941505 in google.golang.org/grpc.(*clientStream).RecvMsg
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/stream.go:758
8 0x0000000000921d3b in google.golang.org/grpc.invoke
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/call.go:73
9 0x0000000000921ad3 in google.golang.org/grpc.(*ClientConn).Invoke
at /root/go/pkg/mod/google.golang.org/grpc@v1.33.2/call.go:37
10 0x0000000000b185f4 in /root/hello/grpc/proto.(*HelloClient).Call
at /root/hello/hello.pb.go:70
- 协程 poll_runtime_pollWait
// poll_runtime_pollWait, which is internal/poll.runtime_pollWait,
// waits for a descriptor to be ready for reading or writing,
// according to mode, which is 'r' or 'w'.
// This returns an error code; the codes are defined above.
//go:linkname poll_runtime_pollWait internal/poll.runtime_pollWait
func poll_runtime_pollWait(pd *pollDesc, mode int) int {
errcode := netpollcheckerr(pd, int32(mode))
if errcode != pollNoError {
return errcode
}
// As for now only Solaris, illumos, and AIX use level-triggered IO.
if GOOS == "solaris" || GOOS == "illumos" || GOOS == "aix" {
netpollarm(pd, mode)
}
for !netpollblock(pd, int32(mode), false) {
errcode = netpollcheckerr(pd, int32(mode))
if errcode != pollNoError {
return errcode
}
// Can happen if timeout has fired and unblocked us,
// but before we had a chance to run, timeout has been reset.
// Pretend it has not happened and retry.
}
return pollNoError
}
函数 runtime.gopark 用于协程的切换
0 0x000000000043bfa5 in runtime.gopark
at /usr/local/go/src/runtime/proc.go:307
1 0x000000000043447b in runtime.netpollblock
at /usr/local/go/src/runtime/netpoll.go:436
2 0x000000000046bdd5 in internal/poll.runtime_pollWait
at /usr/local/go/src/runtime/netpoll.go:220
3 0x00000000004d9685 in internal/poll.(*pollDesc).wait
at /usr/local/go/src/internal/poll/fd_poll_runtime.go:87
4 0x00000000004da6c5 in internal/poll.(*pollDesc).waitRead
at /usr/local/go/src/internal/poll/fd_poll_runtime.go:92
5 0x00000000004da6c5 in internal/poll.(*FD).Read
at /usr/local/go/src/internal/poll/fd_unix.go:159
6 0x00000000005327af in net.(*netFD).Read
at /usr/local/go/src/net/fd_posix.go:55
7 0x000000000054688e in net.(*conn).Read
at /usr/local/go/src/net/net.go:182
8 0x00000000006e14b8 in net/http.(*connReader).backgroundRead
at /usr/local/go/src/net/http/server.go:690
9 0x0000000000471821 in runtime.goexit
at /usr/local/go/src/runtime/asm_amd64.s:1374
- 队列
type itemList struct {
head *itemNode
tail *itemNode
}
type itemNode struct {
it interface{}
next *itemNode
}
// 入队（生产）
//
// 从这可看到，
// 队列没有大小限制，生产（入队）不受限，
// 所以一旦生产速度大于消费速度，就会出现堆积导致内存上涨。
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
}
// 出队（消费）
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
}
// 清空（消费），直接丢弃了
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
}
// 源码所在文件：internal/transport/controlbuf.go
// transport.loopyWriter.run
// -> transport.loopyWriter.handle
// -> transport.loopyWriter.headerHandler
// -> transport.loopyWriter.writeHeader // 阻塞在这了
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) // 阻塞在这了
......
}
// 源码所在文件：internal/transport/controlbuf.go
func (l *loopyWriter) writeHeader(streamID uint32, endStream bool, hf []hpack.HeaderField, onWrite func()) error {
// 阻塞在这儿：
// 结构体 frame 定义在 internal/transport/http_util.go 文件中，
// 成员 fr 的类型为 http2.Framer，定义在 x/net/http2/frame.go 文件中
err = l.framer.fr.WriteHeaders(http2.HeadersFrameParam{
})
}
