socket，connect，listen，getsockopt 都有一个全局函数变量来表示。
hook_unix.go :
// Placeholders for socket system calls.
socketFunc func(int, int, int) (int, error) = syscall.Socket
connectFunc func(int, syscall.Sockaddr) error = syscall.Connect
listenFunc func(int, int) error = syscall.Listen
getsockoptIntFunc func(int, int, int) (int, error) = syscall.GetsockoptInt这些 hook 主要是为了能够写测试，在测试代码中，socketFunc，connectFunc ... 都会被替换成测试专用函数，main_unix_test.go:
func installTestHooks() {
socketFunc = sw.Socket
poll.CloseFunc = sw.Close
connectFunc = sw.Connect
listenFunc = sw.Listen
poll.AcceptFunc = sw.Accept
getsockoptIntFunc = sw.GetsockoptInt
for _, fn := range extraTestHookInstallers {
fn()
}
}用这种全局函数 hook，或者叫注册表的方式，可以实现类似于面向对象中的 interface 功能。不过因为不同平台提供的网络编程函数差别有些大，所以这里这些全局网络函数也就只是用来方便测试。
// 整体的 network poller(平台无关部分)
// 具体的实现需要定义下面这些函数:
// func netpollinit() // 初始化 poller
// func netpollopen(fd uintptr, pd *pollDesc) int32 // 用来装备边缘触发的通知
// 并且将 fd 和 pd 做好关联
// 具体实现中，必须调用 下面的函数来表示 pd 已经 ready
// func netpollready(gpp **g, pd *pollDesc, mode int32)
// pollDesc 包含两个二元信号量, rg 和 wg, 分别用来 park 读、写的 goroutine
// 信号量可以是下面几种状态:
// pdReady - io readiness notification is pending;
// a goroutine consumes the notification by changing the state to nil.
// pdWait - a goroutine prepares to park on the semaphore, but not yet parked;
// the goroutine commits to park by changing the state to G pointer,
// or, alternatively, concurrent io notification changes the state to READY,
// or, alternatively, concurrent timeout/close changes the state to nil.
// G pointer - the goroutine is blocked on the semaphore;
// io notification or timeout/close changes the state to READY or nil respectively
// and unparks the goroutine.
// nil - nothing of the above.
// wg 和 rg 这两个字段用法实在有点特殊
// 既会被用来当作状态值，又会被用来存储被 park 的 g 列表
// 如果没有 g 在等，也没事件，那就是 0
// 官方这里说的 nil 其实就是 0
const (
pdReady uintptr = 1
pdWait uintptr = 2
)
const pollBlockSize = 4 * 1024
// Network file descriptor.
type netFD struct {
pfd poll.FD
// 下面这些元素在 Close 之前都是不可变的
family int
sotype int
isConnected bool
net string
laddr Addr
raddr Addr
}
// FD 是对 file descriptor 的一个包装，内部的 Sysfd 就是 linux 下的
// file descriptor。net 和 os 包中使用这个类型来代表一个网络连接或者一个 OS 文件
type FD struct {
// 对 sysfd 加锁，以使 Read 和 Write 方法串行执行
fdmu fdMutex
// 操作系统的 file descriptor。在关闭之前是不可变的
Sysfd int
// I/O poller.
pd pollDesc
// Writev cache.
iovecs *[]syscall.Iovec
// Semaphore signaled when file is closed.
csema uint32
// 不可变。表示当前这个 fd 是否是一个流，或者是一个基于包的 fd
// 用来区分是 TCP 还是 UDP
IsStream bool
// Whether a zero byte read indicates EOF. This is false for a
// message based socket connection.
ZeroReadIsEOF bool
// 这个 fd 表示的是不是一个文件，如果 false 的话就是一个 network socket
isFile bool
// 这个文件是否被设置为了 blocking 模式
isBlocking bool
}
// Network poller descriptor.
//
// No heap pointers.
//
//go:notinheap
type pollDesc struct {
link *pollDesc // in pollcache, protected by pollcache.lock
// The lock protects pollOpen, pollSetDeadline, pollUnblock and deadlineimpl operations.
// This fully covers seq, rt and wt variables. fd is constant throughout the PollDesc lifetime.
// pollReset, pollWait, pollWaitCanceled and runtime·netpollready (IO readiness notification)
// proceed w/o taking the lock. So closing, rg, rd, wg and wd are manipulated
// in a lock-free way by all operations.
// NOTE(dvyukov): the following code uses uintptr to store *g (rg/wg),
// that will blow up when GC starts moving objects.
lock mutex // protects the following fields
fd uintptr
closing bool
seq uintptr // protects from stale timers and ready notifications
rg uintptr // pdReady, pdWait, G waiting for read or nil
rt timer // read deadline timer (set if rt.f != nil)
rd int64 // read deadline
wg uintptr // pdReady, pdWait, G waiting for write or nil
wt timer // write deadline timer
wd int64 // write deadline
user uint32 // user settable cookie
}graph LR
ListenTCP --> listenTCP
listenTCP --> internetSocket
internetSocket --> socket
socket --> listenStream
func ListenTCP(network string, laddr *TCPAddr) (*TCPListener, error) {
switch network {
case "tcp", "tcp4", "tcp6":
default:
return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: laddr.opAddr(), Err: UnknownNetworkError(network)}
}
if laddr == nil {
laddr = &TCPAddr{}
}
ln, err := listenTCP(context.Background(), network, laddr)
if err != nil {
return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: laddr.opAddr(), Err: err}
}
return ln, nil
}func listenTCP(ctx context.Context, network string, laddr *TCPAddr) (*TCPListener, error) {
fd, err := internetSocket(ctx, network, laddr, nil, syscall.SOCK_STREAM, 0, "listen")
if err != nil {
return nil, err
}
return &TCPListener{fd}, nil
}func internetSocket(ctx context.Context, net string, laddr, raddr sockaddr, sotype, proto int, mode string) (fd *netFD, err error) {
if (runtime.GOOS == "windows" || runtime.GOOS == "openbsd" || runtime.GOOS == "nacl") && mode == "dial" && raddr.isWildcard() {
raddr = raddr.toLocal(net)
}
family, ipv6only := favoriteAddrFamily(net, laddr, raddr, mode)
return socket(ctx, net, family, sotype, proto, ipv6only, laddr, raddr)
}// socket returns a network file descriptor that is ready for
// asynchronous I/O using the network poller.
func socket(ctx context.Context, net string, family, sotype, proto int, ipv6only bool, laddr, raddr sockaddr) (fd *netFD, err error) {
s, err := sysSocket(family, sotype, proto)
if err != nil {
return nil, err
}
if err = setDefaultSockopts(s, family, sotype, ipv6only); err != nil {
poll.CloseFunc(s)
return nil, err
}
if fd, err = newFD(s, family, sotype, net); err != nil {
poll.CloseFunc(s)
return nil, err
}
if laddr != nil && raddr == nil {
switch sotype {
// 基于流的协议
case syscall.SOCK_STREAM, syscall.SOCK_SEQPACKET:
if err := fd.listenStream(laddr, listenerBacklog); err != nil {
fd.Close()
return nil, err
}
return fd, nil
// 基于数据报的协议
case syscall.SOCK_DGRAM:
if err := fd.listenDatagram(laddr); err != nil {
fd.Close()
return nil, err
}
return fd, nil
}
}
if err := fd.dial(ctx, laddr, raddr); err != nil {
fd.Close()
return nil, err
}
return fd, nil
}func (fd *netFD) listenStream(laddr sockaddr, backlog int) error {
if err := setDefaultListenerSockopts(fd.pfd.Sysfd); err != nil {
return err
}
if lsa, err := laddr.sockaddr(fd.family); err != nil {
return err
} else if lsa != nil {
// bind()
if err := syscall.Bind(fd.pfd.Sysfd, lsa); err != nil {
return os.NewSyscallError("bind", err)
}
}
// listenFunc 是全局函数值，在 linux 下非测试环境被绑定到 syscall.Listen
if err := listenFunc(fd.pfd.Sysfd, backlog); err != nil {
return os.NewSyscallError("listen", err)
}
if err := fd.init(); err != nil {
return err
}
lsa, _ := syscall.Getsockname(fd.pfd.Sysfd)
fd.setAddr(fd.addrFunc()(lsa), nil)
return nil
}Go 的 listenTCP 一个函数就把 c 网络编程中 socket()，bind()，listen() 三步都完成了。大大减小了用户的心智负担。
这里有一点需要注意，listenStream 虽然提供了 backlog 的参数，但用户层是没有办法通过 Go 的代码来修改 listen 的 backlog 的。
func maxListenerBacklog() int {
fd, err := open("/proc/sys/net/core/somaxconn")
if err != nil {
return syscall.SOMAXCONN
}
defer fd.close()
l, ok := fd.readLine()
if !ok {
return syscall.SOMAXCONN
}
f := getFields(l)
n, _, ok := dtoi(f[0])
if n == 0 || !ok {
return syscall.SOMAXCONN
}
// Linux stores the backlog in a uint16.
// Truncate number to avoid wrapping.
// See issue 5030.
if n > 1<<16-1 {
n = 1<<16 - 1
}
return n
}如上，在 linux 中，如果配置了 /proc/sys/net/core/somaxconn，那么就用这个值，如果没有配置，那么就使用 syscall 中的 SOMAXCONN:
const (
SOMAXCONN = 0x80 // 128
)社区里有很多人吐槽，希望能有手段能修改这个值，不过看起来官方并不打算支持。所以现阶段只能通过修改 /proc/sys/net/core/somaxconn 来修改 listen 的 backlog。
在上面的 listen 流程的 socket 函数中会调用 newFD 来初始化一个 fd。
func newFD(sysfd, family, sotype int, net string) (*netFD, error) {
ret := &netFD{
pfd: poll.FD{
Sysfd: sysfd,
IsStream: sotype == syscall.SOCK_STREAM,
ZeroReadIsEOF: sotype != syscall.SOCK_DGRAM && sotype != syscall.SOCK_RAW,
},
family: family,
sotype: sotype,
net: net,
}
return ret, nil
}在 socket、bind、listen 三连发，都没有出错的情况下，会调用 fd.init():
func (fd *netFD) init() error {
return fd.pfd.Init(fd.net, true)
}// Init initializes the FD. The Sysfd field should already be set.
// This can be called multiple times on a single FD.
// The net argument is a network name from the net package (e.g., "tcp"),
// or "file".
// Set pollable to true if fd should be managed by runtime netpoll.
func (fd *FD) Init(net string, pollable bool) error {
// We don't actually care about the various network types.
if net == "file" {
fd.isFile = true
}
if !pollable {
fd.isBlocking = true
return nil
}
return fd.pd.init(fd)
}func (pd *pollDesc) init(fd *FD) error {
serverInit.Do(runtime_pollServerInit)
ctx, errno := runtime_pollOpen(uintptr(fd.Sysfd))
if errno != 0 {
if ctx != 0 {
runtime_pollUnblock(ctx)
runtime_pollClose(ctx)
}
return syscall.Errno(errno)
}
pd.runtimeCtx = ctx
return nil
}//go:linkname poll_runtime_pollOpen internal/poll.runtime_pollOpen
func poll_runtime_pollOpen(fd uintptr) (*pollDesc, int) {
pd := pollcache.alloc()
lock(&pd.lock)
if pd.wg != 0 && pd.wg != pdReady {
throw("runtime: blocked write on free polldesc")
}
if pd.rg != 0 && pd.rg != pdReady {
throw("runtime: blocked read on free polldesc")
}
pd.fd = fd
pd.closing = false
pd.seq++
pd.rg = 0
pd.rd = 0
pd.wg = 0
pd.wd = 0
unlock(&pd.lock)
var errno int32
errno = netpollopen(fd, pd)
return pd, int(errno)
}每一个 fd 对会都应一个 pollDesc 结构，可以看到有 pollcache 提供一定程度的复用。
func netpollopen(fd uintptr, pd *pollDesc) int32 {
var ev epollevent
ev.events = _EPOLLIN | _EPOLLOUT | _EPOLLRDHUP | _EPOLLET
*(**pollDesc)(unsafe.Pointer(&ev.data)) = pd
return -epollctl(epfd, _EPOLL_CTL_ADD, int32(fd), &ev)
}pollDesc 初始化好之后，会当作 epoll event 的数据存储到 ev.data 中。 当有事件就续时，会取 ev.data，以判断是哪个 fd 可读/可写。
type conn struct 是在 accept 阶段赋值给 type Conn interface 的。
// Accept implements the Accept method in the Listener interface; it
// waits for the next call and returns a generic Conn.
func (l *TCPListener) Accept() (Conn, error) {
if !l.ok() {
return nil, syscall.EINVAL
}
c, err := l.accept()
if err != nil {
return nil, &OpError{Op: "accept", Net: l.fd.net, Source: nil, Addr: l.fd.laddr, Err: err}
}
return c, nil
}func (ln *TCPListener) accept() (*TCPConn, error) {
fd, err := ln.fd.accept()
if err != nil {
return nil, err
}
return newTCPConn(fd), nil
}func newTCPConn(fd *netFD) *TCPConn {
c := &TCPConn{conn{fd}}
setNoDelay(c.fd, true)
return c
}func (fd *netFD) accept() (netfd *netFD, err error) {
d, rsa, errcall, err := fd.pfd.Accept()
if err != nil {
if errcall != "" {
err = wrapSyscallError(errcall, err)
}
return nil, err
}
if netfd, err = newFD(d, fd.family, fd.sotype, fd.net); err != nil {
poll.CloseFunc(d)
return nil, err
}
if err = netfd.init(); err != nil {
fd.Close()
return nil, err
}
lsa, _ := syscall.Getsockname(netfd.pfd.Sysfd)
netfd.setAddr(netfd.addrFunc()(lsa), netfd.addrFunc()(rsa))
return netfd, nil
}// Accept wraps the accept network call.
func (fd *FD) Accept() (int, syscall.Sockaddr, string, error) {
if err := fd.readLock(); err != nil {
return -1, nil, "", err
}
defer fd.readUnlock()
if err := fd.pd.prepareRead(fd.isFile); err != nil {
return -1, nil, "", err
}
for {
s, rsa, errcall, err := accept(fd.Sysfd)
if err == nil {
return s, rsa, "", err
}
switch err {
case syscall.EAGAIN:
if fd.pd.pollable() {
if err = fd.pd.waitRead(fd.isFile); err == nil {
continue
}
}
case syscall.ECONNABORTED:
// This means that a socket on the listen
// queue was closed before we Accept()ed it;
// it's a silly error, so try again.
continue
}
return -1, nil, errcall, err
}
}// Wrapper around the accept system call that marks the returned file
// descriptor as nonblocking and close-on-exec.
func accept(s int) (int, syscall.Sockaddr, string, error) {
ns, sa, err := Accept4Func(s, syscall.SOCK_NONBLOCK|syscall.SOCK_CLOEXEC)
// On Linux the accept4 system call was introduced in 2.6.28
// kernel and on FreeBSD it was introduced in 10 kernel. If we
// get an ENOSYS error on both Linux and FreeBSD, or EINVAL
// error on Linux, fall back to using accept.
switch err {
case nil:
return ns, sa, "", nil
default: // errors other than the ones listed
return -1, sa, "accept4", err
case syscall.ENOSYS: // syscall missing
case syscall.EINVAL: // some Linux use this instead of ENOSYS
case syscall.EACCES: // some Linux use this instead of ENOSYS
case syscall.EFAULT: // some Linux use this instead of ENOSYS
}
// See ../syscall/exec_unix.go for description of ForkLock.
// It is probably okay to hold the lock across syscall.Accept
// because we have put fd.sysfd into non-blocking mode.
// However, a call to the File method will put it back into
// blocking mode. We can't take that risk, so no use of ForkLock here.
ns, sa, err = AcceptFunc(s)
if err == nil {
syscall.CloseOnExec(ns)
}
if err != nil {
return -1, nil, "accept", err
}
if err = syscall.SetNonblock(ns, true); err != nil {
CloseFunc(ns)
return -1, nil, "setnonblock", err
}
return ns, sa, "", nil
}可以看到，最终还是用 syscall 中的 accept4 或 accept 完成了系统调用。accept4 对比 accept 的优势是，可以通过一次系统调用完成 accept 和 nonblock flag 的两个目的。而使用 accept 的话，还要手动 syscall.SetNonblock。
func (c *conn) ok() bool { return c != nil && c.fd != nil }
// Implementation of the Conn interface.
// Read implements the Conn Read method.
func (c *conn) Read(b []byte) (int, error) {
if !c.ok() {
return 0, syscall.EINVAL
}
n, err := c.fd.Read(b)
if err != nil && err != io.EOF {
err = &OpError{Op: "read", Net: c.fd.net, Source: c.fd.laddr, Addr: c.fd.raddr, Err: err}
}
return n, err
}func (fd *netFD) Read(buf []byte) (int, error) {
n, err := fd.pfd.Read(buf)
runtime.KeepAlive(fd)
return n, wrapSyscallError("wsarecv", err)
}// Read implements io.Reader.
func (fd *FD) Read(p []byte) (int, error) {
if err := fd.readLock(); err != nil {
return 0, err
}
defer fd.readUnlock()
if len(p) == 0 {
return 0, nil
}
if err := fd.pd.prepareRead(fd.isFile); err != nil {
return 0, err
}
if fd.IsStream && len(p) > maxRW {
p = p[:maxRW]
}
for {
// 第一次调用 syscall.Read 之后，如果读到了数据
// 那么直接就返回了
n, err := syscall.Read(fd.Sysfd, p)
if err != nil {
n = 0
// 如果 os 返回 EAGAIN，说明可能暂时没数据
// 判断 fd 是 pollable 的话，说明可以走 poll 流程
if err == syscall.EAGAIN && fd.pd.pollable() {
if err = fd.pd.waitRead(fd.isFile); err == nil {
continue
}
}
}
err = fd.eofError(n, err)
return n, err
}
}func (pd *pollDesc) waitRead(isFile bool) error {
return pd.wait('r', isFile)
}
func (pd *pollDesc) wait(mode int, isFile bool) error {
if pd.runtimeCtx == 0 {
return errors.New("waiting for unsupported file type")
}
res := runtime_pollWait(pd.runtimeCtx, mode)
return convertErr(res, isFile)
}runtime_pollWait 是用 go:linkname 来链接期链接到的函数，实现在 runtime/netpoll.go 中:
//go:linkname poll_runtime_pollWait internal/poll.runtime_pollWait
func poll_runtime_pollWait(pd *pollDesc, mode int) int {
err := netpollcheckerr(pd, int32(mode))
if err != 0 {
return err
}
for !netpollblock(pd, int32(mode), false) {
err = netpollcheckerr(pd, int32(mode))
if err != 0 {
return err
}
// 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 0
}// returns true if IO is ready, or false if timedout or closed
// waitio - wait only for completed IO, ignore errors
func netpollblock(pd *pollDesc, mode int32, waitio bool) bool {
gpp := &pd.rg
if mode == 'w' {
gpp = &pd.wg
}
// set the gpp semaphore to WAIT
for {
old := *gpp
if old == pdReady {
*gpp = 0
return true
}
if old != 0 {
throw("runtime: double wait")
}
if atomic.Casuintptr(gpp, 0, pdWait) {
break
}
}
// need to recheck error states after setting gpp to WAIT
// this is necessary because runtime_pollUnblock/runtime_pollSetDeadline/deadlineimpl
// do the opposite: store to closing/rd/wd, membarrier, load of rg/wg
if waitio || netpollcheckerr(pd, mode) == 0 {
gopark(netpollblockcommit, unsafe.Pointer(gpp), "IO wait", traceEvGoBlockNet, 5)
}
// be careful to not lose concurrent READY notification
old := atomic.Xchguintptr(gpp, 0)
if old > pdWait {
throw("runtime: corrupted polldesc")
}
return old == pdReady
}gopark 将当前 g 挂起，等待就绪事件到达之后再继续执行。
// Write implements the Conn Write method.
func (c *conn) Write(b []byte) (int, error) {
if !c.ok() {
return 0, syscall.EINVAL
}
n, err := c.fd.Write(b)
if err != nil {
err = &OpError{Op: "write", Net: c.fd.net, Source: c.fd.laddr, Addr: c.fd.raddr, Err: err}
}
return n, err
}func (fd *netFD) Write(buf []byte) (int, error) {
n, err := fd.pfd.Write(buf)
runtime.KeepAlive(fd)
return n, wrapSyscallError("wsasend", err)
}// Write implements io.Writer.
func (fd *FD) Write(p []byte) (int, error) {
if err := fd.writeLock(); err != nil {
return 0, err
}
defer fd.writeUnlock()
if err := fd.pd.prepareWrite(fd.isFile); err != nil {
return 0, err
}
var nn int
for {
max := len(p)
if fd.IsStream && max-nn > maxRW {
max = nn + maxRW
}
n, err := syscall.Write(fd.Sysfd, p[nn:max])
if n > 0 {
nn += n
}
if nn == len(p) {
return nn, err
}
if err == syscall.EAGAIN && fd.pd.pollable() {
if err = fd.pd.waitWrite(fd.isFile); err == nil {
continue
}
}
if err != nil {
return nn, err
}
if n == 0 {
return nn, io.ErrUnexpectedEOF
}
}
}内核的写缓冲区满，这里的 syscall.Write 就会返回 EAGAIN。
func (pd *pollDesc) waitWrite(isFile bool) error {
return pd.wait('w', isFile)
}
func (pd *pollDesc) wait(mode int, isFile bool) error {
if pd.runtimeCtx == 0 {
return errors.New("waiting for unsupported file type")
}
res := runtime_pollWait(pd.runtimeCtx, mode)
return convertErr(res, isFile)
}后面的流程就和 Read 完全一致了。
// poll 已经就绪的网络连接
// 返回那些已经可以跑的 goroutine 列表
func netpoll(block bool) *g {
if epfd == -1 {
return nil
}
waitms := int32(-1)
if !block {
waitms = 0
}
var events [128]epollevent
retry:
n := epollwait(epfd, &events[0], int32(len(events)), waitms)
if n < 0 {
if n != -_EINTR {
println("runtime: epollwait on fd", epfd, "failed with", -n)
throw("runtime: netpoll failed")
}
goto retry
}
var gp guintptr
for i := int32(0); i < n; i++ {
ev := &events[i]
if ev.events == 0 {
continue
}
var mode int32
if ev.events&(_EPOLLIN|_EPOLLRDHUP|_EPOLLHUP|_EPOLLERR) != 0 {
mode += 'r'
}
if ev.events&(_EPOLLOUT|_EPOLLHUP|_EPOLLERR) != 0 {
mode += 'w'
}
if mode != 0 {
pd := *(**pollDesc)(unsafe.Pointer(&ev.data))
netpollready(&gp, pd, mode)
}
}
if block && gp == 0 {
goto retry
}
return gp.ptr()
}
// 让 pd 就续，新的可以运行的 goroutine 会 set 到 wg/rg
// May run during STW, so write barriers are not allowed.
//go:nowritebarrier
func netpollready(gpp *guintptr, pd *pollDesc, mode int32) {
var rg, wg guintptr
if mode == 'r' || mode == 'r'+'w' {
rg.set(netpollunblock(pd, 'r', true))
}
if mode == 'w' || mode == 'r'+'w' {
wg.set(netpollunblock(pd, 'w', true))
}
if rg != 0 {
rg.ptr().schedlink = *gpp
*gpp = rg
}
if wg != 0 {
wg.ptr().schedlink = *gpp
*gpp = wg
}
}
// 按照 mode 把 pollDesc 的 wg 或者 rg 捞出来，返回
func netpollunblock(pd *pollDesc, mode int32, ioready bool) *g {
gpp := &pd.rg
if mode == 'w' {
gpp = &pd.wg
}
for {
old := *gpp
if old == pdReady {
return nil
}
if old == 0 && !ioready {
// Only set READY for ioready. runtime_pollWait
// will check for timeout/cancel before waiting.
return nil
}
var new uintptr
if ioready {
new = pdReady
}
if atomic.Casuintptr(gpp, old, new) {
if old == pdReady || old == pdWait {
old = 0
}
return (*g)(unsafe.Pointer(old))
}
}
}三个函数配合完成就续后唤醒对应的 g 的工作，netpollunblock 从 pollDesc 中捞出 rg/wg，netpollready 然后再把所有的 rg/wg 通过 schedlink 串成一个链表。findrunnable 之类需要 g 的场景下，调度器会主动调用 netpoll 函数来寻找是否有已经就绪的网络事件对应的 g。
netpoll 这个函数是平台相关的，实现在对应的 netpoll_epoll、netpoll_kqueue 文件中。
在上面读写流程，syscall.Read 或者 syscall.Write 返回 EAGAIN 时，会挂起当前正在进行这个读/写操作的 g，具体是调用 gopark，并执行 netpollblockcommit，并将 gpp 挂起，netpollblockcommit 比较简单:
func netpollblockcommit(gp *g, gpp unsafe.Pointer) bool {
r := atomic.Casuintptr((*uintptr)(gpp), pdWait, uintptr(unsafe.Pointer(gp)))
if r {
// Bump the count of goroutines waiting for the poller.
// The scheduler uses this to decide whether to block
// waiting for the poller if there is nothing else to do.
atomic.Xadd(&netpollWaiters, 1)
}
return r
}EAGAIN 的时候:
gopark(netpollblockcommit, unsafe.Pointer(gpp), "IO wait", traceEvGoBlockNet, 5)// Puts the current goroutine into a waiting state and calls unlockf.
// If unlockf returns false, the goroutine is resumed.
// unlockf must not access this G's stack, as it may be moved between
// the call to gopark and the call to unlockf.
func gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason string, traceEv byte, traceskip int) {
mp := acquirem()
gp := mp.curg
status := readgstatus(gp)
if status != _Grunning && status != _Gscanrunning {
throw("gopark: bad g status")
}
mp.waitlock = lock
mp.waitunlockf = *(*unsafe.Pointer)(unsafe.Pointer(&unlockf)) // unlockf = netpollblockcommit
gp.waitreason = reason
mp.waittraceev = traceEv
mp.waittraceskip = traceskip
releasem(mp)
// can't do anything that might move the G between Ms here.
mcall(park_m)
}// park continuation on g0.
func park_m(gp *g) {
_g_ := getg()
casgstatus(gp, _Grunning, _Gwaiting)
dropg()
if _g_.m.waitunlockf != nil {
// fn = netpollblockcommit
fn := *(*func(*g, unsafe.Pointer) bool)(unsafe.Pointer(&_g_.m.waitunlockf))
ok := fn(gp, _g_.m.waitlock)
_g_.m.waitunlockf = nil
_g_.m.waitlock = nil
// 原子操作没成功，那还得先继续执行当前的逻辑
// 回到最外面的读/写循环
if !ok {
casgstatus(gp, _Gwaiting, _Grunnable)
execute(gp, true) // Schedule it back, never returns.
}
}
// g 成功挂起，m 成功解绑，当前的 m 进入调度循环，去找其它可以执行的 g
schedule()
}
// 切换到 m->g0 的栈，然后调用 fn(g)
// fn 必须不能返回
// 这个 g 之后应该用 gogo(&g->sched) 来恢复执行
TEXT runtime·mcall(SB), NOSPLIT, $0-8
MOVQ fn+0(FP), DI
get_tls(CX)
MOVQ g(CX), AX // save state in g->sched
MOVQ 0(SP), BX // caller's PC
// 把当前 g 的运行现场保存到 g.gobuf 中
MOVQ BX, (g_sched+gobuf_pc)(AX)
LEAQ fn+0(FP), BX // caller's SP
MOVQ BX, (g_sched+gobuf_sp)(AX)
MOVQ AX, (g_sched+gobuf_g)(AX)
MOVQ BP, (g_sched+gobuf_bp)(AX)
// switch to m->g0 & its stack, call fn
MOVQ g(CX), BX
MOVQ g_m(BX), BX
MOVQ m_g0(BX), SI
CMPQ SI, AX // if g == m->g0 call badmcall
JNE 3(PC)
MOVQ $runtime·badmcall(SB), AX
JMP AX
MOVQ SI, g(CX) // g = m->g0
MOVQ (g_sched+gobuf_sp)(SI), SP // sp = m->g0->sched.sp
PUSHQ AX
MOVQ DI, DX
MOVQ 0(DI), DI
CALL DI
POPQ AX
MOVQ $runtime·badmcall2(SB), AX
JMP AX
RET
// 把 g 当前所在的 m 的指针都清 0
// 将 m 腾出来干别的事去
func dropg() {
_g_ := getg()
setMNoWB(&_g_.m.curg.m, nil)
setGNoWB(&_g_.m.curg, nil)
}上面就是挂起的所有流程了，可见所谓的挂起，就是把 g 当前的运行状态: bp、sp、pc 寄存器，以及 g 的地址保存起来。
至于唤醒流程，当调度器在 findrunnable、startTheWorldWithSema 或者 sysmon 中调用 netpoll 函数时，会获取到上面说的就绪的 g 列表。把这些 g 的 bp/sp/pc 都从 g.gobuf 中恢复出来，就可以继续执行它们的 Read/Write 操作了。
因为调度中有讲，这里就不赘述了。
// 设置底层连接的读超时
// 超时时间是 0 值的话永远都不会超时
func (c *Conn) SetReadDeadline(t time.Time) error {
return c.conn.SetReadDeadline(t)
}
// 设置底层连接的读超时
// 超时时间是 0 值的话永远都不会超时
// 写超时发生之后， TLS 状态会被破坏，未来的所有写都会返回相同的错误
func (c *Conn) SetWriteDeadline(t time.Time) error {
return c.conn.SetWriteDeadline(t)
}// 实现 Conn 接口中的方法
func (c *conn) SetReadDeadline(t time.Time) error {
if !c.ok() {
return syscall.EINVAL
}
if err := c.fd.pfd.SetReadDeadline(t); err != nil {
return &OpError{Op: "set", Net: c.fd.net, Source: nil, Addr: c.fd.laddr, Err: err}
}
return nil
}
// 实现 Conn 接口中的方法
func (c *conn) SetWriteDeadline(t time.Time) error {
if !c.ok() {
return syscall.EINVAL
}
if err := c.fd.pfd.SetWriteDeadline(t); err != nil {
return &OpError{Op: "set", Net: c.fd.net, Source: nil, Addr: c.fd.laddr, Err: err}
}
return nil
}// 设置关联 fd 的读取 deadline
func (fd *FD) SetReadDeadline(t time.Time) error {
return setDeadlineImpl(fd, t, 'r')
}
// 设置关联 fd 的写入 deadline
func (fd *FD) SetWriteDeadline(t time.Time) error {
return setDeadlineImpl(fd, t, 'w')
}func setDeadlineImpl(fd *FD, t time.Time, mode int) error {
diff := int64(time.Until(t))
d := runtimeNano() + diff
if d <= 0 && diff > 0 {
// 如果用户提供了未来的 deadline，但是 delay 计算溢出了，那么设置 dealine 到最大的可能的值
d = 1<<63 - 1
}
if t.IsZero() {
// IsZero reports whether t represents the zero time instant,
// January 1, year 1, 00:00:00 UTC.
// func (t Time) IsZero() bool {
// return t.sec() == 0 && t.nsec() == 0
// }
d = 0
}
if err := fd.incref(); err != nil {
return err
}
defer fd.decref()
if fd.pd.runtimeCtx == 0 {
return ErrNoDeadline
}
runtime_pollSetDeadline(fd.pd.runtimeCtx, d, mode)
return nil
}//go:linkname poll_runtime_pollSetDeadline internal/poll.runtime_pollSetDeadline
func poll_runtime_pollSetDeadline(pd *pollDesc, d int64, mode int) {
lock(&pd.lock)
if pd.closing {
unlock(&pd.lock)
return
}
pd.seq++ // invalidate current timers
// 重置当前的 timer
if pd.rt.f != nil {
// 删除 pollDesc 相关的 read timer
deltimer(&pd.rt)
pd.rt.f = nil
}
if pd.wt.f != nil {
// 删除 pollDesc 相关的 write timer
deltimer(&pd.wt)
pd.wt.f = nil
}
// 设置新 timer
if d != 0 && d <= nanotime() {
d = -1
}
if mode == 'r' || mode == 'r'+'w' {
// 记录 read deadline
pd.rd = d
}
if mode == 'w' || mode == 'r'+'w' {
// 记录 write deadline
pd.wd = d
}
if pd.rd > 0 && pd.rd == pd.wd {
pd.rt.f = netpollDeadline
pd.rt.when = pd.rd
// Copy current seq into the timer arg.
// Timer func will check the seq against current descriptor seq,
// if they differ the descriptor was reused or timers were reset.
pd.rt.arg = pd
pd.rt.seq = pd.seq
// 插入 timer 到时间堆
addtimer(&pd.rt)
} else {
if pd.rd > 0 {
pd.rt.f = netpollReadDeadline
pd.rt.when = pd.rd
pd.rt.arg = pd
pd.rt.seq = pd.seq
addtimer(&pd.rt)
}
if pd.wd > 0 {
pd.wt.f = netpollWriteDeadline
pd.wt.when = pd.wd
pd.wt.arg = pd
pd.wt.seq = pd.seq
addtimer(&pd.wt)
}
}
// If we set the new deadline in the past, unblock currently pending IO if any.
var rg, wg *g
atomicstorep(unsafe.Pointer(&wg), nil) // full memory barrier between stores to rd/wd and load of rg/wg in netpollunblock
if pd.rd < 0 {
rg = netpollunblock(pd, 'r', false)
}
if pd.wd < 0 {
wg = netpollunblock(pd, 'w', false)
}
unlock(&pd.lock)
if rg != nil {
netpollgoready(rg, 3)
}
if wg != nil {
netpollgoready(wg, 3)
}
}根据 read deadline 和 write deadline 给要插入时间堆的 timer 设置不同的回调函数。
func netpollDeadline(arg interface{}, seq uintptr) {
netpolldeadlineimpl(arg.(*pollDesc), seq, true, true)
}
func netpollReadDeadline(arg interface{}, seq uintptr) {
netpolldeadlineimpl(arg.(*pollDesc), seq, true, false)
}
func netpollWriteDeadline(arg interface{}, seq uintptr) {
netpolldeadlineimpl(arg.(*pollDesc), seq, false, true)
}调用最终的实现函数:
func netpolldeadlineimpl(pd *pollDesc, seq uintptr, read, write bool) {
lock(&pd.lock)
// Seq arg is seq when the timer was set.
// If it's stale, ignore the timer event.
if seq != pd.seq {
// The descriptor was reused or timers were reset.
unlock(&pd.lock)
return
}
var rg *g
if read {
if pd.rd <= 0 || pd.rt.f == nil {
throw("runtime: inconsistent read deadline")
}
pd.rd = -1
atomicstorep(unsafe.Pointer(&pd.rt.f), nil) // full memory barrier between store to rd and load of rg in netpollunblock
rg = netpollunblock(pd, 'r', false)
}
var wg *g
if write {
if pd.wd <= 0 || pd.wt.f == nil && !read {
throw("runtime: inconsistent write deadline")
}
pd.wd = -1
atomicstorep(unsafe.Pointer(&pd.wt.f), nil) // full memory barrier between store to wd and load of wg in netpollunblock
wg = netpollunblock(pd, 'w', false)
}
unlock(&pd.lock)
// rg 和 wg 是通过 netpollunblock 从 pollDesc 结构中捞出来的
if rg != nil {
// 恢复 goroutine 执行现场
// 继续执行
netpollgoready(rg, 0)
}
if wg != nil {
// 恢复 goroutine 执行现场
// 继续执行
netpollgoready(wg, 0)
}
}func netpollgoready(gp *g, traceskip int) {
atomic.Xadd(&netpollWaiters, -1)
goready(gp, traceskip+1)
}
func goready(gp *g, traceskip int) {
// switch -> g0
systemstack(func() {
ready(gp, traceskip, true)
})
}
// Mark gp ready to run.
func ready(gp *g, traceskip int, next bool) {
status := readgstatus(gp)
// 标记为 runnable
_g_ := getg()
_g_.m.locks++ // disable preemption because it can be holding p in a local var
if status&^_Gscan != _Gwaiting {
dumpgstatus(gp)
throw("bad g->status in ready")
}
// status is Gwaiting or Gscanwaiting, make Grunnable and put on runq
casgstatus(gp, _Gwaiting, _Grunnable)
// 放到执行队列里
runqput(_g_.m.p.ptr(), gp, next)
if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 {
// 如果有空闲的 p，那么就叫起床干活
wakep()
}
_g_.m.locks--
if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in Case we've cleared it in newstack
_g_.stackguard0 = stackPreempt
}
}// Close closes the connection.
func (c *conn) Close() error {
if !c.ok() {
return syscall.EINVAL
}
err := c.fd.Close()
if err != nil {
err = &OpError{Op: "close", Net: c.fd.net, Source: c.fd.laddr, Addr: c.fd.raddr, Err: err}
}
return err
}func (fd *netFD) Close() error {
runtime.SetFinalizer(fd, nil)
return fd.pfd.Close()
}// Close closes the FD. The underlying file descriptor is closed by the
// destroy method when there are no remaining references.
func (fd *FD) Close() error {
if !fd.fdmu.increfAndClose() {
return errClosing(fd.isFile)
}
// Unblock any I/O. Once it all unblocks and returns,
// so that it cannot be referring to fd.sysfd anymore,
// the final decref will close fd.sysfd. This should happen
// fairly quickly, since all the I/O is non-blocking, and any
// attempts to block in the pollDesc will return errClosing(fd.isFile).
fd.pd.evict()
// The call to decref will call destroy if there are no other
// references.
err := fd.decref()
// Wait until the descriptor is closed. If this was the only
// reference, it is already closed. Only wait if the file has
// not been set to blocking mode, as otherwise any current I/O
// may be blocking, and that would block the Close.
if !fd.isBlocking {
runtime_Semacquire(&fd.csema)
}
return err
}// Evict evicts fd from the pending list, unblocking any I/O running on fd.
func (pd *pollDesc) evict() {
if pd.runtimeCtx == 0 {
return
}
runtime_pollUnblock(pd.runtimeCtx)
}//go:linkname poll_runtime_pollUnblock internal/poll.runtime_pollUnblock
func poll_runtime_pollUnblock(pd *pollDesc) {
lock(&pd.lock)
if pd.closing {
throw("runtime: unblock on closing polldesc")
}
pd.closing = true
pd.seq++
var rg, wg *g
atomicstorep(unsafe.Pointer(&rg), nil) // full memory barrier between store to closing and read of rg/wg in netpollunblock
rg = netpollunblock(pd, 'r', false)
wg = netpollunblock(pd, 'w', false)
if pd.rt.f != nil {
deltimer(&pd.rt)
pd.rt.f = nil
}
if pd.wt.f != nil {
deltimer(&pd.wt)
pd.wt.f = nil
}
unlock(&pd.lock)
if rg != nil {
netpollgoready(rg, 3)
}
if wg != nil {
netpollgoready(wg, 3)
}
}