到这里所有的疑问都集中到了pollDesc上，它到底是什么呢？
const (
pdReady uintptr = 1
pdWait uintptr = 2
)
// Network poller descriptor.
type pollDesc struct {
link *pollDesc // in pollcache, protected by pollcache.lock
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
}
type pollCache struct {
lock mutex
first *pollDesc
}
pollDesc网络轮询器是Golang中针对每个socket文件描述符建立的轮询机制。 此处的轮询并不是一般意义上的轮询，而是Golang的runtime在调度goroutine或者GC完成之后或者指定时间之内，调用epoll_wait获取所有产生IO事件的socket文件描述符。当然在runtime轮询之前，需要将socket文件描述符和当前goroutine的相关信息加入epoll维护的数据结构中，并挂起当前goroutine，当IO就绪后，通过epoll返回的文件描述符和其中附带的goroutine的信息，重新恢复当前goroutine的执行。这里我们可以看到pollDesc中有两个变量wg和rg，其实我们可以把它们看作信号量，这两个变量有几种不同的状态：
继续接着上面的WaitRead调用说起，go在这里到底做了什么让当前的goroutine挂起了呢。
func net_runtime_pollWait(pd *pollDesc, mode int) int {
err := netpollcheckerr(pd, int32(mode))
if err != 0 {
return err
}
// As for now only Solaris uses level-triggered IO.
if GOOS == "solaris" {
netpollarm(pd, mode)
}
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 {
//根据读写模式获取相应的pollDesc中的读写信号量
gpp := &pd.rg
if mode == 'w' {
gpp = &pd.wg
}
for {
old := *gpp
//已经准备好直接返回true
if old == pdReady {
*gpp = 0
return true
}
if old != 0 {
throw("netpollblock: double wait")
}
//设置gpp pdWait
if atomic.Casuintptr(gpp, 0, pdWait) {
break
}
}
if waitio || netpollcheckerr(pd, mode) == 0 {
gopark(netpollblockcommit, unsafe.Pointer(gpp), "IO wait", traceEvGoBlockNet, 5)
}
old := atomic.Xchguintptr(gpp, 0)
if old > pdWait {
throw("netpollblock: corrupted state")
}
return old == pdReady
}
