Channel的作用

协程间通信
数据传递：channel可以让一个goroutine发送数据，另一个goroutine接收数据。例如，在一个网络服务器程序中，一个goroutine可能负责接收客户端连接并将请求数据放入channel，另一个goroutine从channel中取出请求数据进行处理。

数据结构

type hchan struct {
    qcount   uint           // total data in the queue 队列中数据总数
    dataqsiz uint           // size of the circular queue  环形队列size大小
    buf      unsafe.Pointer // points to an array of dataqsiz elements (环形队列)
    elemsize uint16 // 元素大小
    closed   uint32 // 关闭
    timer    *timer // timer feeding this chan
    elemtype *_type // element type 元素类型
    sendx    uint   // send index 已发送数据在环形队列的位置
    recvx    uint   // receive index 已接受数据在环形队列的位置
    recvq    waitq  // list of recv waiters 接收者等待队列
    sendq    waitq  // list of send waiters 发送者等待队列

    // lock protects all fields in hchan, as well as several
    // fields in sudogs blocked on this channel.
    //
    // Do not change another G's status while holding this lock
    // (in particular, do not ready a G), as this can deadlock
    // with stack shrinking.
    lock mutex
}

type waitq struct {
    first *sudog
    last  *sudog
}

如何保证协程安全

lock mutex

无缓冲区的Channel

reveq：从Channel中接受元素，如果没有元素则会直接阻塞等待

// Signal to anyone trying to shrink our stack that we're about
// to park on a channel. The window between when this G's status
// changes and when we set gp.activeStackChans is not safe for
// stack shrinking.
gp.parkingOnChan.Store(true)
gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanSend, traceBlockChanSend, 2)

sendq：向Channel中发送元素，如果没有接收者则会直接阻塞等待

goready(gp, skip+1)

lock 读写锁

有缓冲区的Channel

从缓冲区中读取数据，用数组建立的一个环形链表

不会产生阻塞操作