协程调度时机二:Channel读写

func chansend(t *chantype, c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
 ......
 ......
 // 对应缓冲区大小为0的channel 
 if c.dataqsiz == 0 {
 // 如果有接收者在等着 
 // 这时候可以写入成功,将数据拷贝至 
 // 接收者的缓冲区,唤醒接收者即可 
 sg := c.recvq.dequeue()
 if sg != nil { 
 ......
 }
 // 如果尚未有接收者,需要将其block 
 // no receiver available: block on this channel. 
 gp := getg()
 mysg := acquireSudog()
 mysg.releasetime = 0 
 if t0 != 0 {
 mysg.releasetime = -1 
 }
 mysg.elem = ep
 mysg.waitlink = nil
 gp.waiting = mysg
 mysg.g = gp
 mysg.selectdone = nil
 gp.param = nil
 c.sendq.enqueue(mysg)
 // 在这里将发送者协程block 
 goparkunlock(&c.lock, "chan send")
 }
 // 对于有缓冲区的channel,流程相似 
 ......
}

func goparkunlock(lock *mutex, reason string) {
 gopark(unsafe.Pointer(&parkunlock_c), unsafe.Pointer(lock), reason)
}
func gopark(unlockf unsafe.Pointer, lock unsafe.Pointer, reason string) {
 mp := acquirem()
 gp := mp.curg
 status := readgstatus(gp)
 if status != _Grunning && status != _Gscanrunning {
 gothrow("gopark: bad g status")
 }
 mp.waitlock = lock
 mp.waitunlockf = unlockf
 gp.waitreason = reason
 releasem(mp)
 // 最终调用函数park_m 
 mcall(park_m)
}
void runtime·park_m(G *gp)
{
 bool ok;
 // 首先将g的status变为Gwaiting 
 runtime·casgstatus(gp, Grunning, Gwaiting);
 dropg();
 if(g->m->waitunlockf) {
 ok = g->m->waitunlockf(gp, g->m->waitlock);
 g->m->waitunlockf = nil;
 g->m->waitlock = nil;
 if(!ok) {
 runtime·casgstatus(gp, Gwaiting, Grunnable);
 execute(gp); // Schedule it back, never returns. 
 }
 }
 // 发起一次调度,该m不再执行当前的g,而是选择一个新的g重新执行 
 // schedule的具体实现不在这里描述 
 schedule();
}

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