channel 相关操作

(channel)	send	receive	close
nil	blocks forever	blocks forever	run-time panic
unbuffered	can proceed if a receiver is ready	can proceed if a sender is ready	ok if the channel is bidirectional or send-only
buffered	can proceed if the buffer is not full	can proceed if the buffer is not empty	ok if the channel is bidirectional or send-only
send-only	ok	error	ok
receive-only	error	ok	error
bidirectional	ok	ok	ok
closed	run-time panic	proceed immediately, yielding the element type's zero value after any previously sent values have been received	run-time panic

个人认为需要注意的点

如果关闭 channel 时，有因为发送操作而阻塞的 goroutine，channel 关闭后，他们会 panic^1。这是测试代码。

channel 关闭后，还向 channel 发送，会 panic，这个不用多说。

在 select 语句中，如果多个 case 中的交流都可进行，select 会（伪）随机选择其中的一个执行^3，随机的引入是为了避免饥饿问题。