阻塞：
发生一直阻塞时，会报如下deadlock错误：
fatal error: all goroutines are asleep - deadlock!无缓存channel：
- 通道中无数据，但执行读通道。
- 通道中无数据，向通道写数据，但无协程读取。
有缓存channel：
- 通道的缓存无数据，但执行读通道。
- 通道的缓存已经占满，向通道写数据，但无协程读。
Panic：
1.向已经关闭的channel写。
2.关闭已经关闭的channel。
close channel：
You needn't close every channel when you've finished with it.It's only necessary to close a channel when it is important to tell the receiving goroutines that all data have been sent.
Note that it is only necessary to close a channel if the receiver is looking for a close. Closing the channel is a control signal on the channel indicating that no more data follows.
It's OK to leave a Go channel open forever and never close it. When the channel is no longer used, it will be garbage collected.
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