坑，以下例子中，原本以为close掉done channel后，循环会退出。但事实上会进入死循环

for {
        select {
        case packet := <-s.avPacketChan:
            s.sendPacket(packet)
        case  <-s.done: //终止channel
            break
        }
    }

原因:
[A "break" statement terminates execution of the innermost "for", "switch" or "select" statement.
官方文档](https://golang.org/ref/spec#Break_statements)
BreakStmt = "break" [ Label ] .
If there is a label, it must be that of an enclosing "for", "switch" or "select" statement, and that is the one whose execution terminates (§For statements, §Switch statements, §Select statements).

L:
  for i < n {
      switch i {
      case 5:
          break L // 从L标签处开始执行
      }
  }