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      }  }