go语言学习--channel的关闭

在使用Go channel的时候，一个适用的原则是不要从接收端关闭channel，也不要在多个并发发送端中关闭channel。换句话说，如果sender(发送者)只是唯一的sender或者是channel最后一个活跃的sender，那么你应该在sender的goroutine关闭channel，从而通知receiver(s)(接收者们)已经没有值可以读了。维持这条原则将保证永远不会发生向一个已经关闭的channel发送值或者关闭一个已经关闭的channel。
（我们将会称上面的原则为channel closing principle）

保持channel closing principle的优雅方案

channel closing principle要求我们只能在发送端进行channel的关闭，对于日常遇到的可以归结为三类

1、m个receivers，一个sender.

2、一个receiver,n个sender

3、m个receivers,n个sender

1、m个receivers，一个sender

M个receivers，一个sender，sender通过关闭data channel说“不再发送”

这是最简单的场景了，就只是当sender不想再发送的时候让sender关闭data 来关闭channel：

 package main

 import (
     "time"
     "math/rand"
     "sync"
     "log"
 )

 func main() {
     rand.Seed(time.Now().UnixNano())
     log.SetFlags()

     // ...
     const MaxRandomNumber =
     const NumReceivers = 

     wgReceivers := sync.WaitGroup{}
     wgReceivers.Add(NumReceivers)

     // ...
     dataCh := make(chan int, )

     // the sender
     go func() {
         for {
             if value := rand.Intn(MaxRandomNumber); value ==  {
                 // the only sender can close the channel safely.
                 close(dataCh)
                 return
             } else {
                 dataCh <- value
             }
         }
     }()

     // receivers
     for i := ; i < NumReceivers; i++ {
         go func() {
             defer wgReceivers.Done()

             // receive values until dataCh is closed and
             // the value buffer queue of dataCh is empty.
             for value := range dataCh {
                 log.Println(value)
             }
         }()
     }

     wgReceivers.Wait()
 }

2、一个receiver,n个senders

一个receiver，N个sender，receiver通过关闭一个额外的signal channel说“请停止发送”
这种场景比上一个要复杂一点。我们不能让receiver关闭data channel，因为这么做将会打破channel closing principle。但是我们可以让receiver关闭一个额外的signal channel来通知sender停止发送值：

 package main

 import (
     "time"
     "math/rand"
     "sync"
     "log"
 )

 func main() {
     rand.Seed(time.Now().UnixNano())
     log.SetFlags()

     // ...
     const MaxRandomNumber =
     const NumSenders = 

     wgReceivers := sync.WaitGroup{}
     wgReceivers.Add()

     // ...
     dataCh := make(chan int, )
     stopCh := make(chan struct{})
         // stopCh is an additional signal channel.
         // Its sender is the receiver of channel dataCh.
         // Its reveivers are the senders of channel dataCh.

     // senders
     for i := ; i < NumSenders; i++ {
         go func() {
             for {
                 value := rand.Intn(MaxRandomNumber)

                 select {
                 case <- stopCh:
                     return
                 case dataCh <- value:
                 }
             }
         }()
     }

     // the receiver
     go func() {
         defer wgReceivers.Done()

         for value := range dataCh {
             if value == MaxRandomNumber- {
                 // the receiver of the dataCh channel is
                 // also the sender of the stopCh cahnnel.
                 // It is safe to close the stop channel here.
                 close(stopCh)
                 return
             }

             log.Println(value)
         }
     }()

     // ...
     wgReceivers.Wait()
 }

3、m个receivers,n个sender

M个receiver，N个sender，它们当中任意一个通过通知一个moderator（仲裁者）关闭额外的signal channel来说“让我们结束游戏吧”
这是最复杂的场景了。我们不能让任意的receivers和senders关闭data channel，也不能让任何一个receivers通过关闭一个额外的signal channel来通知所有的senders和receivers退出游戏。这么做的话会打破channel closing principle。但是，我们可以引入一个moderator来关闭一个额外的signal channel。这个例子的一个技巧是怎么通知moderator去关闭额外的signal channel：

 package main

 import (
     "time"
     "math/rand"
     "sync"
     "log"
     "strconv"
 )

 func main() {
     rand.Seed(time.Now().UnixNano())
     log.SetFlags()

     // ...
     const MaxRandomNumber =
     const NumReceivers =
     const NumSenders = 

     wgReceivers := sync.WaitGroup{}
     wgReceivers.Add(NumReceivers)

     // ...
     dataCh := make(chan int, )
     stopCh := make(chan struct{})
         // stopCh is an additional signal channel.
         // Its sender is the moderator goroutine shown below.
         // Its reveivers are all senders and receivers of dataCh.
     toStop := make(chan string, )
         // the channel toStop is used to notify the moderator
         // to close the additional signal channel (stopCh).
         // Its senders are any senders and receivers of dataCh.
         // Its reveiver is the moderator goroutine shown below.

     var stoppedBy string

     // moderator
     go func() {
         stoppedBy = <- toStop // part of the trick used to notify the moderator
                               // to close the additional signal channel.
         close(stopCh)
     }()

     // senders
     for i := ; i < NumSenders; i++ {
         go func(id string) {
             for {
                 value := rand.Intn(MaxRandomNumber)
                 if value ==  {
                     // here, a trick is used to notify the moderator
                     // to close the additional signal channel.
                     select {
                     case toStop <- "sender#" + id:
                     default:
                     }
                     return
                 }

                 // the first select here is to try to exit the
                 // goroutine as early as possible.
                 select {
                 case <- stopCh:
                     return
                 default:
                 }

                 select {
                 case <- stopCh:
                     return
                 case dataCh <- value:
                 }
             }
         }(strconv.Itoa(i))
     }

     // receivers
     for i := ; i < NumReceivers; i++ {
         go func(id string) {
             defer wgReceivers.Done()

             for {
                 // same as senders, the first select here is to
                 // try to exit the goroutine as early as possible.
                 select {
                 case <- stopCh:
                     return
                 default:
                 }

                 select {
                 case <- stopCh:
                     return
                 case value := <-dataCh:
                     if value == MaxRandomNumber- {
                         // the same trick is used to notify the moderator
                         // to close the additional signal channel.
                         select {
                         case toStop <- "receiver#" + id:
                         default:
                         }
                         return
                     }

                     log.Println(value)
                 }
             }
         }(strconv.Itoa(i))
     }

     // ...
     wgReceivers.Wait()
     log.Println("stopped by", stoppedBy)
 }

打破channel closing principle

有没有一个内置函数可以检查一个channel是否已经关闭。如果你能确定不会向channel发送任何值，那么也确实需要一个简单的方法来检查channel是否已经关闭：

 package main

 import "fmt"

 type T int

 func IsClosed(ch <-chan T) bool {
     select {
     case <-ch:
         return true
     default:
     }

     return false
 }

 func main() {
     c := make(chan T)
     fmt.Println(IsClosed(c)) // false
     close(c)
     fmt.Println(IsClosed(c)) // true
 }

上面已经提到了，没有一种适用的方式来检查channel是否已经关闭了。但是，就算有一个简单的 closed(chan T) bool函数来检查channel是否已经关闭，它的用处还是很有限的，就像内置的len函数用来检查缓冲channel中元素数量一样。原因就在于，已经检查过的channel的状态有可能在调用了类似的方法返回之后就修改了，因此返回来的值已经不能够反映刚才检查的channel的当前状态了。
尽管在调用closed(ch)返回true的情况下停止向channel发送值是可以的，但是如果调用closed(ch)返回false，那么关闭channel或者继续向channel发送值就不安全了（会panic）。

The Channel Closing Principle

在使用Go channel的时候，一个适用的原则是不要从接收端关闭channel，也不要在多个并发发送端中关闭channel。换句话说，如果sender(发送者)只是唯一的sender或者是channel最后一个活跃的sender，那么你应该在sender的goroutine关闭channel，从而通知receiver(s)(接收者们)已经没有值可以读了。维持这条原则将保证永远不会发生向一个已经关闭的channel发送值或者关闭一个已经关闭的channel。
（下面，我们将会称上面的原则为channel closing principle

打破channel closing principle的解决方案

如果你因为某种原因从接收端（receiver side）关闭channel或者在多个发送者中的一个关闭channel，那么你应该使用列在Golang panic/recover Use Cases的函数来安全地发送值到channel中（假设channel的元素类型是T）

 func SafeSend(ch chan T, value T) (closed bool) {
     defer func() {
         if recover() != nil {
             // the return result can be altered
             // in a defer function call
             closed = true
         }
     }()

     ch <- value // panic if ch is closed
     return false // <=> closed = false; return
 }

如果channel ch没有被关闭的话，那么这个函数的性能将和ch <- value接近。对于channel关闭的时候，SafeSend函数只会在每个sender goroutine中调用一次，因此程序不会有太大的性能损失。
同样的想法也可以用在从多个goroutine关闭channel中：

 func SafeClose(ch chan T) (justClosed bool) {
     defer func() {
         if recover() != nil {
             justClosed = false
         }
     }()

     // assume ch != nil here.
     close(ch) // panic if ch is closed
     return true
 }

很多人喜欢用sync.Once来关闭channel：

 type MyChannel struct {
     C    chan T
     once sync.Once
 }

 func NewMyChannel() *MyChannel {
     return &MyChannel{C: make(chan T)}
 }

 func (mc *MyChannel) SafeClose() {
     mc.once.Do(func(){
         close(mc.C)
     })
 }

当然了，我们也可以用sync.Mutex来避免多次关闭channel：

 type MyChannel struct {
     C      chan T
     closed bool
     mutex  sync.Mutex
 }

 func NewMyChannel() *MyChannel {
     return &MyChannel{C: make(chan T)}
 }

 func (mc *MyChannel) SafeClose() {
     mc.mutex.Lock()
     if !mc.closed {
         close(mc.C)
         mc.closed = true
     }
     mc.mutex.Unlock()
 }

 func (mc *MyChannel) IsClosed() bool {
     mc.mutex.Lock()
     defer mc.mutex.Unlock()
     return mc.closed
 }

我们应该要理解为什么Go不支持内置SafeSend和SafeClose函数，原因就在于并不推荐从接收端或者多个并发发送端关闭channel。Golang甚至禁止关闭只接收（receive-only）的channel。