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：
1 package main 2 3 import ( 4 "time" 5 "math/rand" 6 "sync" 7 "log" 8 ) 9 10 func main() { 11 rand.Seed(time.Now().UnixNano()) 12 log.SetFlags(0) 13 14 // ... 15 const MaxRandomNumber = 100000 16 const NumReceivers = 100 17 18 wgReceivers := sync.WaitGroup{} 19 wgReceivers.Add(NumReceivers) 20 21 // ... 22 dataCh := make(chan int, 100) 23 24 // the sender 25 go func() { 26 for { 27 if value := rand.Intn(MaxRandomNumber); value == 0 { 28 // the only sender can close the channel safely. 29 close(dataCh) 30 return 31 } else { 32 dataCh <- value 33 } 34 } 35 }() 36 37 // receivers 38 for i := 0; i < NumReceivers; i++ { 39 go func() { 40 defer wgReceivers.Done() 41 42 // receive values until dataCh is closed and 43 // the value buffer queue of dataCh is empty. 44 for value := range dataCh { 45 log.Println(value) 46 } 47 }() 48 } 49 50 wgReceivers.Wait() 51 }
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：
1 package main 2 3 import ( 4 "time" 5 "math/rand" 6 "sync" 7 "log" 8 ) 9 10 func main() { 11 rand.Seed(time.Now().UnixNano()) 12 log.SetFlags(0) 13 14 // ... 15 const MaxRandomNumber = 100000 16 const NumSenders = 1000 17 18 wgReceivers := sync.WaitGroup{} 19 wgReceivers.Add(1) 20 21 // ... 22 dataCh := make(chan int, 100) 23 stopCh := make(chan struct{}) 24 // stopCh is an additional signal channel. 25 // Its sender is the receiver of channel dataCh. 26 // Its reveivers are the senders of channel dataCh. 27 28 // senders 29 for i := 0; i < NumSenders; i++ { 30 go func() { 31 for { 32 value := rand.Intn(MaxRandomNumber) 33 34 select { 35 case <- stopCh: 36 return 37 case dataCh <- value: 38 } 39 } 40 }() 41 } 42 43 // the receiver 44 go func() { 45 defer wgReceivers.Done() 46 47 for value := range dataCh { 48 if value == MaxRandomNumber-1 { 49 // the receiver of the dataCh channel is 50 // also the sender of the stopCh cahnnel. 51 // It is safe to close the stop channel here. 52 close(stopCh) 53 return 54 } 55 56 log.Println(value) 57 } 58 }() 59 60 // ... 61 wgReceivers.Wait() 62 }
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：
1 package main 2 3 import ( 4 "time" 5 "math/rand" 6 "sync" 7 "log" 8 "strconv" 9 ) 10 11 func main() { 12 rand.Seed(time.Now().UnixNano()) 13 log.SetFlags(0) 14 15 // ... 16 const MaxRandomNumber = 100000 17 const NumReceivers = 10 18 const NumSenders = 1000 19 20 wgReceivers := sync.WaitGroup{} 21 wgReceivers.Add(NumReceivers) 22 23 // ... 24 dataCh := make(chan int, 100) 25 stopCh := make(chan struct{}) 26 // stopCh is an additional signal channel. 27 // Its sender is the moderator goroutine shown below. 28 // Its reveivers are all senders and receivers of dataCh. 29 toStop := make(chan string, 1) 30 // the channel toStop is used to notify the moderator 31 // to close the additional signal channel (stopCh). 32 // Its senders are any senders and receivers of dataCh. 33 // Its reveiver is the moderator goroutine shown below. 34 35 var stoppedBy string 36 37 // moderator 38 go func() { 39 stoppedBy = <- toStop // part of the trick used to notify the moderator 40 // to close the additional signal channel. 41 close(stopCh) 42 }() 43 44 // senders 45 for i := 0; i < NumSenders; i++ { 46 go func(id string) { 47 for { 48 value := rand.Intn(MaxRandomNumber) 49 if value == 0 { 50 // here, a trick is used to notify the moderator 51 // to close the additional signal channel. 52 select { 53 case toStop <- "sender#" + id: 54 default: 55 } 56 return 57 } 58 59 // the first select here is to try to exit the 60 // goroutine as early as possible. 61 select { 62 case <- stopCh: 63 return 64 default: 65 } 66 67 select { 68 case <- stopCh: 69 return 70 case dataCh <- value: 71 } 72 } 73 }(strconv.Itoa(i)) 74 } 75 76 // receivers 77 for i := 0; i < NumReceivers; i++ { 78 go func(id string) { 79 defer wgReceivers.Done() 80 81 for { 82 // same as senders, the first select here is to 83 // try to exit the goroutine as early as possible. 84 select { 85 case <- stopCh: 86 return 87 default: 88 } 89 90 select { 91 case <- stopCh: 92 return 93 case value := <-dataCh: 94 if value == MaxRandomNumber-1 { 95 // the same trick is used to notify the moderator 96 // to close the additional signal channel. 97 select { 98 case toStop <- "receiver#" + id: 99 default: 100 } 101 return 102 } 103 104 log.Println(value) 105 } 106 } 107 }(strconv.Itoa(i)) 108 } 109 110 // ... 111 wgReceivers.Wait() 112 log.Println("stopped by", stoppedBy) 113 }
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：
1 package main 2 3 import "fmt" 4 5 type T int 6 7 func IsClosed(ch <-chan T) bool { 8 select { 9 case <-ch: 10 return true 11 default: 12 } 13 14 return false 15 } 16 17 func main() { 18 c := make(chan T) 19 fmt.Println(IsClosed(c)) // false 20 close(c) 21 fmt.Println(IsClosed(c)) // true 22 }
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）
1 func SafeSend(ch chan T, value T) (closed bool) { 2 defer func() { 3 if recover() != nil { 4 // the return result can be altered 5 // in a defer function call 6 closed = true 7 } 8 }() 9 10 ch <- value // panic if ch is closed 11 return false // <=> closed = false; return 12 }
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：
1 func SafeClose(ch chan T) (justClosed bool) { 2 defer func() { 3 if recover() != nil { 4 justClosed = false 5 } 6 }() 7 8 // assume ch != nil here. 9 close(ch) // panic if ch is closed 10 return true 11 }
很多人喜欢用sync.Once来关闭channel：
1 type MyChannel struct { 2 C chan T 3 once sync.Once 4 } 5 6 func NewMyChannel() *MyChannel { 7 return &MyChannel{C: make(chan T)} 8 } 9 10 func (mc *MyChannel) SafeClose() { 11 mc.once.Do(func(){ 12 close(mc.C) 13 }) 14 }
当然了，我们也可以用sync.Mutex来避免多次关闭channel：
1 type MyChannel struct { 2 C chan T 3 closed bool 4 mutex sync.Mutex 5 } 6 7 func NewMyChannel() *MyChannel { 8 return &MyChannel{C: make(chan T)} 9 } 10 11 func (mc *MyChannel) SafeClose() { 12 mc.mutex.Lock() 13 if !mc.closed { 14 close(mc.C) 15 mc.closed = true 16 } 17 mc.mutex.Unlock() 18 } 19 20 func (mc *MyChannel) IsClosed() bool { 21 mc.mutex.Lock() 22 defer mc.mutex.Unlock() 23 return mc.closed 24 }
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。
