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）

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。