如何 优雅的关闭 channel

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(0)

	// ...
	const MaxRandomNumber = 100000
	const NumReceivers = 100

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

	// ...
	dataCh := make(chan int, 100)

	// the sender
	go func() {
		for {
			if value := rand.Intn(MaxRandomNumber); value == 0 {
				// the only sender can close the channel safely.
				close(dataCh)
				return
			} else {
				dataCh <- value
			}
		}
	}()

	// receivers
	for i := 0; 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(0)

    // ...
    const MaxRandomNumber = 100000
    const NumSenders = 1000

    wgReceivers := sync.WaitGroup{}
    wgReceivers.Add(1)

    // ...
    dataCh := make(chan int, 100)
    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 := 0; 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-1 {
                // 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(0)

    // ...
    const MaxRandomNumber = 100000
    const NumReceivers = 10
    const NumSenders = 1000

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

    // ...
    dataCh := make(chan int, 100)
    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, 1)
        // 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 := 0; i < NumSenders; i++ {
        go func(id string) {
            for {
                value := rand.Intn(MaxRandomNumber)
                if value == 0 {
                    // 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 := 0; 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-1 {
                        // 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。

感谢阅读！

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