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。