内容：优雅关闭channel

Channel关闭原则

1、不要在消费端关闭channel

2、不要在有多个并行的生产者时对channel执行关闭操作。

3、只在唯一的或者最后唯一剩下的生产者协程中关闭channel，来通知消费者
   已经没有值可以继续读了。只要坚持这个原则，就可以确保向一个已经关闭
   的channel发送数据的情况不可能发生。

暴力关闭channel的正确方法

如果想要在消费端关闭channel，或者在多个生产者端关闭channel，可以使用recover机制来避免程序
因为panic而崩溃。

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 // <=> justClosed = true; return
}

同理，生产者端使用：

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
}

礼貌的关闭channel方法

使用sync.Once来关闭channel，这样可以确保只会关闭一次

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)
	})
}

使用sync.Mutex达到同样的目的：


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
}

优雅的关闭channel的方法

多个消费者，单个生产者。直接让生产者关闭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()
}

多个生产者，单个消费者，不能在消费端关闭channel，因为这违背了channel关闭原则。
但是我们可以让消费端关闭一个附加的信号来通知发送端停止生产数据。

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 {
				// The first select here is to try to exit the goroutine
				// as early as possible. In fact, it is not essential
				// for this example, so it can be omitted.
				select {
				case <- stopCh:
					return
				default:
				}
				
				// Even if stopCh is closed, the first branch in the
				// second select may be still not selected for some
				// loops if the send to dataCh is also unblocked.
				// But this is acceptable, so the first select
				// can be omitted.
				select {
				case <- stopCh:
					return
				case dataCh <- rand.Intn(MaxRandomNumber):
				}
			}
		}()
	}
	
	// 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()
}


生产者同时也是退出信号channel的接受者，退出信号channel仍然是由它的生产端关闭的，
所以这仍然没有违背channel关闭原则。值得注意的是，这个例子中生产端和接受端都没有
关闭消息数据的channel，channel在没有goroutine引用的时候会自行关闭，而不需显示进行关闭。

多个生产者，多个消费者

不能让接受端或发送端关闭channel。我们甚至都不能让接受者关闭一个退出信号来通知生产者停止生产。
因为不能违反channel关闭原则。但可以引入一个额外的协调者来关闭附加的退出信号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
		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. This select blocks with one
				// receive operation case and one default branches will
				// be optimized as a try-receive operation by the
				// official Go compiler.
				select {
				case <- stopCh:
					return
				default:
				}
				
				// Even if stopCh is closed, the first branch in the
				// second select may be still not selected for some
				// loops (and for ever in theory) if the send to
				// dataCh is also unblocked.
				// This is why the first select block is needed.
				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 the sender goroutine, the first select here
				// is to try to exit the goroutine as early as possible.
				select {
				case <- stopCh:
					return
				default:
				}
				
				// Even if stopCh is closed, the first branch in the
				// second select may be still not selected for some
				// loops (and for ever in theory) if the receive from
				// dataCh is also unblocked.
				// This is why the first select block is needed.
				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)
}