golang并发三板斧系列之一：channel用于通信和同步

某不知名程序员曾说过：

我们爱golang，很大程度是因为爱它的并发。

我们来总结下golang的并发三板斧，channel、pool、context。一路看下来，套路也就那么几个。

Concurrency is not parallelism

首先必须牢记一个重要的概念：并发不是并行。

并发是描述代码架构，是指在模型上可以同时处理多件事务。并行是描述执行状态，除了代码限制外，如果没有多CPU多核心，谈何并行呢？对于单CPU单核心的硬件（当然现在基本上很少了），并发往往带来更多的进程间切换，反而会拖累效率。

那到底要不要并行？或者说什么情况下并行的收益最大？关键在于我们的模型是CPU密集型还是IO密集型的，一般来讲前者更适用于并行。听起来好像有点反常识，这么来思考一下：

• CPU密集型不会主动释放系统线程，人为释放的话会有时间成本。利用并行的话，可以尽量让CPU密集型的任务独占一个系统线程。
• IO密集型本身遇到系统调用或者网络阻塞就会释放系统线程，所以单纯的并发模型就能满足需要了，可以有效利用单一的系统线程。

并发

goroutine

这大概是golang中最吸引人的地方了。

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启动goroutine

go

func testBoring(msg string) {
    for i := 0; ; i++ {
        log.Printf("%s %d", msg, i)
        time.Sleep(time.Duration(rand.Intn(1e3)) * time.Millisecond)
    }
}

func TestBasicGo(t *testing.T) {
    go testBoring("boring!")
}

主进程在启动的goroutine并没有运行到就退出了，因此没有效果，不会有打印。

等待goroutine执行

func testBoring(msg string) {
    for i := 0; ; i++ {
        log.Printf("%s %d", msg, i)
        time.Sleep(time.Duration(rand.Intn(1e3)) * time.Millisecond)
    }
}

func TestGoAndWait(t *testing.T) {
    go testBoring("boring!")

    log.Printf("I'm listening.")
    time.Sleep(time.Second)
    log.Printf("I'm leaving.")
}

time.Sleep

2019/02/17 17:55:33 I'm listening.
2019/02/17 17:55:33 boring! 0
2019/02/17 17:55:34 boring! 1
2019/02/17 17:55:34 boring! 2
2019/02/17 17:55:34 I'm leaving.

Process finished with exit code 0

goroutine执行时机

goGPMWaitingExecutingMtime.Sleep()runtime.Gosched()

因此单核心或者多核心在调度goroutine的时候可能会有很大差异，如下示例：

func TestGoFuncParam(t *testing.T) {
    for i := 0; i < 10; i++ {
        go func() {
            log.Println(i)
        }()
    }
    time.Sleep(100 * time.Millisecond)
    log.Print("xxxxxxxxxxxxxx")

    for i := 0; i < 10; i++ {
        go func(n *int) {
            log.Println(*n)
        }(&i)
    }
    time.Sleep(100 * time.Millisecond)
}

-cpu 1

C02S259EFVH3:go_concurrency baixiao$ go test -cpu 1 -run TestGoFuncParam
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 xxxxxxxxxxxxxx
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
2019/02/17 18:20:10 10
PASS
ok      _/Users/baixiao/Go/src/github.com/baixiaoustc/go_concurrency    0.214s

在多核心时则不同，for循环里的第一个goroutine可能在主进程走完for代码之前就执行到（每次运行可能结果不同）：

C02S259EFVH3:go_concurrency baixiao$ go test -cpu 8 -run TestGoFuncParam
2019/02/17 18:20:24 7
2019/02/17 18:20:24 10
2019/02/17 18:20:24 10
2019/02/17 18:20:24 10
2019/02/17 18:20:24 10
2019/02/17 18:20:24 10
2019/02/17 18:20:24 10
2019/02/17 18:20:24 10
2019/02/17 18:20:24 10
2019/02/17 18:20:24 10
2019/02/17 18:20:25 xxxxxxxxxxxxxx
2019/02/17 18:20:25 1
2019/02/17 18:20:25 10
2019/02/17 18:20:25 10
2019/02/17 18:20:25 10
2019/02/17 18:20:25 10
2019/02/17 18:20:25 10
2019/02/17 18:20:25 10
2019/02/17 18:20:25 10
2019/02/17 18:20:25 10
2019/02/17 18:20:25 10
PASS
ok      _/Users/baixiao/Go/src/github.com/baixiaoustc/go_concurrency    0.219s

channel：goroutine间的通信

搞并发编程，怎么能没有进程间（线程间/协程间）通信呢？

goroutine需要通信

TestGoAndWait

func testBoringWithChannel(msg string, c chan string) {
    for i := 0; ; i++ {
        c <- fmt.Sprintf("%s %d", msg, i)
        time.Sleep(time.Duration(rand.Intn(1e3)) * time.Millisecond)
    }
}

func TestGoWithChannel(t *testing.T) {
    ch := make(chan string)
    go testBoringWithChannel("boring!", ch)
    for c := range ch {
        log.Printf("You say: %s", c)
    }
}

关闭channel

抛开死循环，怎么在子goroutine结束时告知主goroutine呢？直接退出是会造成死锁的！

fatal error: all goroutines are asleep - deadlock!

此时需要在写端关闭channel：

func testBoringWithChannelClose(msg string, c chan string) {
    for i := 0; i < 5; i++ {
        c <- fmt.Sprintf("%s %d", msg, i)
        time.Sleep(time.Duration(rand.Intn(1e3)) * time.Millisecond)
    }
    close(c)
}

func TestGoWithChannelClose(t *testing.T) {
    ch := make(chan string)
    go testBoringWithChannelClose("boring!", ch)
    for c := range ch {
        log.Printf("You say: %s", c)
    }
}

2019/02/19 21:35:23 You say: boring! 0
2019/02/19 21:35:23 You say: boring! 1
2019/02/19 21:35:24 You say: boring! 2
2019/02/19 21:35:24 You say: boring! 3
2019/02/19 21:35:24 You say: boring! 4

Process finished with exit code 0

for range

func TestGoWithChannelCloseThenWrite(t *testing.T) {
    ch := make(chan string)
    go testBoringWithChannelClose("boring!", ch)
    for c := range ch {
        log.Printf("You say: %s", c)
    }
    ch <- "after close"
}

2019/02/18 23:12:31 receive boring! 0
2019/02/18 23:12:31 receive boring! 1
2019/02/18 23:12:32 receive boring! 2
2019/02/18 23:12:32 receive boring! 3
2019/02/18 23:12:32 receive boring! 4
panic: send on closed channel [recovered]
    panic: send on closed channel

goroutine 5 [running]:
testing.tRunner.func1(0xc4200aa0f0)
    /usr/local/go/src/testing/testing.go:711 +0x2d2
panic(0x1114e40, 0x1151930)
    /usr/local/go/src/runtime/panic.go:491 +0x283
github.com/baixiaoustc/go_concurrency.TestGoWithChannelCloseThenWrite(0xc4200aa0f0)
    /Users/baixiao/Go/src/github.com/baixiaoustc/go_concurrency/first_post_test.go:80 +0x167
testing.tRunner(0xc4200aa0f0, 0x1141a58)
    /usr/local/go/src/testing/testing.go:746 +0xd0
created by testing.(*T).Run
    /usr/local/go/src/testing/testing.go:789 +0x2de

Process finished with exit code 2

还需要记住一点，关闭后的channel不能再写入，但是可以继续读，读出来都是定义类型的空值。

只读channel

如何简单地避免上述误操作呢？可以用只读channel，生产者和消费者严格定义好，生产者只写channel，消费者只读channel，如果消费者有写入操作的话在编译时就会报错。但是只读channel的写法有一点技巧，如下的写法就不行，直接编译失败：

func TestReceiveChannelWrong(t *testing.T) {
    ch := make(<-chan int)
    go func() {
        ch <- 1
    }()
    a := <-ch
    log.Println(a)
}

./first_post_test.go:86:6: invalid operation: ch <- 1 (send to receive-only type <-chan int)

Process finished with exit code 2

正确的写法如下，通过函数的返回值限定「只读属性」：

func TestReceiveChannelRight(t *testing.T) {
    ch := func() <-chan int {
        ch := make(chan int)
        go func() {
            ch <- 2
        }()
        return ch
    }()
    a := <-ch
    log.Println(a)
}

channel with select

那么当主goroutine要启动多个子goroutine干不同的事呢？主goroutine不可能依次阻塞到不同的channel上，串行地等待子goroutine依次完工，这样太丑了：

func TestMultiChannelsSerial(t *testing.T) {
    ch1 := make(chan string)
    go testBoringWithChannelClose("boring!", ch1)
    ch2 := make(chan string)
    go testBoringWithChannelClose("funning!", ch2)

    for b := range ch1 {
        log.Printf("You say: %s", b)
    }
    for b := range ch2 {
        log.Printf("You say: %s", b)
    }
}

select

• 使用_,ok判断channel是否关闭
• 当通道为nil时，对应的case永远为阻塞

func TestMultiChannelsConcurrently(t *testing.T) {
    ch1 := make(chan string)
    go testBoringWithChannelClose("boring!", ch1)
    ch2 := make(chan string)
    go testBoringWithChannelClose("funning!", ch2)

    for {
        select {
        case c1, ok := <-ch1:
            if !ok {
                ch1 = nil
                log.Print("close ch1")
                continue
            }
            log.Printf("You say: %s", c1)
        case c2, ok := <-ch2:
            if !ok {
                ch2 = nil
                log.Print("close ch2")
                continue
            }
            log.Printf("You say: %s", c2)
        default:
            break
        }

        if ch1 == nil && ch2 == nil {
            break
        }
    }

    log.Print("go on")
}

2019/02/19 21:32:18 You say: funning! 0
2019/02/19 21:32:18 You say: boring! 0
2019/02/19 21:32:18 You say: funning! 1
2019/02/19 21:32:19 You say: boring! 1
2019/02/19 21:32:19 You say: funning! 2
2019/02/19 21:32:19 You say: boring! 2
2019/02/19 21:32:19 You say: funning! 3
2019/02/19 21:32:19 You say: boring! 3
2019/02/19 21:32:19 You say: funning! 4
2019/02/19 21:32:20 You say: boring! 4
2019/02/19 21:32:20 close ch2
2019/02/19 21:32:20 close ch1

2019/02/19 21:32:20 go on

selectcase

构造定时器

channel + select 的另一个经典运用是超时管理：

func TestTimeOutEach(t *testing.T) {
    ch := make(chan string)
    go testBoringWithChannel("boring!", ch)
    for i := 0; i < 5; i++ {
        select {
        case c := <-ch:
            log.Printf("You say: %s", c)
        case <-time.After(500 * time.Millisecond):
            log.Println("You talk too slow.")
        }
    }
}

2019/02/19 21:08:14 You say: boring! 0
2019/02/19 21:08:14 You say: boring! 1
2019/02/19 21:08:15 You talk too slow.
2019/02/19 21:08:15 You say: boring! 2
2019/02/19 21:08:15 You talk too slow.

Process finished with exit code 0

上述是针对每次循环内部的超时，如果要对整个会话进行管理：

func TestTimeOutWhole(t *testing.T) {
    ch := make(chan string)
    timeout := time.After(1 * time.Second)
    go testBoringWithChannel("boring!", ch)
    for i := 0; i < 5; i++ {
        select {
        case c := <-ch:
            log.Printf("You say: %s", c)
        case <-timeout:
            log.Println("You talk too much.")
            return
        }
    }
}

channel生成器

还有一个比较常用的方法在上面的只读channel也提到了，雅称channel生成器：

func TestChannelGenerator(t *testing.T) {
    ch := testBoringWithChannelGenerate("boring!")
    for i := 0; i < 5; i++ {
        log.Printf("You say: %q\n", <-ch)
    }
    log.Println("I'm leaving.")
}

2019/02/19 21:20:40 You say: "boring! 0"
2019/02/19 21:20:40 You say: "boring! 1"
2019/02/19 21:20:41 You say: "boring! 2"
2019/02/19 21:20:41 You say: "boring! 3"
2019/02/19 21:20:42 You say: "boring! 4"

2019/02/19 21:20:42 I'm leaving.

channel用于退出

扩展上个示例，如果想在主goroutine退出之前告知子goroutine也退出呢？还是利用channel，只不过发送者和接受者对调了：

func testBoringWithChannelQuit(msg string, quit chan bool) <-chan string {
    c := make(chan string)
    go func() {
        for i := 0; ; i++ {
            select {
            case c <- fmt.Sprintf("%s %d", msg, i):
                time.Sleep(time.Duration(rand.Intn(1e3)) * time.Millisecond)
            case <-quit:
                return
            }

        }
    }()
    return c
}

func TestChannelQuit(t *testing.T) {
    quit := make(chan bool)
    c := testBoringWithChannelQuit("boring!", quit)
    for i := 0; i < 5; i++ {
        log.Printf("You say: %q\n", <-c)
    }
    log.Println("I'm leaving.")
    quit <- true
}

通信和同步

上面提到的channel都是没有buffer的，很大程度上起了「同步」的作用。发送者和接受者都必须等双方都准备好才能通信，实际上是一个同步模型。

后面会再提带buffer的channel。

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本篇的模型可以类比为你现在是个师傅，带了个小学徒一起做事情。一开始双方没有通信，各搞各的，指挥学徒很麻烦。后面用上了channel，师傅能从学徒那里收到回馈了，也能定时触发一些事情了，也能由师傅指定学徒该下班了。小作坊就越开越顺了。

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