带buffer的chan能同步吗?

具体看下面2个例子:

无 buffer 的: http://play.golang.org/p/RwPbCcueWh

func main() {
	ch := make(chan bool)
	go func() {
		println("fuck buffer channel")
		<-ch
	}()
	ch <- true
}

有buffer的: http://play.golang.org/p/YV9H6WPYuJ

func main() {
	ch := make(chan bool,1)
	go func() {
		println("fuck buffer channel")
		<-ch
	}()
	ch <- true
}

第二个程序(有buffer的chan)无法保证收/发同步,导致main退出时 println 还未执行. 说明带buffer的chan是不能保证收/发同步.

下面是一个构造的测试例子 http://play.golang.org/p/vQQCv9nG15

package main

import (
	"fmt"
	"sync"
	"time"
)

func main() {
	testNoBufferChan()
	testBufferChan()
}

func testNoBufferChan() {
	wg := new(sync.WaitGroup)
	ch := make(chan bool)

	wg.Add(1)
	go func() {
		defer wg.Done()

		time.Sleep(10 * time.Second)

		fmt.Println(time.Now(),"NoBufferChan recv begin")
		<-ch
		fmt.Println(time.Now(),"NoBufferChan recv end")
	}()

	fmt.Println(time.Now(),"NoBufferChan send begin")
	ch <- true
	fmt.Println(time.Now(),"NoBufferChan send end")

	wg.Wait()
}

func testBufferChan() {
	wg := new(sync.WaitGroup)
	ch := make(chan bool,1)

	wg.Add(1)
	go func() {
		defer wg.Done()

		time.Sleep(10 * time.Second)

		fmt.Println(time.Now(),"BufferChan recv begin")
		<-ch
		fmt.Println(time.Now(),"BufferChan recv end")
	}()

	fmt.Println(time.Now(),"BufferChan send begin")
	ch <- true
	fmt.Println(time.Now(),"BufferChan send end")

	wg.Wait()
}

在 play 上的结果(play上的是假时间):

2009-11-10 23:00:00 +0000 UTC NoBufferChan send begin
2009-11-10 23:00:10 +0000 UTC NoBufferChan recv begin
2009-11-10 23:00:10 +0000 UTC NoBufferChan recv end
2009-11-10 23:00:10 +0000 UTC NoBufferChan send end
2009-11-10 23:00:10 +0000 UTC BufferChan send begin
2009-11-10 23:00:10 +0000 UTC BufferChan send end
2009-11-10 23:00:20 +0000 UTC BufferChan recv begin
2009-11-10 23:00:20 +0000 UTC BufferChan recv end

在本机运行的结果:

2014-05-09 10:41:17.5611304 +0800 CST NoBufferChan send begin
2014-05-09 10:41:27.5637025 +0800 CST NoBufferChan recv begin
2014-05-09 10:41:27.5637025 +0800 CST NoBufferChan recv end
2014-05-09 10:41:27.5637025 +0800 CST NoBufferChan send end
2014-05-09 10:41:27.5647026 +0800 CST BufferChan send begin
2014-05-09 10:41:27.5647026 +0800 CST BufferChan send end
2014-05-09 10:41:37.5662746 +0800 CST BufferChan recv begin
2014-05-09 10:41:37.5662746 +0800 CST BufferChan recv end

可以发现带Buffer的chan收/发完成时间相差了10秒.

（编辑：李大同）

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