golang coroutine 的等待与死锁

直接上代码：

1. 第一种情况， 如果没有select{}， main 主线程不会等待coroutine运行，导致coroutine得不到机会运行。

You are requesting eventual scheduling (using the two go statements)
of two goroutines and then you exit main without giving the scheduler
a chance to do anything.

有了select， 程序正常运行。

package main

import (
	"fmt"
        "time"
)


func main() {
	go func1()
	go func2()
	select{}
}

func func1() {
       for{
	    fmt.Println("here1")
            time.Sleep(10 * time.Minute)
        }

}

func func2() {
       for{
	   fmt.Println("here2")
           time.Sleep(10 * time.Minute)
       }

}

2. coroutine有机会运行，但是会发生死锁，Fatal error: all goroutines are asleep - deadlock!

The goroutine executing func1 exited,ditto for func2. The maingoroutine is blocked with no hope to recover while no other goroutinecan be scheduled.

package main

import (
	"fmt"
	//"time"
)

func main() {
	go func1()
	go func2()
	select {
	}
}

func func1() {
	fmt.Println("here1")

}

func func2() {
	fmt.Println("here2")
}

3. 第三种情况， 正常运行。

package main

import (
	"fmt"
)

var c = make(chan int,2)

func main() {
	go func1()
	go func2()
	<-c
	<-c
	fmt.Println("ok")
}

func func1() {
	fmt.Println("here1")
	c <- 1
}

func func2() {
	fmt.Println("here2")
	c <- 1
}

4. 实现上面的目的的另外一种方法：

  var wg sync.WaitGroup
    var urls = []string{
            "http://www.golang.org/","http://www.google.com/","http://www.somestupidname.com/",}
    for _,url := range urls {
            // Increment the WaitGroup counter.
            wg.Add(1)
            // Launch a goroutine to fetch the URL.
            go func(url string) {
                    // Decrement the counter when the goroutine completes.
                    defer wg.Done()
                    // Fetch the URL.
                    http.Get(url)
            }(url)
    }
    // Wait for all HTTP fetches to complete.
    wg.Wait()