golang：goroute with select不会停止,除非我添加了fmt.Print()

如果它运行像go run 71_hang.go ok,它运行正常.

但是,如果你使用go run 71_hang.go nogood,它将永远运行.

唯一的区别是select语句中默认的额外fmt.Print(“”).

我不确定,但我怀疑某种无限循环和竞争条件？这是我的解决方案.

注意：Go没有抛出并不是死锁：所有的goroutines都睡着了 – 僵局！

package main

import (
    "fmt"
    "os"
)

type Fetcher interface {
    // Fetch returns the body of URL and
    // a slice of URLs found on that page.
    Fetch(url string) (body string,urls []string,err error)
}

func crawl(todo Todo,fetcher Fetcher,todoList chan Todo,done chan bool) {
    body,urls,err := fetcher.Fetch(todo.url)
    if err != nil {
        fmt.Println(err)
    } else {
        fmt.Printf("found: %s %qn",todo.url,body)
        for _,u := range urls {
            todoList <- Todo{u,todo.depth - 1}
        }
    }
    done <- true
    return
}

type Todo struct {
    url   string
    depth int
}

// Crawl uses fetcher to recursively crawl
// pages starting with url,to a maximum of depth.
func Crawl(url string,depth int,fetcher Fetcher) {
    visited := make(map[string]bool)
    doneCrawling := make(chan bool,100)
    toDoList := make(chan Todo,100)
    toDoList <- Todo{url,depth}

    crawling := 0
    for {
        select {
        case todo := <-toDoList:
            if todo.depth > 0 && !visited[todo.url] {
                crawling++
                visited[todo.url] = true
                go crawl(todo,fetcher,toDoList,doneCrawling)
            }
        case <-doneCrawling:
            crawling--
        default:
            if os.Args[1]=="ok" {   // *
                fmt.Print("")
            }
            if crawling == 0 {
                goto END
            }
        }
    }
END:
    return
}

func main() {
    Crawl("http://golang.org/",4,fetcher)
}

// fakeFetcher is Fetcher that returns canned results.
type fakeFetcher map[string]*fakeResult

type fakeResult struct {
    body string
    urls []string
}

func (f *fakeFetcher) Fetch(url string) (string,[]string,error) {
    if res,ok := (*f)[url]; ok {
        return res.body,res.urls,nil
    }
    return "",nil,fmt.Errorf("not found: %s",url)
}

// fetcher is a populated fakeFetcher.
var fetcher = &fakeFetcher{
    "http://golang.org/": &fakeResult{
        "The Go Programming Language",[]string{
            "http://golang.org/pkg/","http://golang.org/cmd/",},"http://golang.org/pkg/": &fakeResult{
        "Packages",[]string{
            "http://golang.org/","http://golang.org/pkg/fmt/","http://golang.org/pkg/os/","http://golang.org/pkg/fmt/": &fakeResult{
        "Package fmt","http://golang.org/pkg/","http://golang.org/pkg/os/": &fakeResult{
        "Package os",}