Web Crawler go go不同输出相同的代码

Question

我一直在努力做所有的go教程，我被困在网络爬虫。我以为我完成了它，但输出不一致，我没有足够的并发经验来弄清楚原因。

这是我的代码：

package main

import (
    "fmt"
    "sync"
)

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)
}
var cache = struct {
    fetched map[string]bool
    sync.Mutex
}{fetched: make(map[string]bool)}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher, c chan []string, quit chan int) {
    if depth <= 0 {
        return
    }
    go safeVisit(url, c, quit, fetcher)
    for {
        select {
        case <- quit:
            return
        case u:= <-c:
            for _, v:= range u {
                go Crawl(v, depth -1, fetcher, c, quit)
            }
        }
    }
}
func main() {
    c := make(chan []string)
    quit := make(chan int)
    Crawl("http://golang.org/", 4, fetcher, c, quit)
}

func safeVisit(url string, c chan []string, quit chan int, fetcher Fetcher) {
    cache.Lock()
    defer cache.Unlock()
    if _, ok := cache.fetched[url] ; ok {
        quit <- 0
        return
    }
    body, urls, err := fetcher.Fetch(url)
    cache.fetched[url] = true
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Printf("Visited : %s, %q 
", url, body)
    c <- urls

}

// 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/cmd/",
            "http://golang.org/pkg/fmt/",
            "http://golang.org/pkg/os/",
        },
    },
    "http://golang.org/pkg/fmt/": &fakeResult{
        "Package fmt",
        []string{
            "http://golang.org/",
            "http://golang.org/pkg/",
        },
    },
    "http://golang.org/pkg/os/": &fakeResult{
        "Package os",
        []string{
            "http://golang.org/",
            "http://golang.org/pkg/",
        },
    },
}

这是一些示例输出

Visited : http://golang.org/, "The Go Programming Language" 
not found: http://golang.org/cmd/
Visited : http://golang.org/pkg/, "Packages" 
Visited : http://golang.org/pkg/os/, "Package os" 
**Visited : http://golang.org/pkg/fmt/, "Package fmt"** 

Process finished with exit code 0

与第一个不同的最后一个包不同（故意在上面的星号）

Visited : http://golang.org/, "The Go Programming Language" 
not found: http://golang.org/cmd/
Visited : http://golang.org/pkg/, "Packages" 
Visited : http://golang.org/pkg/os/, "Package os"

最后，甚至在某些运行中出现僵局：

Visited : http://golang.org/, "The Go Programming Language" 
not found: http://golang.org/cmd/
Visited : http://golang.org/pkg/, "Packages" 
Visited : http://golang.org/pkg/os/, "Package os" 
Visited : http://golang.org/pkg/fmt/, "Package fmt" 
fatal error: all goroutines are asleep - deadlock!

goroutine 1 [select]:
main.Crawl(0x4bfdf9, 0x12, 0x4, 0x524220, 0xc420088120, 0xc420092000, 0xc420092060)
    /home/kostas/development/challenges/go/helloWorld.go:26 +0x201
main.main()
    /home/kostas/development/challenges/go/helloWorld.go:39 +0xab

goroutine 23 [select]:
main.Crawl(0x4bfdf9, 0x12, 0x3, 0x524220, 0xc420088120, 0xc420092000, 0xc420092060)
    /home/kostas/development/challenges/go/helloWorld.go:26 +0x201
created by main.Crawl
    /home/kostas/development/challenges/go/helloWorld.go:31 +0x123

goroutine 24 [select]:
main.Crawl(0x4c09f9, 0x16, 0x3, 0x524220, 0xc420088120, 0xc420092000, 0xc420092060)
    /home/kostas/development/challenges/go/helloWorld.go:26 +0x201
created by main.Crawl
    /home/kostas/development/challenges/go/helloWorld.go:31 +0x123

goroutine 5 [select]:
main.Crawl(0x4bfdf9, 0x12, 0x3, 0x524220, 0xc420088120, 0xc420092000, 0xc420092060)
    /home/kostas/development/challenges/go/helloWorld.go:26 +0x201
created by main.Crawl
    /home/kostas/development/challenges/go/helloWorld.go:31 +0x123

goroutine 6 [select]:
main.Crawl(0x4c0a0f, 0x16, 0x3, 0x524220, 0xc420088120, 0xc420092000, 0xc420092060)
    /home/kostas/development/challenges/go/helloWorld.go:26 +0x201
created by main.Crawl
    /home/kostas/development/challenges/go/helloWorld.go:31 +0x123

我假设它与并发和递归有关。我已经在git hub中看到过使用等待组等的其他解决方案，但它没有在教程中使用 - 到目前为止的游览，所以我宁愿不使用它。

UPDATE

我弄清楚发生了什么，并在处理这个问题。基本上有时候select语句会陷入无限循环，因为通道退出并且c并不总是以预期的顺序执行。我添加了一个打印的默认案例（“无事可做”），程序有时会永远循环，有时候会以正确的方式运气。我的退出条件不对