"A Tour of Go" http://tour.golang.org/#70 共有リソースの排他がなく、問題あり

tour70.go

/*
* http://tour.golang.org/#70
* OR
* http://http://go-tour-jp.appspot.com/#69
*/

package main

import (
	"os"
	"fmt"
	"runtime"
)

type dispResult struct {
	url  string
	body string
}

var checkUrl map[string]int

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 os.Error)
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, ch chan dispResult, fetcher Fetcher) {
	// TODO: Fetch URLs in parallel.
	// TODO: Don't fetch the same URL twice.
	// This implementation doesn't do either:
	if depth <= 0 {
		return
	}

	// URLのチェック回数を記録
	checkUrl[url]++
	// 2度目のチェックはしない
	if checkUrl[url] > 1 {
		return
	}

	body, urls, err := fetcher.Fetch(url)
	if err != nil {
		fmt.Println(err)
		return
	}
	ch <- dispResult{url, body}
	for _, u := range urls {
		go Crawl(u, depth-1, ch, fetcher)
	}
	return
}

func main() {
	ch := make(chan dispResult)
	checkUrl = make(map[string]int)

	go Crawl("http://golang.org/", 4, ch, fetcher)

	for {
		if (runtime.Goroutines() == 1) {
			break
		} else {
			select {
			case v := <- ch:
				fmt.Printf("found: %s %q\n", v.url, v.body)
			}
		}
	}
}

// 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, os.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/",
		},
	},
}