My solution for web crawler exercise in Go tour: http://tour.golang.org/concurrency/10

exercise-web-crawler.go

package main

import (
	"fmt"
)

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)
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(crawlq chan *CrawlState, fetchq chan *CrawlState, fetcher Fetcher) {
	for {
		in := <-fetchq
		body, urls, err := fetcher.Fetch(in.url)
		if err == nil {
			fmt.Printf("found: %s %q\n", in.url, body)
			in.urls = urls
		} else {
			fmt.Println(err)
			in.urls = []string{}
		}
		crawlq <- in
	}
}

type CrawlState struct {
	depth int
	url   string
	urls  []string
}

func main() {
	history := make(map[string][]string)
	crawlq := make(chan *CrawlState, 100)
	fetchq := make(chan *CrawlState, 4)
	go Crawl(crawlq, fetchq, fetcher)
	go Crawl(crawlq, fetchq, fetcher)
	go Crawl(crawlq, fetchq, fetcher)
	go Crawl(crawlq, fetchq, fetcher)
	count := 1
	crawlq <- &CrawlState{4, "http://golang.org/", nil}
	for count > 0 {
		state := <-crawlq
		count--
		if state.urls != nil {
			history[state.url] = state.urls
		}
		if state.depth > 0 {
			urls, ok := history[state.url]
			if ok {
				for _, url := range urls {
					crawlq <- &CrawlState{state.depth - 1, url, nil}
					count++
				}
			} else {
				fetchq <- state
				count++
			}
		}

	}
}

// 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/",
		},
	},
}

I assume that the interface (call structure, definition) of the Crawl function shall be retained, as provided in the template for the task. This is not done here.