My solution to the Tour of Go Web Crawler (Hint: read the sync package docs at https://golang.org/pkg/sync/.)

tour_of_go_web_crawler.go

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)
}

// Crawl uses fetcher to recursively crawl
// pages starting with url.
func Crawl(url string, fetcher Fetcher, visited *sync.Map, wg *sync.WaitGroup) {
	defer wg.Done()
	if _, ok := visited.Load(url); ok {
		//fmt.Printf("skipping: %s\n", url)
		return
	}
	body, foundUrls, err := fetcher.Fetch(url)
	visited.Store(url, true)
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Printf("found: %s %q\n", url, body)
	for _, newUrl := range foundUrls {
		wg.Add(1)
		go Crawl(newUrl, fetcher, visited, wg)
	}
}

func main() {
	var visited sync.Map
	var wg sync.WaitGroup
	wg.Add(1)
	go Crawl("https://golang.org/", fetcher, &visited, &wg)
	wg.Wait()
}

// 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{
	"https://golang.org/": &fakeResult{
		"The Go Programming Language",
		[]string{
			"https://golang.org/pkg/",
			"https://golang.org/cmd/",
		},
	},
	"https://golang.org/pkg/": &fakeResult{
		"Packages",
		[]string{
			"https://golang.org/",
			"https://golang.org/cmd/",
			"https://golang.org/pkg/fmt/",
			"https://golang.org/pkg/os/",
		},
	},
	"https://golang.org/pkg/fmt/": &fakeResult{
		"Package fmt",
		[]string{
			"https://golang.org/",
			"https://golang.org/pkg/",
		},
	},
	"https://golang.org/pkg/os/": &fakeResult{
		"Package os",
		[]string{
			"https://golang.org/",
			"https://golang.org/pkg/",
		},
	},
}