A solution for the Web Crawler concurrency exercise in the Tour of Go (https://go.dev/tour/concurrency/10)

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)
}

type History struct {
    mu sync.Mutex
    set map[string]struct{}
}

func (hist *History) HasSeen(url string) bool {
    hist.mu.Lock()
    _, seen := hist.set[url]
    hist.set[url] = struct{}{}
    hist.mu.Unlock()
    return seen
}

var hist History = History{set: make(map[string]struct{})}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher, ret chan string) {
    defer close(ret)
    if depth <= 0 || hist.HasSeen(url) {
        return
    }
    body, urls, err := fetcher.Fetch(url)
    if err != nil {
        ret <-err.Error()
        return
    }

    ret <-fmt.Sprintf("found: %s %q", url, body)

    result := make([]chan string, len(urls))
    for i, u := range urls {
        result[i] = make(chan string)
        go Crawl(u, depth-1, fetcher, result[i])
    }

    for i := range result {
        for s := range result[i] {
            ret <-s
        }
    }
}

func main() {
    result := make(chan string)
    go Crawl("https://golang.org/", 4, fetcher, result)

    for s := range result {
        fmt.Println(s)
    }
}

// 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/",
		},
	},
}