My first concurrent program with goroutine (for commemoration purposes)

crawler.go

// https://tour.golang.org/concurrency/10

package main

import (
	"fmt"
	"sync"
)

type SafeMap struct {
  mu sync.Mutex
  v  []string
}

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)
}

func contains(search string, s []string) bool {
	for _, a := range s {
		if a == search {
			return true
		}
	}
	return false
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func (c *SafeMap) Crawl(url string, depth int, fetcher Fetcher, wg *sync.WaitGroup) {
	// TODO: Fetch URLs in parallel.
	// TODO: Don't fetch the same URL twice.
	// This implementation doesn't do either:
	defer wg.Done()

	//fmt.Printf("crawling: %s\n", url)
	if depth <= 0 {
		return
	}
	
	c.mu.Lock()
	if contains(url, c.v) {
		c.mu.Unlock()
		return
	}

	c.v = append(c.v, url)
	body, urls, err := fetcher.Fetch(url)

	if err != nil {
		fmt.Println(err)

		c.mu.Unlock()
		return
	}

	c.mu.Unlock()

	fmt.Printf("found: %s %q\n", url, body)
	for _, u := range urls {
	    //fmt.Printf("firing next: %s\n", u)
		wg.Add(1)
		go c.Crawl(u, depth-1, fetcher, wg)
	}

	return
}

func main() {
	c := SafeMap{v: make([]string, 100)}
	var wg sync.WaitGroup
	wg.Add(1)
	go c.Crawl("https://golang.org/", 4, fetcher, &wg)
	wg.Wait()
	fmt.Printf("Complete")
}

// 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/",
		},
	},
}