https://go-tour-jp.appspot.com/concurrency/10

exercise-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 Page struct {
	body string
	urls []string
	err  error
}

type PageCache struct {
	mu sync.Mutex
	v  map[string]Page
}

type CrawlerTasks struct {
	mu    sync.Mutex
	v int
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
	tasks := CrawlerTasks{v: 1}

	cache := PageCache{v: make(map[string]Page)}

	CrawlInternal(url, depth, fetcher, &tasks, &cache)

	for { // TODO: fix busy wait
		if tasks.v == 0 {
			return
		}
	}
}

func CrawlInternal(url string, depth int, fetcher Fetcher, tasks *CrawlerTasks, cache *PageCache) {
	defer func() {
		tasks.mu.Lock()
		tasks.v = tasks.v - 1
		tasks.mu.Unlock()
	}()

	if depth <= 0 {
		return
	}

	cache.mu.Lock()
	page, hasCache := cache.v[url]

	if hasCache {
		cache.mu.Unlock()
	} else {
		body, urls, err := fetcher.Fetch(url)
		if err == nil {
			page = Page{body: body, urls: urls}
		} else {
			page = Page{err: err}
		}

		cache.v[url] = page
		// TODO: fix too long lock time
		cache.mu.Unlock()
	}

	if page.err == nil {
		fmt.Printf("found: %s %q\n", url, page.body)
	} else {
		fmt.Println(page.err)
	}

	tasks.mu.Lock()
	tasks.v = tasks.v + len(page.urls)
	tasks.mu.Unlock()

	for _, u := range page.urls {
		go CrawlInternal(u, depth-1, fetcher, tasks, cache)
	}
}

func main() {
	Crawl("https://golang.org/", 4, fetcher)
}

// 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/",
		},
	},
}

xLICENSE

Copyright (c) 2011 The Go Authors. All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

   * Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
   * Redistributions in binary form must reproduce the above
copyright notice, this list of conditions and the following disclaimer
in the documentation and/or other materials provided with the
distribution.
   * Neither the name of Google Inc. nor the names of its
contributors may be used to endorse or promote products derived from
this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.