duplicated url are crawled twice

[Minyar2004]

What is the current behavior?

Duplicated urls are not skipped. The same url is crawled twice.

If the current behavior is a bug, please provide the steps to reproduce

const HCCrawler = require('./lib/hccrawler');

(async () => {
  const crawler = await HCCrawler.launch({
    evaluatePage: () => ({
      title: document.title,
    }),
    onSuccess: (result => {
      /console.log(result);
    }),
    skipDuplicates: true,
    jQuery: false,
    maxDepth: 3,
    args: ['--no-sandbox']
  });
  
  await crawler.queue([{
        url: 'https://www.example.com/'
      }, {
        url: 'https://www.example.com/'
  }]);

  await crawler.onIdle(); 
  await crawler.close(); 
})();

What is the expected behavior?

Crawled urls should be skipped even if they come from the queue.

Please tell us about your environment:

• Version: lastest
• Platform / OS version: Centos 7.1
• Node.js version: v8.4.0

[davidebaldini]

The reason might lie in helper.js:

static generateKey(options) {
    const json = JSON.stringify(pick(options, PICKED_OPTION_FIELDS), Helper.jsonStableReplacer);
    return Helper.hash(json).substring(0, MAX_KEY_LENGTH);
  }

Uniqueness is assessed from a hash generated on the result of JSON.stringify(), but this method doesn't guarantee constant order.

I'm looking for opinions. See https://github.com/substack/json-stable-stringify

[BubuAnabelas]

Same as #299
@yujiosaka should look into this.

[SuperFireFoxy]

headless 模式下一直报302

[popstas]

I found two reasons:

1. maxConcurrency > 1, same page requested in parallel threads.
2. Page that redirected will deduplicate source url, not target. You can skip these urls by setting skipRequestedRedirect: true

[kulikalov]

is anyone consider creating a PR?

[iamprageeth]

Just posting here hoping this would help someone. This is true it crawls duplicate URLs when concurrency > 1. So here is what I did.

1. First created a sqlite database.
2. Then in RequestStarted event, insert the current url.
3. In preRequest function (You can pass this function along with options object) , just check whether there is a record of current url. If it is there that means url has crawler or still crawling. so return false. It will skip the url
4. In RequestRetried, RequestFailed events, delete the url. So that will allows crawler to try it again.