Feature proposal: Ability to pass browserUrl to puppeteer.connect

Description

Add support for new browserUrl property in options object passed to puppeteer.connect(options). It would accept url in form of http://{host}:{port} which should point to the existing Chromium instance. For example:

puppeteer.connect({
    browserUrl: http://localhost:9222
})

If this option is passed to puppeteer.connect, then browserWSEndpoint is retrieved from http://{host}:{port}/json/version and passed new Connection(). The rest of the flow remains unchanged.

If you’re OK with the proposal I could write up the code and the tests

Background

Currently to attach Puppeteer to an existing Chrome instance, one must provide url of browser websocket endpoint. For example:

puppeteer.connect({
    browserWSEndpoint: ws://localhost:9222/devtools/browser/b0b8a4fb-bb17-4359-9533-a8d9f3908bd8
})

However, as many issues raised here show (#2790, #1645, #940) browserWSEndpoint is often not known to the code calling puppeteer.connect, for example because Chrome was lunched in a docker container.

In such a case, a common and repeating scenario is to:

1. make HTTP request to http://{host}:{port}/json/version
2. parse the response JSON
3. get browserWSEndpoint from and use it in puppeteer.connect

The steps above could be done by puppeteer resulting in less code in the user-land.

[lukaszfiszer]

@andrejska my PR #3558 is ready and awaiting review

[sublimator]

How does one do this for Firefox? (After say using web-ext run to launch Firefox with an extension)