Make it possible to create a session using an empty object

[andreastt]

At minimum, the New Session command mandates the presence of a capabilities field with an empty object for creating a new session:

{"capabilities": {}}

This is due to how the processing capabilities algorithm is phrased:

1. Let capabilities request be the result of getting the property "capabilities" from parameters.
   1. If capabilities request is not a JSON object, return error with error code invalid argument.

The logical consequence of this is that using an empty top-level JSON Object, like this, is illegal:

{}

It was @whimboo who discovered this, and first reported it in https://bugzilla.mozilla.org/show_bug.cgi?id=1470976.

[shs96c]

Is this a desirable property? Having the requirement of the empty object implies that someone has made a meaningful decision to start a session, rather than just POSTing to random URLs.

[andreastt]

geckodriver currently accepts POST /session with body {} because it still supports Selenium-styled desired/required capabilities. Per the spec it is wrong of geckodriver to take an empty object and we would have to drop support for legacy desired/required in order to be conforming. I would rather that we didn’t have the idiosyncrasy that forces {"capabilities": {}} to be used.

[shs96c]

I've no strong feeling either way, but I do have a slight preference for making session initialisation more deliberate.

FWIW, I think all the local ends send {"capabilities": {}} so it should be safe to drop in geckodriver. If they don't, they should be following the spec: these things aren't just written for remote-end implementors.

[andreastt]

I would rather not that the input for New Session be a JSON special case when other commands allow optional parameters to be dropped.

E.g. Set Window Rect allows {}, {width: 200, height: 200}, as well as {width: 200, height: 200, x: 0, y: 0} as input.

Saying that session creation must use {capabilities: {}} for no apparent reason seems counter intuitive to anyone to works with WebDriver at the HTTP request/response level. Why not {capabilities: {alwaysMatch: {}}} or {capabilities: {alwaysMatch: {}, firstMatch: []}} if you want to exhaust the data structure?

[shs96c]

{} is fine as well, I just don't want a POST to the new session end point start a session unless it has some form of payload.