New xdist APIs

[tucked]

pytest-dev/pytest-xdist#505

Seems like an upstreamed version of _is_master:
https://github.com/dbader/pytest-mypy/blob/3dc75dfc5c4ae6ee6dc0d35b88f7898221c3d089/src/pytest_mypy.py#L53-L58

Maybe this project should take advantage of them 🤔

[dmtucker]

is_xdist_master is not a drop-in replacement for _is_master for a couple reasons:

• It takes request_or_session instead of config.
  The only way I know of to get the former from the latter is:

  config.pluginmanager.getplugin("session")

• It returns False if xdist is not active.
  We could get around this by using not is_xdist_worker.

Even though we could make it work, I see 2 problems with proceeding:

1. No APIs can replace _get_xdist_workerinput (which isn't surprising, since it's a compatibility shim -- but even ignoring that, we still have to know about config.workerinput).
2. Accessing the APIs requires accessing xdist, which we'd probably want a helper function for:

   xdist = config.pluginmanager.getplugin("xdist")
   if not (xdist and xdist.is_xdist_worker(config.pluginmanager.getplugin("session"))):

   That basically just means re-implementing _is_master in a slower (though, perhaps more correct) way.

I think 1 is really the deal-breaker. The only way we can justify making _is_master slower is if it is indeed more correct. By that I mean: xdist could redefine how to identify an xdist worker without breaking us. If they did that, though, _get_xdist_workerinput would most likely still break.