tox and pytest runs can behave differently when "-p something.conftest" is specified

Did a little bit of debugging and wanted to share my findings here:

I have an sdist muacrypt which contains a test_muacrypt package and a conftest file inside.
In another project i have a pytest.ini containing addopts = -p test_muacrypt.conftest. This makes all fixtures and command line options available in a normal pytest run. fine.

Problem: When i run tox the fixtures from that plugin are not loaded.
This is surprising from a pure user perspective (tox and pytest runs should not have such differences).

Turns out, the problematic code is in this FixtureManager method:

def pytest_plugin_registered(self, plugin):
    nodeid = None
    try:
        p = py.path.local(plugin.__file__)
    except AttributeError:
        pass
    else:
        # construct the base nodeid which is later used to check
        # what fixtures are visible for particular tests (as denoted
        # by their test id)
        if p.basename.startswith("conftest.py"):
            nodeid = p.dirpath().relto(self.config.rootdir)
            if p.sep != nodes.SEP:
                nodeid = nodeid.replace(p.sep, nodes.SEP)
    self.parsefactories(plugin, nodeid)

in my case, test_muacrypt.conftest has a basename of conftest.py and it's path (inside .tox) is relative to the project root. So the plugin gets a "nodeid" and lookups for fixtures from that plugin fail, because the requesting side will not be a child id of that '.tox/py27/.../conftest.py`. Without tox, Plugins which reside outside a project rootdir will get an empty nodeid and are thus found properly.

Not sure how to fix this in pytest properly yet.

A work aground for me is to put the plugin into test_muacrypt.testing_plugin or so and import everything from the conftest file there.

Start by examining FixtureManager.pytest_plugin_registered and parsefactories, especially how conftest.py paths under tox are converted into nodeids. Reproduce the difference between a normal pytest run and tox with -p test_muacrypt.conftest, then verify that fixtures and command-line options load consistently in both environments.