Autouse session-scoped fixtures are sometimes torn down in the wrong order

[jordanlibrande]

In the pytest documentation https://docs.pytest.org/en/6.2.x/fixture.html#fixture-instantiation-order, fixtures with the autouse flag are ordered specially vs non-autouse fixtures (and the ordering for instantiating fixtures should be reversed when tearing down fixtures https://docs.pytest.org/en/6.2.x/fixture.html#yield-fixtures-recommended).

Teardown ordering is incorrect sometimes (both different from vanilla pytest, and not following the "reverse ordering for teardown" rule) for a session where you have async tests and both explicitly depended session-scoped fixtures and autouse session-scoped fixtures.

Following the autouse ordering, we should also probably ensure not to run autouse fixtures concurrently with explicitly-selected fixtures.

Example code (with pytest-trio and trio mode enabled):

import pytest


@pytest.fixture(scope="session")
def f1():
    print("f1 initialization")
    yield "f1"
    print("f1 teardown")


@pytest.fixture(scope="session")
def f2():
    print("f2 initialization")
    yield "f2"
    print("f2 teardown")


@pytest.fixture(scope="session", autouse=True)
def f3():
    print("f3 initialization")
    yield "f3"
    print("f3 teardown")


async def test_foo(f1: str):
    print("Running test_foo")


async def test_bar(f2: str):
    print("Running test_bar")

Output:

tests/repro.py::test_bar
f3 initialization   <<<<<<<<<<<< initialization ordering is fine
f2 initialization

Running test_bar
PASSED
tests/repro.py::test_foo f1 initialization

Running test_foo
PASSEDf1 teardown
f3 teardown        <<<<<<<<<<< Unexpected, f3 teardown should happen last
f2 teardown

[jordanlibrande]

This is a bug with pytest itself, see issue pytest-dev/pytest#1489