Understand the changes of event-loop when moving from 0.21

[jefflieu]

Hello,
We've been stuck with version 0.21 for a while and now got some time to move on and update to latest version.
I think we've got it working but not quite understand the concept of the changes behind it. What's the changes in the event-loop mechanism before and after version 0.21

Before:

@pytest.fixture(scope="session")
def event_loop():
     try:
         event_loop = asyncio.get_running_loop()
     except RuntimeError:
         event_loop = asyncio.new_event_loop()
     yield event_loop
     event_loop.close()

After:

from pytest_asyncio import is_async_test
def pytest_collection_modifyitems(items):
    pytest_asyncio_tests = (item for item in items if is_async_test(item))
    session_scope_marker = pytest.mark.asyncio(loop_scope="session")
    for async_test in pytest_asyncio_tests:
        async_test.add_marker(session_scope_marker, append=False)

And

[tool.pytest.ini_options]
asyncio_default_fixture_loop_scope = "session"

I understand that these set the the event loop for the test and the fixture to the same "session" loop.
I did some readings and discussion,
but I still don't understand exactly what's the changes that broke it and requires this setting. The changes doesn't seem to be explicitly documented anywhere, it may be embedded in discussions that I might have missed.
I can see documents describing a migration path, but not explain the mechanics of the changes behind the scene.

Without pytest_collection_modifyitems .. I do some experiment and it shows me that background tasks that I launch in the setup of the fixture doesn't run when the test function itself starts.

For example i have this file

import asyncio
import pytest
import pytest_asyncio
import logging


def fork_task(coro):
    # loop = asyncio.get_event_loop()
    loop = asyncio.get_running_loop()
    return loop.create_task(coro)


async def background_task():
    for i in range(10):
        logging.info(f"i={i}")
        await asyncio.sleep(1)


@pytest_asyncio.fixture(scope="package")
async def test_fixture():
    fork_task(background_task())
    yield None


@pytest.mark.asyncio
async def test_queue_writing(test_fixture):
    logging.info("Test starts")
    await asyncio.sleep(10)
    logging.info("Test ends")

Without pytest_collection_modifyitems or (@pytest.mark.asyncio(loop_scope="session")) --> the background_task doesn't run when it enters the test_queue_writing

Could you please point me back to the basic of pytest-asyncio?
Thank you very much!