Incomplete async support?

[oliverhaas]

For vaguely related reasons I looked through the internals of pghistory (great package btw 😃) last week, and since I have the topic of async django on the back of my mind more and more, I noticed the use of threading.local and was trying to wrap my head around how or how well async is supported in pghistory.

I found this issue #217, which supposedly fixes async support, but my initial thought was that any solution with threading.local instead of ContextVar would be incomplete, but I'm definitely not well versed in the topic.

I think pghistory supports async only as far as one does not have real concurrency with async and "accidentally" still serializes due to async_to_sync / sync_to_async, but breaks otherwise. The failure mode is actually not intuitive, apparently since merging the context and event loop and thread interaction. Here a simple test repro oliverhaas#2

@pytest.mark.asyncio
@pytest.mark.django_db(transaction=True)
async def test_concurrent_async_contexts():
    """Two concurrent async tasks each opening their own pghistory.context
    must produce two independent Contexts with disjoint metadata.
    """

    async def handler(label):
        async with pghistory.context(label=label):
            await test_models.BigAutoFieldModel.objects.acreate()

    await asyncio.gather(handler("a"), handler("b"))

    contexts = [
        c
        async for c in pghistory.models.Context.objects.order_by("metadata__label").all()
    ]
    assert [c.metadata for c in contexts] == [{"label": "a"}, {"label": "b"}]

    await test_models.BigAutoFieldModel.pgh_event_model.objects.all().adelete()
    await test_models.BigAutoFieldModel.objects.all().adelete()
    await pghistory.models.Context.objects.all().adelete()

AssertionError: assert [{'label': 'a'}] == [{'label': 'a'}, {'label': 'b'}]

Right contains one more item: {'label': 'b'}

Full diff:
  [
      {
          'label': 'a',
      },
-     {
-         'label': 'b',
-     },
  ]

Obviously this is not a realistic django request cycle, and I know there are a lot of cases where the context would just work, but it also looks to me that there are big gaps.

I'm still not certain about a lot of things and I still wouldn't be surprised if I was missing something obvious, but in any case I wanted to take a small step and ask here whether someone has more information. If I'm correct I would be looking forward into making a PR with a ContextVar solution (so the internal parts), plus probably a second PR for stuff like async middleware.