explicitly start ASGI run with empty context

[pmeier]

Summary

One possible solution for #2167.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Kludex]

This does make sense, but do you know why it happens?

[pmeier]

I think so, yes. asyncio does not clean up the context when starting a new task. This is why they added the context keyword argument in 3.11 to make this simpler. The documentation states

> The current context copy is created when no context is provided.

So far I only have a not-so-minimal reproducer: https://github.com/pmeier/asyncio-contextvars-pollution. Using the patch in this PR, the pollution is resolved.

How exactly it happens in the first place, I don't know yet. My goal is to get to a reproducer that just depends on uvicorn that I hopefully can put into a unittest. This could serve as testing ground to find the underlying reason.

Finally, my working hypothesis as for why this is not happening by uvloop is that they are not a drop-in replacement for this specific behavior of asyncio. On the surface it seems that they are replicating it correctly, but I haven't dug any deeper just yet.

@Kludex I was wrong. This is a bug in asyncio. See #2167 (comment) for details.

[pmeier]

Ping @Kludex

[Kludex]

We need to fix it in httptools as well, and do you think you can create a test that reproduces the issue?

[pmeier]

@Kludex

We need to fix it in httptools as well

Can you explain that? Are you talking about

uvicorn/pyproject.toml

Lines 41 to 44 in 8ae0bcb

	[project.optional-dependencies]
	standard = [
	"colorama>=0.4; sys_platform == 'win32'",
	"httptools>=0.6.3",

https://github.com/MagicStack/httptools

do you think you can create a test that reproduces the issue?

Yeah, likely. I haven't written one, because the solution is not the only one that is possible to fix the issue. See #2167 (comment).

Can you decide if you want to go with the solution before I put in more work?

[Kludex]

Can you decide if you want to go with the solution before I put in more work?

It's a bit unfortunate we need to do it here, but I guess it's fine.

---

Can you explain that? Are you talking about

We have a h11_impl.py and httptools_impl.py that has analogous logic.

[pmeier]

@Kludex

It's a bit unfortunate we need to do it here, but I guess it's fine.

There is an open PR python/cpython#141158 that'll hopefully fix this for Python >=3.15. But until that there is no way around doing this in uvicorn if we don't want to tell users to use uvloop by default.

Worse, while writing the test I realized that my attempted fix, i.e. being explicit about an empty context does not work. At least we now know that the test will actually fail on a regression.