exceptions raised before calling `task_status.started` loses cause & context

[jakkdl]

This snippet eats cause&context

trio/src/trio/_core/_run.py

Lines 1387 to 1389 in 9c909fa

	except BaseExceptionGroup as exc:
	if len(exc.exceptions) == 1:
	raise exc.exceptions[0] from None

a la https://flake8-async.readthedocs.io/en/latest/rules.html#async123

we currently only run the ruff reimplementation of flake8-async, which hasn't added that rule. So we should maybe consider running flake8-async as well, disabling any rules that ruff handles [better], or are noisy.

It should make use of trio._util.raise_saving_context.

repro:

import trio


async def task(task_status: trio.TaskStatus) -> None:
    e = ValueError("foo")
    e.__cause__ = SyntaxError("bar")
    e.__context__ = TypeError("baz")
    raise e

async def main() -> None:
    async with trio.open_nursery() as nursery:
        await nursery.start(task)

try:
    trio.run(main)
except ExceptionGroup as excgroup:
    # should be SyntaxError, is None
    print(excgroup.exceptions[0].__cause__)
    # should be TypeError, is the exceptiongroup
    print(repr(excgroup.exceptions[0].__context__))

[A5rocks]

I was looking at flake8-async:

• we would need to bring in flake8 if we want to ignore the test directories?
• I think the tags are out of sync -- would pre-commit really work?

If those are bugs I can go make issues. (also generally theres a lot of errors raised...)

[jakkdl]

we would need to bring in flake8 if we want to ignore the test directories?

we can use the pre-commit files or exclude directives to exclude them

I think the tags are out of sync -- would pre-commit really work?

if that is a problem I would love to know so I can fix it!

[A5rocks]

we can use the pre-commit files or exclude directives to exclude them

I was hoping to maybe be able to run it as a command, but this makes enough sense to me.

if that is a problem I would love to know so I can fix it!

Yeah for instance the tag referenced on the docs doesn't exist:

(.venv) PS C:\Users\A5rocks\Documents\trio> pre-commit run -a
[INFO] Initializing environment for https://github.com/python-trio/flake8-async.
An unexpected error has occurred: CalledProcessError: command: ('C:\\Program Files\\Git\\cmd\\git.EXE', 'checkout', '25.5.2')
return code: 1
stdout: (none)
stderr:
    error: pathspec '25.5.2' did not match any file(s) known to git
Check the log at C:\Users\A5rocks\.cache\pre-commit\pre-commit.log

This isn't really a problem (we can just use old versions in pre-commit, or maybe even commit hashes?) but it's suboptimal.

[jakkdl]

whelp, it appears I broke git tagging in python-trio/flake8-async#321
thank you, will fix!

EDIT: fixing in python-trio/flake8-async#385