ASYNC102 await-in-finally-or-cancelled false alarms on nurseries

[jakkdl]

flake8-async/tests/eval_files/async102.py

Lines 117 to 125 in 225f15a

	try:
	pass
	finally:
	# false alarm, open_nursery does not block/checkpoint on entry.
	async with trio.open_nursery() as nursery: # error: 8, Statement("try/finally", lineno-4)
	nursery.cancel_scope.deadline = trio.current_time() + 10
	nursery.cancel_scope.shield = True
	# false alarm, we currently don't handle nursery.cancel_scope.[deadline/shield]
	await foo() # error: 12, Statement("try/finally", lineno-8)

TODO:

• Special-case async with open_nursery(...), and async with anyio.create_task_group(...) as not being checkpoints. (also affects other rules)
• handle [nursery_name].cancel_scope.[deadline/shield]

[jakkdl]

The other rules that could be relevant are:

• ASYNC100 cancel-scope-no-checkpoint
  • A cancel scope with only a nursery can never be cancelled, so this should ignore open_nursery/create_task_group
• ASYNC120 await-in-except
  • Same as 102, and also already fixed by fix various issues with ASYNC102 #289
• ASYNC91x
  • nurseries can checkpoint, so these should not ignore nurseries, and do not need to be changed.

async def foo(): # should not give ASYNC910
  async with trio.open_nursery() as nursery():
    nursery.start_soon(bar)

tl;dr, ASYNC100 also needs changing

[jakkdl]

Hrm, neither of these statements are cancellation sources

my_nursery = await trio.open_nursery()
async with my_nursery:
    ...

So it's not just async with trio.open_nursery() that needs to be checked against.
Both of those cases are definitely less common, but far from unheard of. The former is an easy fix, the latter is a bit more tricky but we're already type-tracking nurseries for other reasons so shouldn't be too bad.