concurrent.futures.wait() waits forever if given a cancelled future

Bug report

concurrent.futures.wait() hangs if given a future that is already cancelled.

Steps to reproduce:

1. Get a future that is cancellable (create lots of jobs such that the later ones cannot start yet and are therefore cancellable):

>>> import concurrent.futures
>>> pool = concurrent.futures.ThreadPoolExecutor(max_workers=1)
>>> import time
>>> for _ in range(1000): future = pool.submit(time.sleep, 1000)

2. Cancel this future and note that it returns True meaning it has been cancelled.

>>> future.cancel()
True

3. Now waiting on this future hangs:

>>> concurrent.futures.wait([future]) 

The expected behaviour, regardless of the value of return_when is that wait() treats cancelled futures as already completed. This is essentially how return_when=FIRST_COMPLETED and return_when=ALL_COMPLETED are documented to behave:

The function will return when any future finishes or is cancelled.

and

The function will return when all futures finish or are cancelled.

Treating a future that is cancelled prior to wait() as different to one that changes state to cancelled once waiting risks a race condition: it could change state after any possible check but before blocking. i.e. it needs to be level-triggered not edge-triggered. And indeed, it works this way for futures that are done, just not ones that are cancelled.

Your environment

• CPython versions tested on: 3.9.7, 3.10.2
• Operating system and architecture: Linux

Bắt đầu bằng cách tái hiện ví dụ được cung cấp với concurrent.futures.wait() và một future đã bị hủy. Kiểm tra entry point wait() và cách xử lý các future bị hủy, sau đó bổ sung coverage cho hành vi được báo cáo; hoàn thành khi wait() trả về thay vì bị treo đối với các giá trị return_when được tài liệu hóa.