gh-110205: Fix asyncio ThreadedChildWatcher._join_threads()

[vstinner]

ThreadedChildWatcher._join_threads() now clears references to completed threads.

test_asyncio.utils.TestCase now calls _join_threads() of the watcher, uses SHORT_TIMEOUT to join a thread, and then raises an exception if there are still running threads.

Rename also ThreadedChildWatcher threads to add "asyncio-" prefix to the name.

• Issue: test_asyncio.test_subprocess leaked dangling threads on PPC64LE Fedora Stable 3.x: #110205

[gvanrossum]

Is there any advantage to using the internal API _join_threads() instead of the internal variable _threads? You could just add the timeout arg to the join() call in the existing test cleanup code.

[gvanrossum]

Why are you creating a new instance variable self.threads here? Did you intend to clean out completed threads from the dict self._threads?

[vstinner]

I used the list comprehension above as an example, for me it looks easier to remove completed threads rather than having a regular list and removing items from the list.

The purpose of this line is to remove completed threads to save memory and to fix the dangling threads issue.

[vstinner]

Do you mean that you prefer to not create a new list?

[vstinner]

I modified my PR to use self.threads[:] = ..., so the self.threads list object is not replacd.

[gvanrossum]

Do you mean that you prefer to not create a new list?

No I meant that the tests fail because self._threads is a dict, not a list.

[vstinner]

Is there any advantage to using the internal API _join_threads() instead of the internal variable _threads?

It fix the issue related to the PR: dangling thread. The warning can be emitted when the Python object still exist, even if the thread is no longer running. I don't think that copy/paste the _join_threads() logic in the test is worth it.

[gvanrossum]

Please take a break and read the comments and the code. There is no variable self.threads.

[vstinner]

Please take a break and read the comments and the code. There is no variable self.threads.

Ah, I misunderstood your comment. I fixed the typo, the code now sets self._threads.

[gvanrossum]

As I was trying to let you see for yourself, self._threads is a dict, not a list. If you want to update it using a comprehension you can use something like this:

self._threads ={key: thread for key, thread in self._threads.items() if thread.daemon or thread.is_alive()} 

Please test locally before merging.