[issue38591] Deprecate Process Child Watchers

[Kyle Stanley]

Kyle Stanley <aeros167 at gmail.com> added the comment:

> I think FastChildWatcher and SafeChildWatcher should go, ThreadedChildWatcher should be kept default and MultiLoopChildWatcher is an option where ThreadedChildWatcher is not satisfactory.

Okay, I think I can understand the reasoning here. Do you think that FastChildWatcher and SafeChildWatcher could be deprecated starting in 3.9 and removed in 3.11? If so, I'd be glad to start working on adding the deprecation warnings and the 3.9 Whats New entries.

> MultiLoopChildWatcher problems can and should be fixed; there is nothing bad in the idea but slightly imperfect implementation.

Yeah my largest concern was just that the current issues seem especially complex to fix and I interpreted your previous comment as "I'd like to deprecate all the process watchers in the near future". Thus, it seemed to make sense to me that we could start removing them rather than sinking time into fixing something that might be removed soon. 

By "slightly imperfect implementation", do you have any ideas for particularly imperfect parts of it that could use improvement? 

I feel that I've developed a decent understanding of the implementation for ThreadedChildWatcher, but after looking at the race conditions for MultiLoopChildWatcher in https://bugs.python.org/issue38323, I'll admit that I felt a bit lost for where to find a solution. Primarily because my understanding of the signal module is quite limited in comparison to others; it's not an area that I've strongly focused on.

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<https://bugs.python.org/issue38591>
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