Proposed change

Users tend to be fond of command line sensors even though they tend to be extremely inefficient (its compounded by posix_spawn not being detected on alpine which is addressed in #87958). We can make that a bit better by using PidfdChildWatcher when available since we avoid creating a separate thread per child process.

https://docs.python.org/3/library/asyncio-policy.html#asyncio.PidfdChildWatcher

PidfdChildWatcher is a “Goldilocks” child watcher implementation. It doesn’t require signals or threads, doesn’t interfere with any processes launched outside the event loop, and scales linearly with the number of subprocesses launched by the event loop. The main disadvantage is that pidfds are specific to Linux, and only work on recent (5.3+) kernels.

This is a backport from cpython 3.12 python/cpython#98024. Note that there are some additional fixes in cpython 3.12 in python/cpython#94184 when there is no event loop running in the main thread but this is not a problem we have.

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