gh-83069: Use efficient event-driven subprocess.Popen.wait() on Linux / macOS / BSD

[giampaolo]

This patch replaces the busy-loop mechanism used by subprocess.Popen.wait() on Linux, BSD, and macOS when a timeout is specified:

• Linux >= 5.3: uses os.pidfd_open() + select.poll()
• macOS, BSD: uses select.kqueue() + KQ_FILTER_PROC + KQ_NOTE_EXIT

The kernel wakes us up exactly when the PID terminates or the timeout expires. If the fast-wait mechanisms is not available or fails (e.g. "too many open files"), the function falls back to the traditional busy loop. A similar approach was recently implemented in psutil (see giampaolo/psutil#2706). While working on that change, I noticed that the subprocess module used essentially the same busy-loop strategy, which motivated this proposal.

EDIT: I wrote a blog post about this: https://gmpy.dev/blog/2026/event-driven-process-waiting

• Issue: Popen should use pidfd_open to implement a non-busy wait #83069

[vstinner]

Nice change! I like it. Here is a first review.

cc @gpshead

[giampaolo]

Thanks for the review @vstinner. I addressed your comments.

[vstinner]

LGTM. Thanks for the updates!

[giampaolo]

Whops! I fixed the remaining bare assert statements.

[vstinner]

@gpshead: Do you want to double check the change?

[giampaolo]

In the hope of adding BSD / macOS support for asyncio in the future, I have opened #144211.

[vstinner]

Merged, thank you for this nice optimization!