os: use pidfd_open with epoll on Linux instead of blocking wait in blockUntilWaitable

[mitar]

My understanding is that in blockUntilWaitable the waitid call blocks the thread you call it on. On Linux, we could instead use pidfd_open and then monitor the file descriptor with epool:

When the process that it refers to terminates, these interfaces (e.g., epool) indicate the file descriptor as readable.

I think this would then not block the thread?

[ianlancetaylor]

I don't see any API change here, so this doesn't have to be a proposal. Taking it out of the proposal process.

[mitar]

OK, I made a draft of the implementation in #60461 to test how would this work out. It is not yet perfect (see comments on Gerrit), but it gives me/us something to play around. I think it works pretty nice.

[gopherbot]

Change https://go.dev/cl/498615 mentions this issue: os/exec: use pidfd for waiting and signaling of processes