[issue12187] subprocess.wait() with a timeout uses polling on POSIX

[Charles-François Natali]

Charles-François Natali <neologix at free.fr> added the comment:

> For subprocess.wait(), we can do something with signals (SIGCHLD and/or SIGCLD).

There's just one problem: SIGCHLD is ignored by default, which means that sigwait and friends won't return when a child exits.
Well, it actually works on recent Linux kernels, but POSIX makes no such guarantee, and it's at least known to fail on Solaris (see Dave Butenhof's comment):
http://www.multithreadedprogramming.info/sigwait-ing-for-sigchld

To be portable, we would need to set a handler for SIGCHLD, which has the following problems:
- you have to do that from the main thread
- it impacts every thread
- it will make syscalls fail with EINTR
- once you've changed SIGCHLD setting, you can't go back to the original semantic (setting it to SIG_IGN again will prevent children from becoming zombies, and waitpid wait until all children exited and will fail with ECHILD)

Note that even if it does work, there's a problem in multi-threaded programs, because the signal must be blocked by all the threads...

But since we use it with a timeout, we could also consider that this will work on systems that allow ignore signals to be catched by sigtimedwait, and it will wait the full timeout on other systems. I don't know if that's acceptable.

> Why not use signalfd() when available?

It suffers from the same issue, and it's Linux-specific (sigwait and friends are POSIX).

Note that exposing sigtimedwait is probably useful anyway, and I'd like to work on a patch.
Note that I'm not sure that exposing sigtimedwait is necessary (I don't think that the info field is going to be used by Python applications): how about just adding an optional timeout argument to signal_sigwait?

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nosy: +neologix

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<http://bugs.python.org/issue12187>
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