Allow waitpid(-1) to be woken if a waitpid(pid) call is pending

If two threads are running, with one calling waitpid(-1), and another
calling waitpid($some_pid), and then $some_other_pid exits, we would
expect the waitpid(-1) call to retrieve that exit status; however, it
cannot actually do so until $some_pid _also_ exits.

This patch fixes the issue by unconditionally checking for pending
process group waits on SIGCHLD, and then allowing pending pid-only waits
to "steal" the notification.

[Fixes #19387]

0 files changed, 0 insertions, 0 deletions