Jun Wu <quark@fb.com> [Sat, 12 Nov 2016 03:07:22 +0000] rev 30425
worker: add a SIGCHLD handler to collect worker immediately
As planned by previous patches, add a SIGCHLD handler to get notifications
about worker exits, and deals with worker failure immediately.
Note that the SIGCHLD handler gets unregistered before killworkers(), so
SIGCHLD won't interrupt "killworkers" - making it harder to send kill
signals to waited processes.