worker: make sure killworkers() never be interrupted by another SIGCHLD

killworkers() iterates over pids, which can be updated by SIGCHLD handler.
So we should either copy pids or prevent killworkers() from being interrupted
by SIGCHLD. I chose the latter as it is simpler and can make pids handling
more consistent.

This fixes a possible "set changed size during iteration" error at
killworkers() before cleanup().

mercurial/worker.py

@@ -89,6 +89,10 b' def _posixworker(ui, func, staticargs, a'
     signal.signal(signal.SIGINT, signal.SIG_IGN)
     pids, problem = set(), [0]
     def killworkers():
+        # unregister SIGCHLD handler as all children will be killed. This
+        # function shouldn't be interrupted by another SIGCHLD; otherwise pids
+        # could be updated while iterating, which would cause inconsistency.
+        signal.signal(signal.SIGCHLD, oldchldhandler)
         # if one worker bails, there's no good reason to wait for the rest
         for p in pids:
             try:
@@ -115,8 +119,6 b' def _posixworker(ui, func, staticargs, a'
                 st = _exitstatus(st)
             if st and not problem[0]:
                 problem[0] = st
-                # unregister SIGCHLD handler as all children will be killed
-                signal.signal(signal.SIGCHLD, oldchldhandler)
                 killworkers()
     def sigchldhandler(signum, frame):
         waitforworkers(blocking=False)