worker: kill workers after all zombie processes are reaped

Since we now wait child processes in non-blocking way (changed by 7bc25549e084
and e8fb03cfbbde), we don't have to kill them in the middle of the waitpid()
loop. This change will help solving a possible race of waitpid()-pids.discard()
sequence and another SIGCHLD.

waitforworkers() is called by cleanup(), in which case we do killworkers()
beforehand so we can remove killworkers() from waitforworkers().

mercurial/worker.py

@@ -119,9 +119,10 b' def _posixworker(ui, func, staticargs, a'
                 st = _exitstatus(st)
             if st and not problem[0]:
                 problem[0] = st
-                killworkers()
     def sigchldhandler(signum, frame):
         waitforworkers(blocking=False)
+        if problem[0]:
+            killworkers()
     oldchldhandler = signal.signal(signal.SIGCHLD, sigchldhandler)
     for pargs in partition(args, workers):
         pid = os.fork()