worker: allow waitforworkers to be non-blocking

This patch adds a boolean flag to waitforworkers and makes it non-blocking
if set to True.

This is to make it possible that we can reap our workers while keep other
unrelated children untouched, after receiving SIGCHLD.

mercurial/worker.py

@@ -97,9 +97,11 b' def _posixworker(ui, func, staticargs, a'
             except OSError as err:
                 if err.errno != errno.ESRCH:
                     raise
-    def waitforworkers():
+    def waitforworkers(blocking=True):
         for pid in pids:
-            st = _exitstatus(os.waitpid(pid, 0)[1])
+            p, st = os.waitpid(pid, 0 if blocking else os.WNOHANG)
+            if p:
+                st = _exitstatus(st)
             if st and not problem[0]:
                 problem[0] = st
                 killworkers()