worker: wait worker pid explicitly

worker: wait worker pid explicitly

Before this patch, waitforworkers uses os.wait() to collect child workers, and
only wait len(pids) processes. This can have serious issues if other code
spawns new processes and does not reap them: 1. worker.py may get wrong exit
code and kill innocent workers. 2. worker.py may continue without waiting for
all workers to complete.

This patch fixes the issue by using waitpid to wait worker pid explicitly.

However, this patch introduces a new issue: worker failure may not be handled
immediately. The issue will be addressed in next patches.

--- a/mercurial/worker.py	Thu Jul 28 20:49:57 2016 +0100
+++ b/mercurial/worker.py	Thu Jul 28 20:51:20 2016 +0100
@@ -98,8 +98,8 @@
                 if err.errno != errno.ESRCH:
                     raise
     def waitforworkers():
-        for _pid in pids:
-            st = _exitstatus(os.wait()[1])
+        for pid in pids:
+            st = _exitstatus(os.waitpid(pid, 0)[1])
             if st and not problem[0]:
                 problem[0] = st
                 killworkers()