worker: ignore meaningless exit status indication returned by os.waitpid() Before this patch, worker implementation assumes that os.waitpid() with os.WNOHANG returns '(0, 0)' for still running child process. This is explicitly specified as below in Python API document. os.WNOHANG The option for waitpid() to return immediately if no child process status is available immediately. The function returns (0, 0) in this case. On the other hand, POSIX specification doesn't define the "stat_loc" value returned by waitpid() with WNOHANG for such child process. http://pubs.opengroup.org/onlinepubs/9699919799/functions/waitpid.html CPython implementation for os.waitpid() on POSIX doesn't take any care of this gap, and this may cause unexpected "exit status indication" even on POSIX conformance platform. For example, os.waitpid() with os.WNOHANG returns non-zero "exit status indication" on FreeBSD. This implies os.kill() with own pid or sys.exit() with non-zero exit code, even if no child process fails. To ignore meaningless exit status indication returned by os.waitpid(), this patch skips subsequent steps forcibly, if os.waitpid() returns 0 as pid. This patch also arranges examination of 'p' value for readability. FYI, there are some issues below about this behavior reported for CPython. https://bugs.python.org/issue21791 https://bugs.python.org/issue27808

--- a/mercurial/worker.py	Mon Feb 13 11:43:12 2017 -0800
+++ b/mercurial/worker.py	Sat Feb 25 01:07:52 2017 +0900
@@ -120,9 +120,12 @@
                         break
                     else:
                         raise
-            if p:
-                pids.discard(p)
-                st = _exitstatus(st)
+            if not p:
+                # skip subsequent steps, because child process should
+                # be still running in this case
+                continue
+            pids.discard(p)
+            st = _exitstatus(st)
             if st and not problem[0]:
                 problem[0] = st
     def sigchldhandler(signum, frame):