Issue 21491: race condition in SocketServer.py ForkingMixIn collect_children (original) (raw)

collect_children routine in SocketServer.py contains two possible race conditions. First one is in while loop "while len(self.active_children) >= self.max_children:". If status of child is collected outside of Socket server (for example in signal handler or something), then the variable self.active_children will not match actual child processes and the os.waitpid(0, 0) within the while loop will raise os.error, errno=10 (ECHILD) "No Child Processes". self.active_children should be emptied in this case, otherwise you'll end up with an endless loop comsuming 100% CPU (as happened to us).

The second possible race condition is below in the collect_children routine in the "for child in self.active_children" which contains a statement self.active_children.remove(pid) which would modify the iterator. I do not now about python 2.7, but before this would result in "incorrect iteration". Original code: for child in self.active_children: try: pid, status = os.waitpid(child, os.WNOHANG) except os.error: pid = None if not pid: continue try: self.active_children.remove(pid) except ValueError, e: raise ValueError('%s. x=%d and list=%r' % (e.message, pid, self.active_children))

Fixed code: to_remove = [] for child in self.active_children: try: pid, status = os.waitpid(child, os.WNOHANG) except os.error: pid = None if not pid: continue to_remove.append(pid)

    for pid in to_remove:
        try:
            self.active_children.remove(pid)
        except ValueError, e:
            raise ValueError('%s. x=%d and list=%r' % (e.message, pid, self.active_children))