[Python-Dev] Set close-on-exec flag by default in SocketServer

[Charles-François Natali]

> The SocketServer class creates a socket to listen on clients, and a
> new socket per client (only for stream server like TCPServer, not for
> UDPServer).
>
> Until recently (2011-05-24, issue #5715), the listening socket was not
> closed after fork for the ForkingMixIn flavor. This caused two issues:
> it's a security leak, and it causes "address already in use" error if
> the server is restarted (see the first message of #12107 for an
> example with Django).

Note that the server socket is actually still not closed in the child
process: one this gets fixed, setting FD_CLOEXEC will not be useful
anymore (but it would be an extra security it it could be done
atomically, especially against race conditions in multi-threaded
applications). (Same thing for the client socket, which is actually
already closed in the parent process).

As for the backward compatibility issue, here's a thought: subprocess
was changed in 3.2 to close all FDs > 2 in the child process by
default. AFAICT, we didn't get a single report complaining about this
behavior change. OTOH, we did get numerous bug reports due to FDs
inherited by subprocesses before that change. (I know that Python >=
3.2 is less widespread than its predecessors, but still).