[Python-Dev] test_multiprocessing:test_listener_client flakiness (original) (raw)

Trent Nelson tnelson at onresolve.com
Fri Jun 20 07:02:02 CEST 2008

This is a common problem. Binding to '127.0.0.1' will bind to only that address;

Indeed.

binding to "" will bind to all addresses the machine is known by.

Agreed again. I believe what we're dealing with here though is a lack of clarity regarding what role the 'address' attribute exposed by multiprocess.connection.Listener should play. The way test_listener_client() is written, it effectively treats 'address' as an end-point that can be connected to directly (irrespective of the underlying family (i.e. AF_INET, AF_UNIX, AF_PIPE)).

I believe the problems we've run into stem from the fact that the API doesn't provide any guarantees as to what 'address' represents. The test suite assumes it always reflects a connectable end-point, which I think is more than reasonable. Unfortunately, nothing stops us from breaking this invariant by constructing the object as Listener(family='AF_INET', address=('0.0.0.0', 0)).

How do I connect to an AF_INET Listener (i.e. SocketListener) instance whose 'address' attribute reports '0.0.0.0' as the host? I can't.

So, for now, I think we should enforce this invariant by raising an exception in Listener.init() if self._socket.getsockbyname()[0] returns '0.0.0.0'. In effect, tightening up the API such that we can guarantee Listener.address will always represent a connectable end-point. We can look at how to service 'listen on all available interfaces' semantics at a later date -- that adds far less value IMO than being able to depend on the said guarantee.

Thoughts?

    Trent.

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