Add callback/hook when the client connection is closed

[adiroiban]

I am running some automated end to end tests with ldaptor client-side.

My code runs a set of LDAP operation and at the end of each test the connection to the LDAP server is closed.

At the end of each test, I have some extra checks that no connections are accidentally left open and that the reactor is clean.

I used to have this final Deferred.addBoth that will forward the result but will close the client connection.

        def bb_connect_done(result, client):
            """
            Called after all LDAP operations to clean the client.
            """
            if client.connected:
                client.unbind()

            return result

The problem is the client.unbind() is not returning a deferred. Just triggering a transport.loseConnection and hoping for the best :)

This is not a big problem for TCP, but for TLS the TLS shutdown might take a bit longer.

for now, as a workaround I am using this hack ... triggering a disconnection and then calling bind right away to put a request on the wire that will errback when the connection is actually lost.

        def bb_connect_done(result, client):
            """
            Called after all LDAP operations to clean the client.
            """
            if client.connected:
                client.unbind()

            deferred = client.bind(b'', b'')
            deferred.addBoth(lambda _: result)
            return deferred