rlinks: potential loop in invocation forwarding code

[om26er]

Imagine two Crossbar instances

• Instance 1 connects to instance 2 using rlinks with forward_local_invocations and forward_remote_invocations set to true.
• Instance 1 registers procedure foo.bar.1 and instance 2 registers foo.bar.2 on their ends
• Since both RLinkLocalSession and RLinkRemoteSession are subscribed to wamp.subscription.on_create, when instance 1 successfully registers procedure foo.bar.2 due to the rlink connection, it gets an event wamp.subscription.on_create on the local leg, so it then tries to re-register the procedure on the remote leg.

Exception gets raised here and since there is no error handling here, bad things happen

crossbar/crossbar/worker/rlink.py

Line 309 in 712d9da

reg = yield other.register(on_call,

[om26er]

This probably applies to event forwarding code as well.

[oberstet]

In general, breaking loops is supposed to work using forward_for

crossbar/crossbar/router/dealer.py

Line 57 in 712d9da

self.forward_for = forward_for

This WAMP message item collects all node IDs of router nodes a message has traveled when crossing rlinks. And this field can then be used to filter looped back messages.

Now, there will be errors left in the code. I guess we need some CI test approach where we can add specific test scenarios, including the topology between nodes and their config, to robustly fix such issues robustly and prevent them to reappear ...

[oberstet]

partial fix merged via #1913