Adding support of transient federates

[ChadliaJerad]

This PR replaces #1695.
The companion PR in reactor-c is lf-lang/reactor-c#358.

It implements the transient feature in the federation execution. Details of the implementation are documented in Discussion #2212.

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Merged #2609

This PR builds on the centralized transient support and extends the LF federated runtime to support transient federates under decentralized coordination, where all connections are P2P and no RTI message
forwarding occurs. It also adds support for physical connections involving transient federates under centralized coordination, which are P2P connections.

Companion PR in reactor-c: #574

LF compiler (lingua-franca)

Code generation (CExtension.java)

• lf_connect_to_federate() now takes an additional is_transient parameter. The difference in behavior is documented in the companion PR.
• The compiler generates per-federate inbound_p2p_connection_is_transient[NUMBER_OF_FEDERATES] and outbound_p2p_connection_is_transient[NUMBER_OF_FEDERATES], so the runtime can track which outbound peers are transient.

Tests

All transient tests are under test/C/src/federated/transient/. New tests:

• DecentralizedTransientDownstreamWithTimer.lf — transient upstream, persistent outbound, decentralized coordination.
• DecentralizedTransientDownstreamWithTwoUpstream.lf — transient with two inbound federates (one persistent, one transient), authenticated.
• DecentralizedTransientStatePersistence.lf — verifies that state is correctly carried across re-joins.
• DecentralizedTransientHotSwap.lf — hot-swap of a transient federate under decentralized coordination.
• DecentralizedTransientWithPhysicalConnection.lf
• TransientWithPhysicalConnection.lf

Possible future improvements

• Add an icon in the diagram for transient federates.

[edwardalee]

Looks great! The tests are quite nice. I wonder whether they will prove flaky, since there is always a risk of the launched federates taking an indeterminate amount of time to launch. Have you seen any flaky failures?

[cmnrd]

I have significant concerns about this approach, and I don't think that we should merge this lightheartedly without being clear about the benefits and the costs. See my comment in the discussion thread for an explanation.

[ChadliaJerad]

Looks great! The tests are quite nice. I wonder whether they will prove flaky, since there is always a risk of the launched federates taking an indeterminate amount of time to launch. Have you seen any flaky failures?

I agree that there are chances of having the tests prove flaky... But I have not encountered such a case so far on my machine. As a proactive workaround, I increased the timeout in this commit 3503ee1.

[ChadliaJerad]

I have significant concerns about this approach, and I don't think that we should merge this lightheartedly without being clear about the benefits and the costs. See my comment in the discussion thread for an explanation.

Thanks for the feedback, Christian!
Further discussions are in the discussion.

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