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.

[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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[erlingrj]

I have not reviewed the code, but tried a simple example that failed:

target C {
  coordination: decentralized
}


reactor R(id: int = 0)  {
  input in: int
  output out: int

  reaction(startup) -> out {=
    printf("Handling startup!\n");
    lf_set(out, self->id);
  =}

  reaction(in) {=
    printf("Got %d\n", in->value);
  =}
}

federated reactor {
  r1 = new R()

  @transient
  r2 = new R(id=2)

  @transient
  r3 = new R(id=3)

  r2.out -> r3.in
  r3.out -> r2.in
}

Try starting it in the order:

1. RTI -n 3 -nt 2
2. r1
3. r2
4. r3

As expected when r1 is started, he connects to RTI and gets a start tag and executes the startup reaction.

But when r2 is started, it does not start, it is blocked, presumably waiting for r3 (but that's a transient federate). When r3 is started it crashes at once with a segfault, when run in debug mode an assertion fails because it has decoded a nonsense port. Only after r3 starts (and crashes) does r2 finally start

[edwardalee]

I don't think there is support yet for decentralized coordination.

[erlingrj]

Then a compilation error should be issued, a couple of other observations:

• No compiler error/warning if a program with only transients are compiled (we need at least on non-transient right?)
• If a transient federate tries to connect to the RTI before any non-transient has connected, the RTI crashes