FM-259: Sign all incomplete checkpoints

[aakoshh]

Closes consensus-shipyard/ipc#298

The PR changes the end-of-period checkpoint signature broadcasting by validators to apply on all incomplete (a.k.a. pending) checkpoints where they were validators.

So, instead of doing this on node start, it's done on each checkpoint period end. This has the following benefits:

• main.rs stays simpler and the checkpointing logic is kept in the interpreter
• a validator doesn't have to restart their node to fill in their signatures if e.g. they ran out of funds, they just need to get some tokens and it will automatically be retried

[adlrocha]

@cryptoAtwill, can you double-check that this is the case once you migrate to the ipc-solidity-actors bindings?

[aakoshh]

I don't remember seeing that this has been implemented already, we just talked about the fact that the bindings generate as many versions of SubnetID and whatnot as there are facets. So it's just a note for the future that if this gets out of hand, we should try to solve it there.

[adlrocha]

Why is it possibel to do the broadcast of incomplete signatures in parallel here without resulting in nonce races? (I also ask because in the comment above you mention that broadcasts can't be done in parallel).

[aakoshh]

Sorry, it looks like my comment wasn't as clear as it could have been.

What I meant was that we should not try to kick off a separate background task for each pending checkpoint like this:

for cp in pending_checkpoints {
  tokio::spawn(async move {
    checkpoint::broadcast_signature(broadcaster, cp).await;
  });
}

That's because every time the broadcaster is asked to send a transaction it will query the state for the nonce, and in this case that will surely mean all of the checkpoint submissions will get the same nonce and only one will go through.

Instead, what we have is effectively this:

tokio::spawn(async move {
  for cp in pending_checkpoints {
    checkpoint::broadcast_signature(broadcaster, cp).await;
  }
});

So it's not doing the submissions in parallel, they are done one after the other, but in the background.