Gloas vc ptc duty

[shane-moore]

Changes

• added polling of ptc duties to the VC per spec

How This Works

• The VC is currently checking whether one of its registered validators are in a voting committee, such as sync or attestation, or if they are the proposer for a slot. With epbs, it's possible for a validator to also be in a ptc committee for at most one slot of the epoch. More context on how validators are selected to be in a ptc was discussed on discord.

Implemented in the PR

duties_service will now spawn a background polling task for the current and next epoch to check whether any of the registered validators are in a ptc committee. Since re-org is possible, we also poll every slot. The beacon node querying is performed by new request endpoint POST validator/duties/ptc/{epoch} per beacon api spec.

To save on bandwidth similar to attestation committee polling, the ptc polling will make a tiny initial requests to learn the dependent_root, compare with the cached dependent_root, and only fetch full duties for the epoch if the root has changed (or if the root is unknown like for a new epoch). Old duties are pruned based on HISTORICAL_DUTIES_EPOCHS.

Todo

• add tests noted in the diffs
• create beacon node (server side) response that will send the ptc duties to the VC. Some of this work has already been done in other PR's such as the get_ptc helper, but the warp implementation still needs to be complete as to send the response to the VC

@ethDreamer
@eserilev

[ethDreamer]

Since re-org is possible, we also poll every slot.

I don't believe this is correct. Have a look at get_ptc()

def get_ptc(state: BeaconState, slot: Slot) -> Vector[ValidatorIndex, PTC_SIZE]:
    """
    Get the payload timeliness committee for the given ``slot``.
    """
    epoch = compute_epoch_at_slot(slot)
    seed = hash(get_seed(state, epoch, DOMAIN_PTC_ATTESTER) + uint_to_bytes(slot))
    indices: List[ValidatorIndex] = []
    # Concatenate all committees for this slot in order
    committees_per_slot = get_committee_count_per_slot(state, epoch)
    for i in range(committees_per_slot):
        committee = get_beacon_committee(state, slot, CommitteeIndex(i))
        indices.extend(committee)
    return compute_balance_weighted_selection(
        state, indices, seed, size=PTC_SIZE, shuffle_indices=False
    )

The committee selection appears constant for the same seed, and the seed is constant over MIN_SEED_LOOKAHEAD = 1 epoch

[ethDreamer]

ohhh you just mean that you check the cache every slot to ensure we have the same dependent root.

[ethDreamer]

If the beacon node only returns duties for pubkeys that this validator custodies, then this should probably just be a

HashMap<Epoch, (DependentRoot, Vec<PtcDuty>)>

as this Vec will almost always be quite small and iterating through it will be fast