feat(validator_store): fork-gate RegistrationService at GLOAS_FORK_EPOCH

[shane-moore]

Goal

Insert a Gloas short-circuit in RegistrationService::start_validator_registration_service's spawning loop (anchor/validator_store/src/registration_service.rs:60-92) so that the periodic register_validators call is skipped once current_slot >= GLOAS_FORK_EPOCH. The service stays spawned; the periodic loop sleeps to the next epoch instead of issuing a wasted BN call. Pre-Gloas behavior unchanged.

Do NOT delete the service. Pre-Gloas testnets and the Gloas transition window still need it. SIP-94 §4 removes the pre-fork relay-builder mechanism (along with blinded blocks); post_validator_register_validator becomes a no-op at the BN under Gloas. A Gloas-aware skip on the Anchor side avoids the wasted RPC call and operator-log noise.

Context

Per SIP-94 §4 (ssvlabs/SIPs#94), the pre-Gloas relay-builder registration flow is replaced under Gloas by the new proposer_preferences p2p mechanism (handled by #1062, #1063, #1064). The legacy RegistrationService continues to call post_validator_register_validator against the BN every slot; under Gloas the BN ignores those calls. Anchor should short-circuit the periodic loop so:

• Pre-Gloas testnets continue to publish registrations unchanged (Boole and earlier).
• The transition window (Anchor running post-LH-bump but pre-Gloas-activation on a given network) continues to publish.
• Once the slot clock crosses GLOAS_FORK_EPOCH, the periodic loop sleeps without invoking the registration path.

LH's proposer_preferences_service.rs:77-87 and payload_attestation_service.rs:88-100 already use this pattern (with inverted polarity, skip-when-not-Gloas). For RegistrationService, the polarity is reversed: skip when Gloas is enabled.

Suggested approach

anchor/validator_store/src/registration_service.rs:

Location	Change
start_validator_registration_service (around :70-85)	Inside the if let Some(slot) = self.inner.slot_clock.now() branch, add a Gloas fork-gate that sleeps to the next epoch and continues if spec.fork_name_at_slot::<S::E>(slot).gloas_enabled().

Sketch (gate sits before the existing inner.register_validators(slot).await call):

if let Some(slot) = self.inner.slot_clock.now() {
    // SIP-94 §4: under Gloas, the relay-builder registration mechanism is
    // removed; the BN ignores post_validator_register_validator. Skip the
    // periodic publish to avoid wasted RPC + operator-log noise. The
    // service stays spawned so pre-Gloas networks continue unchanged.
    if spec.fork_name_at_slot::<S::E>(slot).gloas_enabled() {
        let sleep_duration = self
            .inner
            .slot_clock
            .duration_to_next_epoch(S::E::slots_per_epoch())
            .unwrap_or_else(|| slot_duration * S::E::slots_per_epoch() as u32);
        sleep(sleep_duration).await;
        continue;
    }

    let inner = self.inner.clone();
    let executor = inner.executor.clone();
    let future = async move {
        if let Err(e) = inner.register_validators(slot).await {
            error!(error = ?e, "Error during validator registration");
        }
    };
    executor.spawn(future, "validator_registration");
} else {
    error!("Slot clock can not return current slot");
}

spec is already in scope (cloned at the top of start_validator_registration_service); no struct changes to Inner required. S::E is the generic already used elsewhere in the file (e.g., S::E::slots_per_epoch() inside register_validators).

Testability

The register_validators function isn't called when the gate skips, so a unit test that exercises the gate directly is awkward (the spawning loop runs forever). Extract the gate decision into a small pure helper so it can be asserted without a runtime:

fn should_skip_for_gloas<E: EthSpec>(spec: &ChainSpec, slot: Slot) -> bool {
    spec.fork_name_at_slot::<E>(slot).gloas_enabled()
}

Tests assert this helper directly (no async, no slot clock, no BN mock).

Acceptance criteria

• Pre-Gloas slot: register_validators is invoked from the spawning loop unchanged (existing behavior).
• Gloas slot: the spawning loop sleeps to the next epoch without invoking register_validators or making any BN call.
• Service stays spawned across the fork transition; the loop re-evaluates the gate each iteration.
• In-code comment references SIP-94 §4 (relay-builder removal) so a future reader understands why the gate is there.
• cargo check --workspace green; existing RegistrationService tests unaffected.

Tests to deliver (against the extracted helper):

#[test]
fn should_skip_for_gloas_returns_true_after_fork() {
    let mut spec = ChainSpec::mainnet();
    spec.gloas_fork_epoch = Some(Epoch::new(0)); // active from genesis
    assert!(should_skip_for_gloas::<MainnetEthSpec>(&spec, Slot::new(0)));
}

#[test]
fn should_skip_for_gloas_returns_false_before_fork() {
    let mut spec = ChainSpec::mainnet();
    spec.gloas_fork_epoch = Some(Epoch::new(100_000)); // far future
    let pre_fork_slot = Slot::new(32); // epoch 1
    assert!(!should_skip_for_gloas::<MainnetEthSpec>(&spec, pre_fork_slot));
}

#[test]
fn should_skip_for_gloas_returns_false_when_unscheduled() {
    let mut spec = ChainSpec::mainnet();
    spec.gloas_fork_epoch = None; // never
    assert!(!should_skip_for_gloas::<MainnetEthSpec>(&spec, Slot::new(100_000)));
}

Risks

• Do not delete the service. Pre-Gloas testnets and transition-window operators still need it. Fork-gating to a sleeping no-op preserves backward compatibility; deleting would break Holesky / Hoodi / any non-Gloas network Anchor is run against.
• Gating choice (gloas_enabled() vs explicit epoch comparison). Using LH's gloas_enabled() helper (consensus/types/src/fork/fork_name.rs:205) mirrors how LH gates its own Gloas-aware code in proposer_preferences_service.rs, payload_attestation_service.rs, block_service.rs, and elsewhere. If LH refactors the helper, Anchor inherits the change via the LH bump. Acceptable; no Anchor-side abstraction required.
• Sleep duration unwrap. duration_to_next_epoch returns Option; on None (slot-clock failure) the fallback is slot_duration * slots_per_epoch, matching LH's proposer_preferences_service.rs:81-83 fallback shape.

Dependencies

Blocked by the LH-bump milestone (M0) for ChainSpec::fork_name_at_slot, ForkName::gloas_enabled, and the gloas_fork_epoch field on ChainSpec. Independent of #1062, #1063, #1064 — can land in parallel with the rest of the M5 arc.