Only allow sync committee period calculation at period boundaries

specs/altair/beacon-chain.md

@@ -276,16 +276,14 @@ def has_flag(flags: ParticipationFlags, flag_index: int) -> bool:
 def get_sync_committee_indices(state: BeaconState, epoch: Epoch) -> Sequence[ValidatorIndex]:
     """
     Return the sequence of sync committee indices (which may include duplicate indices)
-    for a given ``state`` and ``epoch``.
+    for a given ``state`` and ``epoch`` at a sync committee period boundary.
-
-    Note: This function is not stable during a sync committee period as
-    a validator's effective balance may change enough to affect the sampling.
     """
+    assert epoch % EPOCHS_PER_SYNC_COMMITTEE_PERIOD == 0
+
     MAX_RANDOM_BYTE = 2**8 - 1
-    base_epoch = Epoch((max(epoch // EPOCHS_PER_SYNC_COMMITTEE_PERIOD, 1) - 1) * EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
+    active_validator_indices = get_active_validator_indices(state, epoch)
-    active_validator_indices = get_active_validator_indices(state, base_epoch)
     active_validator_count = uint64(len(active_validator_indices))
-    seed = get_seed(state, base_epoch, DOMAIN_SYNC_COMMITTEE)
+    seed = get_seed(state, epoch, DOMAIN_SYNC_COMMITTEE)
     i = 0
     sync_committee_indices: List[ValidatorIndex] = []
     while len(sync_committee_indices) < SYNC_COMMITTEE_SIZE:

specs/altair/fork.md

@@ -81,7 +81,9 @@ def upgrade_to_altair(pre: phase0.BeaconState) -> BeaconState:
         inactivity_scores=[uint64(0) for _ in range(len(pre.validators))],
     )
     # Fill in sync committees
-    post.current_sync_committee = get_sync_committee(post, get_current_epoch(post))
+    current_period = epoch // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
-    post.next_sync_committee = get_sync_committee(post, get_current_epoch(post) + EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
+    base_epoch = current_period * EPOCHS_PER_SYNC_COMMITTEE_PERIOD
+    post.current_sync_committee = get_sync_committee(post, base_epoch)
+    post.next_sync_committee = get_sync_committee(post, base_epoch + EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
     return post
 ```

tests/core/pyspec/eth2spec/test/altair/block_processing/test_process_sync_committee.py

@@ -7,7 +7,6 @@ from eth2spec.test.helpers.block_processing import run_block_processing_to
 from eth2spec.test.helpers.state import (
     state_transition_and_sign_block,
     transition_to,
-    next_epoch,
 )
 from eth2spec.test.helpers.constants import (
     MAINNET, MINIMAL,
@@ -367,43 +366,3 @@ def test_valid_signature_future_committee(spec, state):
     )
 
     yield from run_sync_committee_processing(spec, state, block)
-
-
-@with_altair_and_later
-@spec_state_test
-def test_sync_committee_is_only_computed_at_epoch_boundary(spec, state):
-    """
-    Sync committees can only be computed at sync committee period boundaries.
-    Ensure a client respects the committee in the state (assumed to be derived
-    in the correct way).
-    """
-    current_epoch = spec.get_current_epoch(state)
-
-    # use a "synthetic" committee to simulate the situation
-    # where ``spec.get_sync_committee`` at the sync committee
-    # period epoch boundary would have diverged some epochs into the
-    # period; ``aggregate_pubkey`` is not relevant to this test
-    pubkeys = []
-    committee_indices = []
-    i = 0
-    active_validator_count = len(spec.get_active_validator_indices(state, current_epoch))
-    while len(pubkeys) < spec.SYNC_COMMITTEE_SIZE:
-        v = state.validators[i % active_validator_count]
-        if spec.is_active_validator(v, current_epoch):
-            pubkeys.append(v.pubkey)
-            committee_indices.append(i)
-        i += 1
-
-    synthetic_committee = spec.SyncCommittee(pubkeys=pubkeys, aggregate_pubkey=spec.BLSPubkey())
-    state.current_sync_committee = synthetic_committee
-
-    assert spec.EPOCHS_PER_SYNC_COMMITTEE_PERIOD > 3
-    for _ in range(3):
-        next_epoch(spec, state)
-
-    committee = get_committee_indices(spec, state)
-    assert committee != committee_indices
-    committee_size = len(committee_indices)
-    committee_bits = [True] * committee_size
-
-    yield from run_successful_sync_committee_test(spec, state, committee_indices, committee_bits)

tests/core/pyspec/eth2spec/test/altair/epoch_processing/test_process_sync_committee_updates.py

@@ -39,8 +39,7 @@ def run_sync_committees_progress_test(spec, state):
 
     # Can compute the third committee having computed final balances in the last epoch
     # of this `EPOCHS_PER_SYNC_COMMITTEE_PERIOD`
-    current_epoch = spec.get_current_epoch(state)
+    third_sync_committee = spec.get_sync_committee(state, (next_period + 1) * spec.EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
-    third_sync_committee = spec.get_sync_committee(state, current_epoch + 2 * spec.EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
 
     assert state.current_sync_committee == second_sync_committee
     assert state.next_sync_committee == third_sync_committee

tests/core/pyspec/eth2spec/test/altair/sanity/test_blocks.py

@@ -18,7 +18,8 @@ from eth2spec.test.context import (
 
 
 def run_sync_committee_sanity_test(spec, state, fraction_full=1.0):
-    committee = spec.get_sync_committee_indices(state, spec.get_current_epoch(state))
+    all_pubkeys = [v.pubkey for v in state.validators]
+    committee = [all_pubkeys.index(pubkey) for pubkey in state.current_sync_committee.pubkeys]
     participants = random.sample(committee, int(len(committee) * fraction_full))
 
     yield 'pre', state

tests/core/pyspec/eth2spec/test/altair/unittests/test_sync_protocol.py

@@ -46,7 +46,8 @@ def test_process_light_client_update_not_updated(spec, state):
         body_root=signed_block.message.body.hash_tree_root(),
     )
     # Sync committee signing the header
-    committee = spec.get_sync_committee_indices(state, spec.get_current_epoch(state))
+    all_pubkeys = [v.pubkey for v in state.validators]
+    committee = [all_pubkeys.index(pubkey) for pubkey in state.current_sync_committee.pubkeys]
     sync_committee_bits = [True] * len(committee)
     sync_committee_signature = compute_aggregate_sync_committee_signature(
         spec,
@@ -111,7 +112,8 @@ def test_process_light_client_update_timeout(spec, state):
     )
 
     # Sync committee signing the finalized_block_header
-    committee = spec.get_sync_committee_indices(state, spec.get_current_epoch(state))
+    all_pubkeys = [v.pubkey for v in state.validators]
+    committee = [all_pubkeys.index(pubkey) for pubkey in state.current_sync_committee.pubkeys]
     sync_committee_bits = [True] * len(committee)
     sync_committee_signature = compute_aggregate_sync_committee_signature(
         spec,
@@ -190,7 +192,8 @@ def test_process_light_client_update_finality_updated(spec, state):
     )
 
     # Sync committee signing the finalized_block_header
-    committee = spec.get_sync_committee_indices(state, spec.get_current_epoch(state))
+    all_pubkeys = [v.pubkey for v in state.validators]
+    committee = [all_pubkeys.index(pubkey) for pubkey in state.current_sync_committee.pubkeys]
     sync_committee_bits = [True] * len(committee)
     sync_committee_signature = compute_aggregate_sync_committee_signature(
         spec,