Merge pull request #1742 from ethereum/ffg_lmd_vote_consistency

Added FFG-LMD vote consistency checks

specs/phase0/fork-choice.md

@@ -162,7 +162,7 @@ def get_latest_attesting_balance(store: Store, root: Root) -> Gwei:
     active_indices = get_active_validator_indices(state, get_current_epoch(state))
     return Gwei(sum(
         state.validators[i].effective_balance for i in active_indices
-        if (i in store.latest_messages 
+        if (i in store.latest_messages
             and get_ancestor(store, store.latest_messages[i].root, store.blocks[root].slot) == root)
     ))
 ```
@@ -285,6 +285,10 @@ def validate_on_attestation(store: Store, attestation: Attestation) -> None:
     # Attestations must not be for blocks in the future. If not, the attestation should not be considered
     assert store.blocks[attestation.data.beacon_block_root].slot <= attestation.data.slot
 
+    # FFG and LMD vote must be consistent with each other
+    target_slot = compute_start_slot_at_epoch(target.epoch)
+    assert target.root == get_ancestor(store, attestation.data.beacon_block_root, target_slot)
+
     # Attestations can only affect the fork choice of subsequent slots.
     # Delay consideration in the fork choice until their slot is in the past.
     assert get_current_slot(store) >= attestation.data.slot + 1

tests/core/pyspec/eth2spec/test/fork_choice/test_on_attestation.py

@@ -1,7 +1,7 @@
 from eth2spec.test.context import PHASE0, with_all_phases, spec_state_test
 from eth2spec.test.helpers.block import build_empty_block_for_next_slot
 from eth2spec.test.helpers.attestations import get_valid_attestation, sign_attestation
-from eth2spec.test.helpers.state import transition_to, state_transition_and_sign_block, next_epoch
+from eth2spec.test.helpers.state import transition_to, state_transition_and_sign_block, next_epoch, next_slot
 
 
 def run_on_attestation(spec, state, store, attestation, valid=True):
@@ -116,6 +116,44 @@ def test_on_attestation_mismatched_target_and_slot(spec, state):
     run_on_attestation(spec, state, store, attestation, False)
 
 
+@with_all_phases
+@spec_state_test
+def test_on_attestation_inconsistent_target_and_head(spec, state):
+    store = spec.get_forkchoice_store(state)
+    spec.on_tick(store, store.time + 2 * spec.SECONDS_PER_SLOT * spec.SLOTS_PER_EPOCH)
+
+    # Create chain 1 as empty chain between genesis and start of 1st epoch
+    target_state_1 = state.copy()
+    next_epoch(spec, target_state_1)
+
+    # Create chain 2 with different block in chain from chain 1 from chain 1 from chain 1 from chain 1
+    target_state_2 = state.copy()
+    diff_block = build_empty_block_for_next_slot(spec, target_state_2)
+    signed_diff_block = state_transition_and_sign_block(spec, target_state_2, diff_block)
+    spec.on_block(store, signed_diff_block)
+    next_epoch(spec, target_state_2)
+    next_slot(spec, target_state_2)
+
+    # Create and store block new head block on target state 1
+    head_block = build_empty_block_for_next_slot(spec, target_state_1)
+    signed_head_block = state_transition_and_sign_block(spec, target_state_1, head_block)
+    spec.on_block(store, signed_head_block)
+
+    # Attest to head of chain 1
+    attestation = get_valid_attestation(spec, target_state_1, slot=head_block.slot, signed=False)
+    epoch = spec.compute_epoch_at_slot(attestation.data.slot)
+
+    # Set attestation target to be from chain 2
+    attestation.data.target = spec.Checkpoint(epoch=epoch, root=spec.get_block_root(target_state_2, epoch))
+    sign_attestation(spec, state, attestation)
+
+    assert attestation.data.target.epoch == spec.GENESIS_EPOCH + 1
+    assert spec.compute_epoch_at_slot(attestation.data.slot) == spec.GENESIS_EPOCH + 1
+    assert spec.get_block_root(target_state_1, epoch) != attestation.data.target.root
+
+    run_on_attestation(spec, state, store, attestation, False)
+
+
 @with_all_phases
 @spec_state_test
 def test_on_attestation_target_not_in_store(spec, state):