Add on_attester_slashing() and related test

specs/phase0/fork-choice.md

@@ -101,6 +101,7 @@ class Store(object):
     finalized_checkpoint: Checkpoint
     best_justified_checkpoint: Checkpoint
     proposer_boost_root: Root
+    has_equivocated: Set[ValidatorIndex]
     blocks: Dict[Root, BeaconBlock] = field(default_factory=dict)
     block_states: Dict[Root, BeaconState] = field(default_factory=dict)
     checkpoint_states: Dict[Checkpoint, BeaconState] = field(default_factory=dict)
@@ -129,6 +130,7 @@ def get_forkchoice_store(anchor_state: BeaconState, anchor_block: BeaconBlock) -
         finalized_checkpoint=finalized_checkpoint,
         best_justified_checkpoint=justified_checkpoint,
         proposer_boost_root=proposer_boost_root,
+        has_equivocated=set(),
         blocks={anchor_root: copy(anchor_block)},
         block_states={anchor_root: copy(anchor_state)},
         checkpoint_states={justified_checkpoint: copy(anchor_state)},
@@ -179,6 +181,7 @@ def get_latest_attesting_balance(store: Store, root: Root) -> Gwei:
     attestation_score = Gwei(sum(
         state.validators[i].effective_balance for i in active_indices
         if (i in store.latest_messages
+            and not i in store.has_equivocated
             and get_ancestor(store, store.latest_messages[i].root, store.blocks[root].slot) == root)
     ))
     if store.proposer_boost_root == Root():
@@ -358,8 +361,9 @@ def update_latest_messages(store: Store, attesting_indices: Sequence[ValidatorIn
     target = attestation.data.target
     beacon_block_root = attestation.data.beacon_block_root
     for i in attesting_indices:
-        if i not in store.latest_messages or target.epoch > store.latest_messages[i].epoch:
-            store.latest_messages[i] = LatestMessage(epoch=target.epoch, root=beacon_block_root)
+        if i not in store.has_equivocated:
+            if i not in store.latest_messages or target.epoch > store.latest_messages[i].epoch:
+                store.latest_messages[i] = LatestMessage(epoch=target.epoch, root=beacon_block_root)
 ```
 
 
@@ -459,3 +463,22 @@ def on_attestation(store: Store, attestation: Attestation, is_from_block: bool=F
     # Update latest messages for attesting indices
     update_latest_messages(store, indexed_attestation.attesting_indices, attestation)
 ```
+
+#### `on_attester_slashing`
+
+```python
+def on_attester_slashing(store: Store, attester_slashing: AttesterSlashing) -> None:
+    """
+    Run ``on_attester_slashing`` upon receiving a new ``AttesterSlashing`` from either within a block or directly on the wire.
+    """
+    attestation_1 = attester_slashing.attestation_1
+    attestation_2 = attester_slashing.attestation_2
+    assert is_slashable_attestation_data(attestation_1.data, attestation_2.data)
+    state = store.block_states[store.justified_checkpoint.root]
+    assert is_valid_indexed_attestation(state, attestation_1)
+    assert is_valid_indexed_attestation(state, attestation_2)
+
+    indices = set(attestation_1.attesting_indices).intersection(attestation_2.attesting_indices)
+    for index in sorted(indices):
+        store.has_equivocated.add(index)
+```

tests/core/pyspec/eth2spec/test/phase0/fork_choice/test_get_head.py

@@ -24,6 +24,8 @@ from eth2spec.test.helpers.state import (
     next_epoch,
     state_transition_and_sign_block,
 )
+from tests.core.pyspec.eth2spec.test.helpers.block import apply_empty_block
+from tests.core.pyspec.eth2spec.test.helpers.fork_choice import run_on_attestation
 
 
 rng = random.Random(1001)
@@ -338,3 +340,84 @@ def test_proposer_boost_correct_head(spec, state):
     })
 
     yield 'steps', test_steps
+
+
+@with_all_phases
+@spec_state_test
+def test_discard_equivocations(spec, state):
+    test_steps = []
+    genesis_state = state.copy()
+
+    # Initialization
+    store, anchor_block = get_genesis_forkchoice_store_and_block(spec, state)
+    yield 'anchor_state', state
+    yield 'anchor_block', anchor_block
+    anchor_root = get_anchor_root(spec, state)
+    assert spec.get_head(store) == anchor_root
+    test_steps.append({
+        'checks': {
+            'head': get_formatted_head_output(spec, store),
+        }
+    })
+
+    # Build block that serves as head before discarding equivocations
+    state_1 = genesis_state.copy()
+    next_slots(spec, state_1, 3)
+    block_1 = build_empty_block_for_next_slot(spec, state_1)
+    signed_block_1 = state_transition_and_sign_block(spec, state_1, block_1)
+
+    # Build equivocating attestations to feed to store
+    state_eqv = state_1.copy()
+    block_eqv = apply_empty_block(spec, state_eqv, state_eqv.slot+1)
+    attestation_eqv = get_valid_attestation(spec, state_eqv, slot=block_eqv.slot, signed=True)
+
+    next_slots(spec, state_1, 1)
+    attestation = get_valid_attestation(spec, state_1, slot=block_eqv.slot, signed=True)
+    assert spec.is_slashable_attestation_data(attestation.data, attestation_eqv.data)
+
+    indexed_attestation = spec.get_indexed_attestation(state_1, attestation)
+    indexed_attestation_eqv = spec.get_indexed_attestation(state_eqv, attestation_eqv)
+    attester_slashing = spec.AttesterSlashing(attestation_1=indexed_attestation, attestation_2=indexed_attestation_eqv)
+
+    # Build block that serves as head after discarding equivocations
+    state_2 = genesis_state.copy()
+    next_slots(spec, state_2, 2)
+    block_2 = build_empty_block_for_next_slot(spec, state_2)
+    signed_block_2 = state_transition_and_sign_block(spec, state_2.copy(), block_2)
+    while spec.hash_tree_root(block_1) >= spec.hash_tree_root(block_2):
+        block_2.body.graffiti = spec.Bytes32(hex(rng.getrandbits(8 * 32))[2:].zfill(64))
+        signed_block_2 = state_transition_and_sign_block(spec, state_2.copy(), block_2)
+    assert spec.hash_tree_root(block_1) < spec.hash_tree_root(block_2)
+
+    # Tick to (block_eqv.slot + 2) slot time
+    time = store.genesis_time + (block_eqv.slot + 2) * spec.config.SECONDS_PER_SLOT
+    on_tick_and_append_step(spec, store, time, test_steps)
+
+    # Process block_2
+    yield from add_block(spec, store, signed_block_2, test_steps)
+    assert store.proposer_boost_root == spec.Root()
+    assert spec.get_head(store) == spec.hash_tree_root(block_2)
+
+    # Process block_1
+    # The head should remain block_2
+    yield from add_block(spec, store, signed_block_1, test_steps)
+    assert store.proposer_boost_root == spec.Root()
+    assert spec.get_head(store) == spec.hash_tree_root(block_2)
+
+    # Process attestation
+    # The head should change to block_1
+    run_on_attestation(spec, store, attestation)
+    assert spec.get_head(store) == spec.hash_tree_root(block_1)
+
+    # Process attester_slashing
+    # The head should revert to block_2
+    spec.on_attester_slashing(store, attester_slashing)
+    assert spec.get_head(store) == spec.hash_tree_root(block_2)
+
+    test_steps.append({
+        'checks': {
+            'head': get_formatted_head_output(spec, store),
+        }
+    })
+
+    yield 'steps', test_steps