enforce must match target to match head to avoid perverse incentive path

setup.py

@@ -182,7 +182,17 @@ get_active_validator_indices = cache_this(
 _get_beacon_committee = get_beacon_committee
 get_beacon_committee = cache_this(
     lambda state, slot, index: (state.validators.hash_tree_root(), state.randao_mixes.hash_tree_root(), slot, index),
-    _get_beacon_committee)'''
+    _get_beacon_committee)
+
+_get_matching_target_attestations = get_matching_target_attestations
+get_matching_target_attestations = cache_this(
+    lambda state, epoch: (state.hash_tree_root(), epoch),
+    _get_matching_target_attestations)
+
+_get_matching_head_attestations = get_matching_head_attestations
+get_matching_head_attestations = cache_this(
+    lambda state, epoch: (state.hash_tree_root(), epoch),
+    _get_matching_head_attestations)'''
 
 
 def objects_to_spec(spec_object: SpecObject, imports: str, fork: str) -> str:

specs/phase0/beacon-chain.md

@@ -1235,7 +1235,7 @@ def get_matching_target_attestations(state: BeaconState, epoch: Epoch) -> Sequen
 ```python
 def get_matching_head_attestations(state: BeaconState, epoch: Epoch) -> Sequence[PendingAttestation]:
     return [
-        a for a in get_matching_source_attestations(state, epoch)
+        a for a in get_matching_target_attestations(state, epoch)
         if a.data.beacon_block_root == get_block_root_at_slot(state, a.data.slot)
     ]
 ```

tests/core/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_rewards_and_penalties.py

@@ -97,6 +97,29 @@ def test_full_attestations(spec, state):
             assert state.balances[index] < pre_state.balances[index]
 
 
+@with_all_phases
+@spec_state_test
+def test_full_attestations_random_incorrect_fields(spec, state):
+    attestations = prepare_state_with_full_attestations(spec, state)
+    for i, attestation in enumerate(state.previous_epoch_attestations):
+        if i % 3 == 0:
+            # Mess up some head votes
+            attestation.data.beacon_block_root = b'\x56' * 32
+        if i % 3 == 1:
+            # Message up some target votes
+            attestation.data.target.root = b'\x23' * 32
+        if i % 3 == 2:
+            # Keep some votes 100% correct
+            pass
+
+    yield from run_process_rewards_and_penalties(spec, state)
+
+    attesting_indices = spec.get_unslashed_attesting_indices(state, attestations)
+    assert len(attesting_indices) > 0
+    # No balance checks, non-trivial base on group rewards
+    # Mainly for consensus tests
+
+
 @with_all_phases
 @spec_test
 @with_custom_state(balances_fn=misc_balances, threshold_fn=default_activation_threshold)