re-add crosslink tests and ensure pass

specs/core/0_beacon-chain.md

@@ -1631,8 +1631,8 @@ def get_previous_epoch_matching_head_attestations(state: BeaconState) -> List[Pe
 def get_crosslink_from_attestation_data(state: BeaconState, data: AttestationData) -> Crosslink:
     return Crosslink(
         epoch=min(slot_to_epoch(data.slot), state.current_crosslinks[data.shard].epoch + MAX_CROSSLINK_EPOCHS),
-        crosslink_data_root=data.crosslink_data_root,
         previous_crosslink_root=data.previous_crosslink_root,
+        crosslink_data_root=data.crosslink_data_root,
     )
 ```
 
@@ -1641,8 +1641,9 @@ def get_winning_crosslink_and_attesting_indices(state: BeaconState, epoch: Epoch
     pending_attestations = state.current_epoch_attestations if epoch == get_current_epoch(state) else state.previous_epoch_attestations
     shard_attestations = [a for a in pending_attestations if a.data.shard == shard]
     shard_crosslinks = [get_crosslink_from_attestation_data(state, a.data) for a in shard_attestations]
-    candidate_crosslinks = [c for c in shard_crosslinks if
-        hash_tree_root(state.current_crosslinks[shard]) in (c.previous_crosslink_root, hash_tree_root(c))
+    candidate_crosslinks = [
+        c for c in shard_crosslinks
+        if hash_tree_root(state.current_crosslinks[shard]) in (c.previous_crosslink_root, hash_tree_root(c))
     ]
     if len(candidate_crosslinks) == 0:
         return Crosslink(epoch=GENESIS_EPOCH, previous_crosslink_root=ZERO_HASH, crosslink_data_root=ZERO_HASH), []
@@ -1718,7 +1719,7 @@ Run the following function:
 
 ```python
 def process_crosslinks(state: BeaconState) -> None:
-    state.previous_crosslinks = state.current_crosslinks
+    state.previous_crosslinks = [c for c in state.current_crosslinks]
     previous_epoch = get_previous_epoch(state)
     next_epoch = get_current_epoch(state) + 1
     for slot in range(get_epoch_start_slot(previous_epoch), get_epoch_start_slot(next_epoch)):

test_libs/pyspec/tests/epoch_processing/test_process_crosslinks.py

@@ -0,0 +1,136 @@
+from copy import deepcopy
+import pytest
+
+import eth2spec.phase0.spec as spec
+
+from eth2spec.phase0.state_transition import (
+    state_transition,
+)
+from eth2spec.phase0.spec import (
+    cache_state,
+    get_crosslink_deltas,
+    process_crosslinks,
+)
+from tests.helpers import (
+    add_attestation_to_state,
+    build_empty_block_for_next_slot,
+    fill_aggregate_attestation,
+    get_crosslink_committee_for_attestation,
+    get_valid_attestation,
+    next_epoch,
+    next_slot,
+    set_bitfield_bit,
+)
+
+
+# mark entire file as 'crosslinks'
+pytestmark = pytest.mark.crosslinks
+
+
+def run_process_crosslinks(state, valid=True):
+    # transition state to slot before state transition
+    slot = state.slot + (spec.SLOTS_PER_EPOCH - state.slot % spec.SLOTS_PER_EPOCH) - 1
+    block = build_empty_block_for_next_slot(state)
+    block.slot = slot
+    state_transition(state, block)
+
+    # cache state before epoch transition
+    cache_state(state)
+
+    post_state = deepcopy(state)
+    process_crosslinks(post_state)
+
+    return state, post_state
+
+
+def test_no_attestations(state):
+    pre_state, post_state = run_process_crosslinks(state)
+
+    for shard in range(spec.SHARD_COUNT):
+        assert post_state.previous_crosslinks[shard] == post_state.current_crosslinks[shard]
+
+    return pre_state, post_state
+
+
+def test_single_crosslink_update_from_current_epoch(state):
+    next_epoch(state)
+
+    attestation = get_valid_attestation(state)
+
+    fill_aggregate_attestation(state, attestation)
+    add_attestation_to_state(state, attestation, state.slot + spec.MIN_ATTESTATION_INCLUSION_DELAY)
+
+    assert len(state.current_epoch_attestations) == 1
+
+    pre_state, post_state = run_process_crosslinks(state)
+
+    shard = attestation.data.shard
+    assert post_state.previous_crosslinks[shard] != post_state.current_crosslinks[shard]
+    assert pre_state.current_crosslinks[shard] != post_state.current_crosslinks[shard]
+
+    return pre_state, post_state
+
+
+def test_single_crosslink_update_from_previous_epoch(state):
+    next_epoch(state)
+
+    attestation = get_valid_attestation(state)
+
+    fill_aggregate_attestation(state, attestation)
+    add_attestation_to_state(state, attestation, state.slot + spec.SLOTS_PER_EPOCH)
+
+    assert len(state.previous_epoch_attestations) == 1
+
+    pre_state, post_state = run_process_crosslinks(state)
+    crosslink_deltas = get_crosslink_deltas(state)
+
+    shard = attestation.data.shard
+    assert post_state.previous_crosslinks[shard] != post_state.current_crosslinks[shard]
+    assert pre_state.current_crosslinks[shard] != post_state.current_crosslinks[shard]
+    # ensure rewarded
+    for index in get_crosslink_committee_for_attestation(state, attestation.data):
+        assert crosslink_deltas[0][index] > 0
+        assert crosslink_deltas[1][index] == 0
+
+    return pre_state, post_state
+
+
+def test_double_late_crosslink(state):
+    next_epoch(state)
+    state.slot += 4
+
+    attestation_1 = get_valid_attestation(state)
+    fill_aggregate_attestation(state, attestation_1)
+
+    # add attestation_1 in the next epoch
+    next_epoch(state)
+    add_attestation_to_state(state, attestation_1, state.slot + 1)
+
+    for slot in range(spec.SLOTS_PER_EPOCH):
+        attestation_2 = get_valid_attestation(state)
+        if attestation_2.data.shard == attestation_1.data.shard:
+            break
+        next_slot(state)
+    fill_aggregate_attestation(state, attestation_2)
+
+    # add attestation_2 in the next epoch after attestation_1 has
+    # already updated the relevant crosslink
+    next_epoch(state)
+    add_attestation_to_state(state, attestation_2, state.slot + 1)
+
+    assert len(state.previous_epoch_attestations) == 1
+    assert len(state.current_epoch_attestations) == 0
+
+    pre_state, post_state = run_process_crosslinks(state)
+    crosslink_deltas = get_crosslink_deltas(state)
+
+    shard = attestation_2.data.shard
+
+    # ensure that the current crosslinks were not updated by the second attestation
+    assert post_state.previous_crosslinks[shard] == post_state.current_crosslinks[shard]
+    # ensure no reward, only penalties for the failed crosslink
+    for index in get_crosslink_committee_for_attestation(state, attestation_2.data):
+        assert crosslink_deltas[0][index] == 0
+        assert crosslink_deltas[1][index] > 0
+
+    return pre_state, post_state