remove previous crosslink check from process_crosslinks

specs/core/0_beacon-chain.md

@@ -1041,7 +1041,7 @@ def verify_merkle_branch(leaf: Bytes32, proof: List[Bytes32], depth: int, index:
 
 ```python
 def get_crosslink_committee_for_attestation(state: BeaconState,
-                                        attestation_data: AttestationData) -> List[ValidatorIndex]:
+                                            attestation_data: AttestationData) -> List[ValidatorIndex]:
     # Find the committee in the list with the desired shard
     crosslink_committees = get_crosslink_committees_at_slot(state, attestation_data.slot)
 
@@ -1766,10 +1766,7 @@ def get_winning_root_and_participants(state: BeaconState, slot: Slot, shard: Sha
     attestations = state.current_epoch_attestations if slot_to_epoch(slot) == get_current_epoch(state) else state.previous_epoch_attestations
     crosslinks = state.current_crosslinks if slot_to_epoch(slot) == get_current_epoch(state) else state.previous_crosslinks
 
-    valid_attestations = [
-        a for a in attestations
-        if a.data.previous_crosslink == crosslinks[shard] and a.data.shard == shard
-    ]
+    valid_attestations = [a for a in attestations if a.data.shard == shard]
     all_roots = [a.data.crosslink_data_root for a in valid_attestations]
 
     # handle when no attestations for shard available

tests/phase0/epoch_processing/test_process_crosslinks.py

@@ -0,0 +1,92 @@
+from copy import deepcopy
+import pytest
+
+import build.phase0.spec as spec
+
+from build.phase0.state_transition import (
+    state_transition,
+)
+from build.phase0.spec import (
+    ZERO_HASH,
+    cache_state,
+    get_crosslink_committee_for_attestation,
+    get_current_epoch,
+    process_crosslinks,
+    slot_to_epoch,
+)
+from tests.phase0.helpers import (
+    add_attestation_to_state,
+    build_empty_block_for_next_slot,
+    fill_aggregate_attestation,
+    get_valid_attestation,
+    next_epoch,
+    set_bitfield_bit,
+)
+
+
+# mark entire file as 'crosslinks'
+pytestmark = pytest.mark.crosslinks
+
+
+def run_process_crosslinks(state, valid=True):
+    post_state = deepcopy(state)
+
+    # 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)
+
+    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)
+
+    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

tests/phase0/helpers.py

@@ -4,6 +4,9 @@ from py_ecc import bls
 
 import build.phase0.spec as spec
 from build.phase0.utils.minimal_ssz import signed_root
+from build.phase0.state_transition import (
+    state_transition,
+)
 from build.phase0.spec import (
     # constants
     EMPTY_SIGNATURE,
@@ -47,6 +50,19 @@ pubkeys = [bls.privtopub(privkey) for privkey in privkeys]
 pubkey_to_privkey = {pubkey: privkey for privkey, pubkey in zip(privkeys, pubkeys)}
 
 
+def set_bitfield_bit(bitfield, i):
+    """
+    Set the bit in ``bitfield`` at position ``i`` to ``1``.
+    """
+    byte_index = i // 8
+    bit_index = i % 8
+    return (
+        bitfield[:byte_index] +
+        bytes([bitfield[byte_index] | (1 << bit_index)]) +
+        bitfield[byte_index+1:]
+    )
+
+
 def create_mock_genesis_validator_deposits(num_validators, deposit_data_leaves=None):
     if not deposit_data_leaves:
         deposit_data_leaves = []
@@ -140,7 +156,7 @@ def build_attestation_data(state, slot, shard):
     if epoch_start_slot == slot:
         epoch_boundary_root = block_root
     else:
-        get_block_root(state, epoch_start_slot)
+        epoch_boundary_root = get_block_root(state, epoch_start_slot)
 
     if slot < epoch_start_slot:
         justified_block_root = state.previous_justified_root
@@ -257,7 +273,7 @@ def get_valid_attester_slashing(state):
 def get_valid_attestation(state, slot=None):
     if slot is None:
         slot = state.slot
-    shard = state.latest_start_shard
+    shard = state.latest_start_shard + slot % spec.SLOTS_PER_EPOCH
     attestation_data = build_attestation_data(state, slot, shard)
 
     crosslink_committee = get_crosslink_committee_for_attestation(state, attestation_data)
@@ -309,3 +325,22 @@ def get_attestation_signature(state, attestation_data, privkey, custody_bit=0b0)
             domain_type=spec.DOMAIN_ATTESTATION,
         )
     )
+
+
+def fill_aggregate_attestation(state, attestation):
+    crosslink_committee = get_crosslink_committee_for_attestation(state, attestation.data)
+    for i in range(len(crosslink_committee)):
+        attestation.aggregation_bitfield = set_bitfield_bit(attestation.aggregation_bitfield, i)
+
+
+def add_attestation_to_state(state, attestation, slot):
+    block = build_empty_block_for_next_slot(state)
+    block.slot = slot
+    block.body.attestations.append(attestation)
+    state_transition(state, block)
+
+
+def next_epoch(state):
+    block = build_empty_block_for_next_slot(state)
+    block.slot += spec.SLOTS_PER_EPOCH - (state.slot % spec.SLOTS_PER_EPOCH)
+    state_transition(state, block)