add test case for crosslink tie breaking between epochs

test_libs/pyspec/eth2spec/test/helpers/attestations.py

@@ -138,10 +138,11 @@ def fill_aggregate_attestation(spec, state, attestation):
         attestation.aggregation_bits[i] = True
 
 
-def add_attestation_to_state(spec, state, attestation, slot):
+def add_attestations_to_state(spec, state, attestations, slot):
     block = build_empty_block_for_next_slot(spec, state)
     block.slot = slot
-    block.body.attestations.append(attestation)
+    for attestation in attestations:
+        block.body.attestations.append(attestation)
     spec.process_slots(state, block.slot)
     sign_block(spec, state, block)
     spec.state_transition(state, block)

test_libs/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_crosslinks.py

@@ -7,7 +7,7 @@ from eth2spec.test.helpers.state import (
 )
 from eth2spec.test.helpers.block import apply_empty_block
 from eth2spec.test.helpers.attestations import (
-    add_attestation_to_state,
+    add_attestations_to_state,
     fill_aggregate_attestation,
     get_valid_attestation,
     sign_attestation,
@@ -36,7 +36,7 @@ def test_single_crosslink_update_from_current_epoch(spec, state):
     attestation = get_valid_attestation(spec, state, signed=True)
 
     fill_aggregate_attestation(spec, state, attestation)
-    add_attestation_to_state(spec, state, attestation, state.slot + spec.MIN_ATTESTATION_INCLUSION_DELAY)
+    add_attestations_to_state(spec, state, [attestation], state.slot + spec.MIN_ATTESTATION_INCLUSION_DELAY)
 
     assert len(state.current_epoch_attestations) == 1
 
@@ -57,7 +57,7 @@ def test_single_crosslink_update_from_previous_epoch(spec, state):
     attestation = get_valid_attestation(spec, state, signed=True)
 
     fill_aggregate_attestation(spec, state, attestation)
-    add_attestation_to_state(spec, state, attestation, state.slot + spec.SLOTS_PER_EPOCH)
+    add_attestations_to_state(spec, state, [attestation], state.slot + spec.SLOTS_PER_EPOCH)
 
     assert len(state.previous_epoch_attestations) == 1
 
@@ -95,7 +95,7 @@ def test_double_late_crosslink(spec, state):
 
     # add attestation_1 to next epoch
     next_epoch(spec, state)
-    add_attestation_to_state(spec, state, attestation_1, state.slot + 1)
+    add_attestations_to_state(spec, state, [attestation_1], state.slot + spec.MIN_ATTESTATION_INCLUSION_DELAY)
 
     for _ in range(spec.SLOTS_PER_EPOCH):
         attestation_2 = get_valid_attestation(spec, state)
@@ -110,7 +110,7 @@ def test_double_late_crosslink(spec, state):
     # add attestation_2 in the next epoch after attestation_1 has
     # already updated the relevant crosslink
     next_epoch(spec, state)
-    add_attestation_to_state(spec, state, attestation_2, state.slot + 1)
+    add_attestations_to_state(spec, state, [attestation_2], state.slot + spec.MIN_ATTESTATION_INCLUSION_DELAY)
 
     assert len(state.previous_epoch_attestations) == 1
     assert len(state.current_epoch_attestations) == 0
@@ -130,3 +130,45 @@ def test_double_late_crosslink(spec, state):
             attestation_2.data.crosslink.shard):
         assert crosslink_deltas[0][index] == 0
         assert crosslink_deltas[1][index] > 0
+
+
+@with_all_phases
+@spec_state_test
+def test_tied_crosslink_between_epochs(spec, state):
+    """
+    Addresses scenario found at Interop described by this test case
+    https://github.com/djrtwo/interop-test-cases/tree/master/tests/night_one_16_crosslinks
+
+    Ensure that ties on crosslinks between epochs are broken by previous epoch.
+    """
+    prev_attestation = get_valid_attestation(spec, state, signed=True)
+    fill_aggregate_attestation(spec, state, prev_attestation)
+
+    # add attestation at start of next epoch
+    next_epoch(spec, state)
+    add_attestations_to_state(spec, state, [prev_attestation], state.slot)
+
+    # create attestation from current epoch for same shard
+    for _ in range(spec.SLOTS_PER_EPOCH):
+        cur_attestation = get_valid_attestation(spec, state)
+        if cur_attestation.data.crosslink.shard == prev_attestation.data.crosslink.shard:
+            sign_attestation(spec, state, cur_attestation)
+            break
+        next_slot(spec, state)
+    fill_aggregate_attestation(spec, state, cur_attestation)
+
+    add_attestations_to_state(spec, state, [cur_attestation], state.slot + spec.MIN_ATTESTATION_INCLUSION_DELAY)
+
+    shard = prev_attestation.data.crosslink.shard
+    pre_crosslink = deepcopy(state.current_crosslinks[shard])
+
+    assert prev_attestation.data.crosslink != cur_attestation.data.crosslink
+    assert state.current_crosslinks[shard] == spec.Crosslink()
+    assert len(state.previous_epoch_attestations) == 1
+    assert len(state.current_epoch_attestations) == 1
+
+    yield from run_process_crosslinks(spec, state)
+
+    assert state.previous_crosslinks[shard] != state.current_crosslinks[shard]
+    assert pre_crosslink != state.current_crosslinks[shard]
+    assert state.current_crosslinks[shard] == prev_attestation.data.crosslink