Add eth.beacon.helpers.is_double_vote and implement tests

Author: onyb

eth/beacon/helpers.py

@@ -42,6 +42,7 @@
 if TYPE_CHECKING:
     from eth.beacon.enums import SignatureDomain  # noqa: F401
     from eth.beacon.types.attestation_records import AttestationRecord  # noqa: F401
+    from eth.beacon.types.attestation_data import AttestationData  # noqa: F401
     from eth.beacon.types.blocks import BaseBeaconBlock  # noqa: F401
     from eth.beacon.types.states import BeaconState  # noqa: F401
     from eth.beacon.types.fork_data import ForkData  # noqa: F401
@@ -418,3 +419,14 @@ def get_domain(fork_data: 'ForkData',
         fork_data,
         slot,
     ) * 4294967296 + domain_type
+
+
+def is_double_vote(attestation_data_1: 'AttestationData',
+                   attestation_data_2: 'AttestationData') -> bool:
+    """
+    Assumes ``attestation_data_1`` is distinct from ``attestation_data_2``.
+
+    Returns True if the provided ``AttestationData`` are slashable
+    due to a 'double vote'.
+    """
+    return attestation_data_1.slot == attestation_data_2.slot

tests/beacon/test_helpers.py

@@ -13,6 +13,9 @@
 from eth.beacon.enums import (
     ValidatorStatusCode,
 )
+from eth.beacon.types.attestation_data import (
+    AttestationData,
+)
 from eth.beacon.types.blocks import BaseBeaconBlock
 from eth.beacon.types.fork_data import ForkData
 from eth.beacon.types.shard_committees import ShardCommittee
@@ -32,6 +35,7 @@
     get_new_validator_registry_delta_chain_tip,
     _get_shard_committees_at_slot,
     get_block_committees_info,
+    is_double_vote,
 )
 
 
@@ -729,3 +733,27 @@ def test_get_domain(pre_fork_version,
         slot=current_slot,
         domain_type=domain_type,
     )
+
+
+def test_is_double_vote(sample_attestation_data_params):
+    attestation_data_1_params = {
+        **sample_attestation_data_params,
+        'slot': 12345,
+    }
+    attestation_data_1 = AttestationData(**attestation_data_1_params)
+
+    attestation_data_2_params = {
+        **sample_attestation_data_params,
+        'slot': 12345,
+    }
+    attestation_data_2 = AttestationData(**attestation_data_2_params)
+
+    assert is_double_vote(attestation_data_1, attestation_data_2)
+
+    attestation_data_3_params = {
+        **sample_attestation_data_params,
+        'slot': 54321,
+    }
+    attestation_data_3 = AttestationData(**attestation_data_3_params)
+
+    assert not is_double_vote(attestation_data_1, attestation_data_3)