add mocked eth1data standard

interop/README.md

@@ -3,4 +3,5 @@
 This is a collection of standards to aid in client interoperability testing including:
 
 * [Mocked start](./mocked_start)
+* [Mocked `Eth1Data`](./mocked_eth1data)
 * [Minimal hobbits network testing](./hobbits)

interop/mocked_eth1data/README.md

@@ -0,0 +1,21 @@
+# Mocked Eth1Data votes for interop testing
+
+Most initial interop tests will not have an associated eth1 chain to come to consensus upon via the `Eth1Data` vote mechanism. This is a quick standard on how clients should stub this vote. The goal here is to check that `eth1data` updates but to _not_ update it in such a way that new deposits must be processed.
+
+We may come up with a more clever mechanism that can actually handles deposits in the future, but for now we just operate in such a way that disables deposits.
+
+## Stub standard
+
+The following stub standard allows the eth1data mechanism to update. This should be used by validators whenever they are supposed to retrieve data from the eth1 chain.
+
+This has validators agreeing upon a stubbed `deposit_root` and `block_hash` but always keeps `deposit_count` at `state.eth1_deposit_index` so that the consensus does not force deposits even as eth1data is updated. 
+
+```python
+def get_eth1data_stub(state: BeaconState, current_epoch: Epoch) -> Eth1Data:
+    epochs_per_period = SLOTS_PER_ETH1_VOTING_PERIOD // SLOTS_PER_EPOCH
+    voting_period = current_epoch // epochs_per_period
+    return Eth1Data(
+        deposit_root=hash(voting_period),
+        deposit_count=state.eth1_deposit_index,
+        block_hash=hash(hash(voting_period)),
+    )