Fix + refactor `is_valid_beacon_attestation` and add basic test

specs/core/1_shard-data-chains.md

@@ -555,16 +555,14 @@ def shard_block_transition(state: ShardState,
 
 Let:
 
-- `shard` be a valid `Shard`
 - `pre_state` is the `ShardState` before processing any blocks
-- `shard_blocks_or_state_roots` be the `Union[ShardBlock, Hash]` list such that `shard_blocks[slot]` is the canonical `ShardBlock` for shard `shard` at slot `slot` if a block exists, or the post-state-root of processing state up to and including that slot if a block does not exist.
+- `shard_blocks_or_state_roots` be the `Union[ShardBlock, Hash]` list such that `shard_blocks[slot]` is the canonical `ShardBlock` for shard `pre_state.shard` at slot `slot` if a block exists, or the post-state-root of processing state up to and including that slot if a block does not exist.
 - `beacon_state` be the canonical `BeaconState`
 - `valid_attestations` be the set of valid `Attestation` objects, recursively defined
 - `candidate` be a candidate `Attestation` which is valid under Phase 0 rules, and for which validity is to be determined under Phase 1 rules by running `is_valid_beacon_attestation`
 
 ```python
-def is_valid_beacon_attestation(shard: Shard,
-                                pre_state: ShardState,
+def is_valid_beacon_attestation(pre_state: ShardState,
                                 shard_blocks_or_state_roots: Sequence[Union[ShardBlock, Hash]],
                                 beacon_state: BeaconState,
                                 valid_attestations: Set[Attestation],
@@ -587,7 +585,7 @@ def is_valid_beacon_attestation(shard: Shard,
         assert candidate.data.previous_attestation.epoch < compute_epoch_of_slot(candidate.data.slot)
 
     # Check crosslink data root
-    start_epoch = beacon_state.crosslinks[shard].epoch
+    start_epoch = beacon_state.crosslinks[pre_state.shard].epoch
     end_epoch = min(compute_epoch_of_slot(candidate.data.slot) - CROSSLINK_LOOKBACK,
                     start_epoch + MAX_EPOCHS_PER_CROSSLINK)
     blocks = []
@@ -595,11 +593,14 @@ def is_valid_beacon_attestation(shard: Shard,
         if isinstance(shard_blocks_or_state_roots[slot], ShardBlock):
             blocks.append(shard_blocks_or_state_roots[slot])
         else:
-            blocks.append(ShardBlockHeader(ShardBlockCore(
-                slot=slot,
-                state_root=shard_blocks_or_state_roots[slot],
-                total_bytes=pre_state.total_bytes
-            ), ShardBlockSignatures()))
+            blocks.append(ShardBlock(
+                core=ExtendedShardBlockCore(
+                    slot=slot,
+                    state_root=shard_blocks_or_state_roots[slot],
+                    total_bytes=pre_state.total_bytes,
+                ),
+                signatures=ShardBlockSignatures(),
+            ))
     assert candidate.data.crosslink.data_root == compute_crosslink_data_root(blocks)
 
     return True

test_libs/pyspec/eth2spec/test/phase_1/shard_data_chain/test_beacon_attestation.py

@@ -0,0 +1,48 @@
+from eth2spec.test.context import (
+    with_all_phases_except,
+    spec_state_test,
+    always_bls,
+)
+from eth2spec.test.helpers.phase1.shard_block import (
+    build_empty_shard_block,
+)
+from eth2spec.test.helpers.attestations import get_valid_attestation
+
+
+@with_all_phases_except(['phase0'])
+@always_bls
+@spec_state_test
+def test_process_empty_shard_block(spec, state):
+    beacon_state = state
+
+    shard_slot = spec.PHASE_1_FORK_SLOT
+    beacon_state.slot = spec.Slot(spec.PHASE_1_FORK_EPOCH * spec.SLOTS_PER_EPOCH)
+    shard_state = spec.get_default_shard_state(beacon_state, shard=spec.Shard(0))
+    shard_state.slot = shard_slot
+
+    block = build_empty_shard_block(
+        spec,
+        shard_state,
+        beacon_state,
+        slot=shard_slot + 1,
+        parent_root=spec.Hash(),
+        signed=True,
+        full_attestation=True,
+    )
+
+    yield 'pre', shard_state
+    yield 'beacon_state', beacon_state
+    yield 'block', block
+
+    beacon_attestation = get_valid_attestation(spec, beacon_state, signed=True)
+    yield 'beacon_attestation', beacon_attestation
+
+    is_valid_beacon_attestation = spec.is_valid_beacon_attestation(
+        pre_state=shard_state,
+        shard_blocks_or_state_roots=(block,),
+        beacon_state=beacon_state,
+        valid_attestations=set([beacon_attestation]),
+        candidate=beacon_attestation,
+    )
+    assert is_valid_beacon_attestation
+    yield 'is_valid_beacon_attestation', is_valid_beacon_attestation