Merge pull request #1925 from ethereum/hwwhww/shard_state_transition

Rework `shard_state_transition` interface + fix #1922

specs/phase1/shard-fork-choice.md

@@ -169,14 +169,14 @@ def on_shard_block(store: Store, shard_store: ShardStore, signed_shard_block: Si
     )
 
     # Check the block is valid and compute the post-state
-    assert verify_shard_block_message(beacon_parent_state, shard_parent_state, shard_block)
-    assert verify_shard_block_signature(beacon_parent_state, signed_shard_block)
-
-    post_state = get_post_shard_state(shard_parent_state, shard_block)
+    shard_state = shard_parent_state.copy()
+    shard_state_transition(
+        shard_state, signed_shard_block,
+        validate=True, beacon_parent_state=beacon_parent_state)
 
     # Add new block to the store
     shard_store.blocks[hash_tree_root(shard_block)] = shard_block
 
     # Add new state for this block to the store
-    shard_store.block_states[hash_tree_root(shard_block)] = post_state
+    shard_store.block_states[hash_tree_root(shard_block)] = shard_state
 ```

specs/phase1/shard-transition.md

@@ -50,10 +50,10 @@ def verify_shard_block_message(beacon_parent_state: BeaconState,
 ```
 
 ```python
-def verify_shard_block_signature(beacon_state: BeaconState,
+def verify_shard_block_signature(beacon_parent_state: BeaconState,
                                  signed_block: SignedShardBlock) -> bool:
-    proposer = beacon_state.validators[signed_block.message.proposer_index]
-    domain = get_domain(beacon_state, DOMAIN_SHARD_PROPOSAL, compute_epoch_at_slot(signed_block.message.slot))
+    proposer = beacon_parent_state.validators[signed_block.message.proposer_index]
+    domain = get_domain(beacon_parent_state, DOMAIN_SHARD_PROPOSAL, compute_epoch_at_slot(signed_block.message.slot))
     signing_root = compute_signing_root(signed_block.message, domain)
     return bls.Verify(proposer.pubkey, signing_root, signed_block.signature)
 ```
@@ -62,7 +62,21 @@ def verify_shard_block_signature(beacon_state: BeaconState,
 
 ```python
 def shard_state_transition(shard_state: ShardState,
-                           block: ShardBlock) -> None:
+                           signed_block: SignedShardBlock,
+                           validate: bool = True,
+                           beacon_parent_state: Optional[BeaconState] = None) -> ShardState:
+    if validate:
+        assert beacon_parent_state is not None
+        assert verify_shard_block_message(beacon_parent_state, shard_state, signed_block.message)
+        assert verify_shard_block_signature(beacon_parent_state, signed_block)
+
+    process_shard_block(shard_state, signed_block.message)
+    return shard_state
+```
+
+```python
+def process_shard_block(shard_state: ShardState,
+                        block: ShardBlock) -> None:
     """
     Update ``shard_state`` with shard ``block``.
     """
@@ -77,19 +91,6 @@ def shard_state_transition(shard_state: ShardState,
     shard_state.latest_block_root = latest_block_root
 ```
 
-We have a pure function `get_post_shard_state` for describing the fraud proof verification and honest validator behavior.
-
-```python
-def get_post_shard_state(shard_state: ShardState,
-                         block: ShardBlock) -> ShardState:
-    """
-    A pure function that returns a new post ShardState instead of modifying the given `shard_state`.
-    """
-    post_state = shard_state.copy()
-    shard_state_transition(post_state, block)
-    return post_state
-```
-
 ## Fraud proofs
 
 ### Verifying the proof
@@ -127,7 +128,7 @@ def is_valid_fraud_proof(beacon_state: BeaconState,
     else:
         shard_state = transition.shard_states[offset_index - 1]  # Not doing the actual state updates here.
 
-    shard_state = get_post_shard_state(shard_state, block)
+    shard_state_transition(shard_state, block, validate=False)
     if shard_state != transition.shard_states[offset_index]:
         return True
 

specs/phase1/validator.md

@@ -280,7 +280,6 @@ def get_shard_transition_fields(
     beacon_state: BeaconState,
     shard: Shard,
     shard_blocks: Sequence[SignedShardBlock],
-    validate_signature: bool=True,
 ) -> Tuple[Sequence[uint64], Sequence[Root], Sequence[ShardState]]:
     shard_states = []
     shard_data_roots = []
@@ -299,7 +298,8 @@ def get_shard_transition_fields(
         else:
             shard_block = SignedShardBlock(message=ShardBlock(slot=slot, shard=shard))
             shard_data_roots.append(Root())
-        shard_state = get_post_shard_state(shard_state, shard_block.message)
+        shard_state = shard_state.copy()
+        shard_state_transition(shard_state, shard_block, validate=False)
         shard_states.append(shard_state)
         shard_block_lengths.append(len(shard_block.message.body))