Enforce terminal PoW block to be on the cusp

setup.py

@@ -509,7 +509,7 @@ def sundry_functions(cls) -> str:
 
 
 def get_pow_block(hash: Bytes32) -> PowBlock:
-    return PowBlock(block_hash=hash, is_valid=True, is_processed=True,
+    return PowBlock(block_hash=hash, parent_hash=Bytes32(), is_valid=True, is_processed=True,
                     total_difficulty=uint256(0), difficulty=uint256(0))
 
 

specs/merge/fork-choice.md

@@ -82,6 +82,7 @@ class TransitionStore(object):
 @dataclass
 class PowBlock(object):
     block_hash: Hash32
+    parent_hash: Hash32
     is_processed: boolean
     is_valid: boolean
     total_difficulty: uint256
@@ -99,9 +100,10 @@ Let `get_pow_block(block_hash: Hash32) -> PowBlock` be the function that given t
 Used by fork-choice handler, `on_block`.
 
 ```python
-def is_valid_terminal_pow_block(transition_store: TransitionStore, block: PowBlock) -> bool:
+def is_valid_terminal_pow_block(transition_store: TransitionStore, block: PowBlock, parent: PowBlock) -> bool:
     is_total_difficulty_reached = block.total_difficulty >= transition_store.transition_total_difficulty
-    return block.is_valid and is_total_difficulty_reached
+    is_parent_total_difficulty_valid = parent.total_difficulty < transition_store.transition_total_difficulty
+    return block.is_valid and is_total_difficulty_reached and is_parent_total_difficulty_valid
 ```
 
 ## Updated fork-choice handlers
@@ -130,8 +132,9 @@ def on_block(store: Store, signed_block: SignedBeaconBlock, transition_store: Tr
     if (transition_store is not None) and is_merge_block(pre_state, block):
         # Delay consideration of block until PoW block is processed by the PoW node
         pow_block = get_pow_block(block.body.execution_payload.parent_hash)
+        pow_parent = get_pow_block(pow_block.parent_hash)
         assert pow_block.is_processed
-        assert is_valid_terminal_pow_block(transition_store, pow_block)
+        assert is_valid_terminal_pow_block(transition_store, pow_block, pow_parent)
 
     # Check the block is valid and compute the post-state
     state = pre_state.copy()

specs/merge/validator.md

@@ -19,7 +19,6 @@
   - [Block proposal](#block-proposal)
     - [Constructing the `BeaconBlockBody`](#constructing-the-beaconblockbody)
       - [Execution Payload](#execution-payload)
-        - [`get_pow_chain_head`](#get_pow_chain_head)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 <!-- /TOC -->
@@ -63,13 +62,19 @@ All validator responsibilities remain unchanged other than those noted below. Na
 
 ##### Execution Payload
 
-###### `get_pow_chain_head`
+* Set `block.body.execution_payload = get_execution_payload(state, transition_store, execution_engine, pow_chain)` where:
 
-Let `get_pow_chain_head() -> PowBlock` be the function that returns the head of the PoW chain. The body of the function is implementation specific.
+```python
+def get_pow_block_at_total_difficulty(total_difficulty: uint256, pow_chain: Sequence[PowBlock]) -> Optional[PowBlock]:
+    # `pow_chain` abstractly represents all blocks in the PoW chain
+    for block in pow_chain:
+        parent = get_pow_block(block.parent_hash)
+        if block.total_difficulty >= total_difficulty and parent.total_difficulty < total_difficulty:
+            return block
+
+    return None
 
-* Set `block.body.execution_payload = get_execution_payload(state, transition_store, execution_engine)` where:
 
-```python
 def compute_randao_mix(state: BeaconState, randao_reveal: BLSSignature) -> Bytes32:
     epoch = get_current_epoch(state)
     return xor(get_randao_mix(state, epoch), hash(randao_reveal))
@@ -87,15 +92,16 @@ def produce_execution_payload(state: BeaconState,
 def get_execution_payload(state: BeaconState,
                           transition_store: TransitionStore,
                           randao_reveal: BLSSignature,
-                          execution_engine: ExecutionEngine) -> ExecutionPayload:
+                          execution_engine: ExecutionEngine,
+                          pow_chain: Sequence[PowBlock]) -> ExecutionPayload:
     if not is_merge_complete(state):
-        pow_block = get_pow_chain_head()
-        if not is_valid_terminal_pow_block(transition_store, pow_block):
+        terminal_pow_block = get_pow_block_at_total_difficulty(transition_store.transition_total_difficulty, pow_chain)
+        if terminal_pow_block is None:
             # Pre-merge, empty payload
             return ExecutionPayload()
         else:
             # Signify merge via producing on top of the last PoW block
-            return produce_execution_payload(state, pow_block.block_hash, randao_reveal, execution_engine)
+            return produce_execution_payload(state, terminal_pow_block.block_hash, randao_reveal, execution_engine)
 
     # Post-merge, normal payload
     parent_hash = state.latest_execution_payload_header.block_hash