Merge pull request ethereum#2719 from paulhauner/patch-33

Genesis execution block handling

specs/merge/validator.md

@@ -44,8 +44,11 @@ Please see related Beacon Chain doc before continuing and use them as a referenc
 def get_pow_block_at_terminal_total_difficulty(pow_chain: Dict[Hash32, PowBlock]) -> Optional[PowBlock]:
     # `pow_chain` abstractly represents all blocks in the PoW chain
     for block in pow_chain:
-        parent = pow_chain[block.parent_hash]
         block_reached_ttd = block.total_difficulty >= TERMINAL_TOTAL_DIFFICULTY
+        # If genesis block, no parent exists so reaching TTD alone qualifies as valid terminal block
+        if block_reached_ttd and block.parent_hash == Hash32():
+            return block
+        parent = pow_chain[block.parent_hash]
         parent_reached_ttd = parent.total_difficulty >= TERMINAL_TOTAL_DIFFICULTY
         if block_reached_ttd and not parent_reached_ttd:
             return block