Disallow configuring an invalid base fee on header

Author: carver

eth/vm/forks/london/headers.py

@@ -5,6 +5,9 @@
     Optional,
 )
 
+from eth_utils import (
+    ValidationError,
+)
 from toolz.functoolz import curry
 
 from eth._utils.headers import (
@@ -104,10 +107,14 @@ def create_header_from_parent(difficulty_fn: Callable[[BlockHeaderAPI, int], int
     all_fields = fill_header_params_from_parent(parent_header, **header_params)
 
     # must add the new field *after* filling, because the general fill function doesn't recognize it
-    if 'base_fee_per_gas' in header_params:
-        all_fields['base_fee_per_gas'] = header_params['base_fee_per_gas']
+    base_fee_per_gas = calculate_expected_base_fee_per_gas(parent_header)
+    if 'base_fee_per_gas' in header_params and all_fields['base_fee_per_gas'] != base_fee_per_gas:
+        raise ValidationError(
+            f"Cannot select an invalid base_fee_per_gas of:"
+            f" {all_fields['base_fee_per_gas']!r}, expected: {base_fee_per_gas}"
+        )
     else:
-        all_fields['base_fee_per_gas'] = calculate_expected_base_fee_per_gas(parent_header)
+        all_fields['base_fee_per_gas'] = base_fee_per_gas
 
     new_header = LondonBlockHeader(**all_fields)  # type:ignore
     return new_header