Update usages of BlockHeader() and .from_parent()

Direct invocation of the header object is not allowed anymore, because
different VMs now have different header types (as of EIP-1559 in
London). So we must always invoke header creation through the VM.

- Remove the BlockHeader() init in test_blockchain.
- Remove HeaderChain, which seems to be completely unused in the code
    base.
- Embrace that we need one global type that can decode all headers.
- Add VM.create_genesis_header
    The genesis header now needs to be VM-aware, to handle EIP-1559. Rather
    than pass in a parent of None, it looks a bit cleaner to use the new
    create_genesis_header.
- Create header from VM in opcode test (we need a proper London header
    at genesis)

Author: carver

eth/abc.py

@@ -3315,6 +3315,16 @@ def generate_block_from_parent_header_and_coinbase(cls,
         """
         ...
 
+    @classmethod
+    @abstractmethod
+    def create_genesis_header(cls, **genesis_params: Any) -> BlockHeaderAPI:
+        """
+        Create a genesis header using this VM's rules.
+
+        This is currently equivalent to create_header_from_parent(None, **genesis_params)
+        """
+        ...
+
     @classmethod
     @abstractmethod
     def get_block_class(cls) -> Type[BlockAPI]:

eth/chains/base.py

@@ -247,7 +247,7 @@ def from_genesis(cls,
                 f"Expected {genesis_params['state_root']!r}"
             )
 
-        genesis_header = genesis_vm_class.create_header_from_parent(None, **genesis_params)
+        genesis_header = genesis_vm_class.create_genesis_header(**genesis_params)
         return cls.from_genesis_header(base_db, genesis_header)
 
     @classmethod

eth/constants.py

@@ -161,6 +161,9 @@
 GENESIS_NONCE = b'\x00\x00\x00\x00\x00\x00\x00B'  # 0x42 encoded as big-endian-integer
 GENESIS_MIX_HASH = ZERO_HASH32
 GENESIS_EXTRA_DATA = b''
+GENESIS_BLOOM = 0
+GENESIS_GAS_USED = 0
+
 #
 # Sha3 Keccak
 #

eth/tools/fixtures/__init__.py

@@ -8,6 +8,7 @@
 )
 from .helpers import (  # noqa: F401
     new_chain_from_fixture,
+    genesis_fields_from_fixture,
     genesis_params_from_fixture,
     apply_fixture_block_to_chain,
     setup_state,

eth/tools/fixtures/helpers.py

@@ -24,6 +24,7 @@
     StateAPI,
     VirtualMachineAPI,
 )
+from eth import constants
 from eth.db.atomic import AtomicDB
 from eth.chains.mainnet import (
     MainnetDAOValidatorVM,
@@ -165,7 +166,11 @@ def chain_vm_configuration(fixture: Dict[str, Any]) -> Iterable[Tuple[int, Type[
         raise ValueError(f"Network {network} does not match any known VM rules")
 
 
-def genesis_params_from_fixture(fixture: Dict[str, Any]) -> Dict[str, Any]:
+def genesis_fields_from_fixture(fixture: Dict[str, Any]) -> Dict[str, Any]:
+    """
+    Convert all genesis fields in a fixture to a dictionary of header fields and values.
+    """
+
     return {
         'parent_hash': fixture['genesisBlockHeader']['parentHash'],
         'uncles_hash': fixture['genesisBlockHeader']['uncleHash'],
@@ -185,6 +190,34 @@ def genesis_params_from_fixture(fixture: Dict[str, Any]) -> Dict[str, Any]:
     }
 
 
+def genesis_params_from_fixture(fixture: Dict[str, Any]) -> Dict[str, Any]:
+    """
+    Convert a genesis fixture into a dict of the configurable header fields and values.
+
+    Some fields cannot be explicitly set when creating a new header, like
+    parent_hash, which is automatically set to the empty hash.
+    """
+
+    params = genesis_fields_from_fixture(fixture)
+
+    # Confirm that (currently) non-configurable defaults are set correctly,
+    #   then remove them because they cannot be configured on the header.
+    defaults = (
+        ('parent_hash', constants.GENESIS_PARENT_HASH),
+        ('uncles_hash', constants.EMPTY_UNCLE_HASH),
+        ('bloom', constants.GENESIS_BLOOM),
+        ('block_number', constants.GENESIS_BLOCK_NUMBER),
+        ('gas_used', constants.GENESIS_GAS_USED),
+    )
+
+    for key, default_val in defaults:
+        supplied_val = params.pop(key)
+        if supplied_val != default_val:
+            raise ValueError(f"Unexpected genesis {key}: {supplied_val}, expected: {default_val}")
+
+    return params
+
+
 def new_chain_from_fixture(fixture: Dict[str, Any],
                            chain_cls: Type[ChainAPI] = MainnetChain) -> ChainAPI:
     base_db = AtomicDB()

eth/tools/rlp.py

@@ -8,11 +8,6 @@
 )
 
 
-assert_imported_genesis_header_unchanged = replace_exceptions({
-    ValidationError: AssertionError,
-})(validate_rlp_equal(obj_a_name='genesis header', obj_b_name='imported header'))
-
-
 assert_mined_block_unchanged = replace_exceptions({
     ValidationError: AssertionError,
 })(validate_rlp_equal(obj_a_name='provided block', obj_b_name='executed block'))

eth/vm/base.py

@@ -454,6 +454,11 @@ def generate_block_from_parent_header_and_coinbase(cls,
         )
         return block
 
+    @classmethod
+    def create_genesis_header(cls, **genesis_params) -> BlockHeaderAPI:
+        # Create genesis header by setting the parent to None
+        return cls.create_header_from_parent(None, **genesis_params)
+
     @classmethod
     def get_block_class(cls) -> Type[BlockAPI]:
         if cls.block_class is None: