Add v to special legacy transaction API

Author: carver

eth/abc.py

@@ -380,6 +380,19 @@ def chain_id(self) -> Optional[int]:
         ...
 
 
+class LegacyTransactionFieldsAPI(TransactionFieldsAPI):
+    @property
+    @abstractmethod
+    def v(self) -> int:
+        """
+        In old transactions, this v field combines the y_parity bit and the
+        chain ID. All new usages should prefer accessing those fields directly.
+        But if you must access the original v, then you can cast to this API
+        first (after checking that type_id is None).
+        """
+        ...
+
+
 class UnsignedTransactionAPI(BaseTransactionAPI):
 
     """

eth/rlp/transactions.py

@@ -24,6 +24,7 @@
 from eth.abc import (
     BaseTransactionAPI,
     ComputationAPI,
+    LegacyTransactionFieldsAPI,
     SignedTransactionAPI,
     TransactionBuilderAPI,
     TransactionFieldsAPI,
@@ -103,13 +104,21 @@ def is_signature_valid(self) -> bool:
             return True
 
 
-class BaseTransaction(BaseTransactionFields, SignedTransactionMethods, TransactionBuilderAPI):
+class BaseTransaction(
+        LegacyTransactionFieldsAPI,
+        BaseTransactionFields,
+        SignedTransactionMethods,
+        TransactionBuilderAPI):
     # "Legacy" transactions implemented by BaseTransaction are a combination of
     # the transaction codec (TransactionBuilderAPI) *and* the transaction
     # object (SignedTransactionAPI). In a multi-transaction-type world, that
     # becomes less desirable, and that responsibility splits up. See Berlin
     # transactions, for example.
 
+    # Note that it includes at least one legacy field (v) that is not
+    # explicitly accessible in new transaction types. See the v docstring in
+    # LegacyTransactionFieldsAPI for more.
+
     # this is duplicated to make the rlp library happy, otherwise it complains
     # about no fields being defined but inheriting from multiple `Serializable`
     # bases.

newsfragments/1997.feature.rst

@@ -0,0 +1 @@
+Add new LegacyTransactionFieldsAPI, with a v field for callers that want to access v directly.