Define a new interface for storages that support the new commit protocol.

This allows interface incompatibilities:

- Future version of ZODB that requires the new protocol being
  configured with a storage that doesn't implement the new protocol

- ZEO has a client that uses the old protocol can't use a storage that
  implements the new protocol.

To be caught early.

This commit also clarifies that the serial returned by ``undo()`` is ignored.

src/ZODB/interfaces.py

@@ -811,6 +811,92 @@ class IStorage(Interface):
         either from tpc_vote or store for objects passed to store.
         """
 
+class IMultiCommitStorage(IStorage):
+    """A multi-commit storage can commit multiple transactions at once.
+
+    It's lilely that future versions of ZODB will require all storages
+    to provide this interface.
+    """
+
+    def store(oid, serial, data, version, transaction):
+        """Store data for the object id, oid.
+
+        Arguments:
+
+        oid
+            The object identifier.  This is either a string
+            consisting of 8 nulls or a string previously returned by
+            new_oid.
+
+        serial
+            The serial of the data that was read when the object was
+            loaded from the database.  If the object was created in
+            the current transaction this will be a string consisting
+            of 8 nulls.
+
+        data
+            The data record. This is opaque to the storage.
+
+        version
+            This must be an empty string. It exists for backward compatibility.
+
+        transaction
+            A transaction object.  This should match the current
+            transaction for the storage, set by tpc_begin.
+
+        None is returned.
+
+        Several different exceptions may be raised when an error occurs.
+
+        ConflictError
+          is raised when serial does not match the most recent serial
+          number for object oid and the conflict was not resolved by
+          the storage.
+
+        StorageTransactionError
+          is raised when transaction does not match the current
+          transaction.
+
+        StorageError or, more often, a subclass of it
+          is raised when an internal error occurs while the storage is
+          handling the store() call.
+
+        """
+
+    def tpc_finish(transaction, func = lambda tid: None):
+        """Finish the transaction, making any transaction changes permanent.
+
+        Changes must be made permanent at this point.
+
+        This call raises a StorageTransactionError if the storage
+        isn't participating in two-phase commit.  Failure of this
+        method is extremely serious.
+
+        The second argument is a call-back function that must be
+        called while the storage transaction lock is held.  It takes
+        the new transaction id generated by the transaction.
+
+        The return value must be the committed tid. It is used to set the
+        serial for objects whose ids were passed to previous store calls
+        in the same transaction.
+        """
+
+    def tpc_vote(transaction):
+        """Provide a storage with an opportunity to veto a transaction
+
+        This call raises a StorageTransactionError if the storage
+        isn't participating in two-phase commit or if it is commiting
+        a different transaction.
+
+        If a transaction can be committed by a storage, then the
+        method should return.  If a transaction cannot be committed,
+        then an exception should be raised.  If this method returns
+        without an error, then there must not be an error if
+        tpc_finish or tpc_abort is called subsequently.
+
+        The return value can be either None or a sequence of oids for which
+        a conflict was resolved.
+        """
 
 class IStorageRestoreable(IStorage):
     """Copying Transactions
@@ -948,8 +1034,8 @@ class IStorageUndoable(IStorage):
         This method must only be called in the first phase of
         two-phase commit (after tpc_begin but before tpc_vote). It
         returns a serial (transaction id) and a sequence of object ids
-        for objects affected by the transaction.
-
+        for objects affected by the transaction. The serial is ignored
+        and may be None.
         """
         # Used by DB (Actually, by TransactionalUndo)