Block creation of transaction for session w/ existing txn. (googleapis#3785)

Closes googleapis#3476.

Author: tseaver

spanner/google/cloud/spanner/session.py

@@ -249,6 +249,7 @@ def transaction(self):
 
         if self._transaction is not None:
             self._transaction._rolled_back = True
+            del self._transaction
 
         txn = self._transaction = Transaction(self)
         return txn

spanner/google/cloud/spanner/transaction.py

@@ -24,12 +24,24 @@
 
 
 class Transaction(_SnapshotBase, _BatchBase):
-    """Implement read-write transaction semantics for a session."""
+    """Implement read-write transaction semantics for a session.
+
+    :type session: :class:`~google.cloud.spanner.session.Session`
+    :param session: the session used to perform the commit
+
+    :raises ValueError: if session has an existing transaction
+    """
     committed = None
     """Timestamp at which the transaction was successfully committed."""
     _rolled_back = False
     _multi_use = True
 
+    def __init__(self, session):
+        if session._transaction is not None:
+            raise ValueError("Session has existing transaction.")
+
+        super(Transaction, self).__init__(session)
+
     def _check_state(self):
         """Helper for :meth:`commit` et al.
 

spanner/tests/unit/test_transaction.py

@@ -47,6 +47,12 @@ def _make_one(self, session, *args, **kwargs):
         session._transaction = transaction
         return transaction
 
+    def test_ctor_session_w_existing_txn(self):
+        session = _Session()
+        session._transaction = object()
+        with self.assertRaises(ValueError):
+            transaction = self._make_one(session)
+
     def test_ctor_defaults(self):
         session = _Session()
         transaction = self._make_one(session)