Adding system test for datastore failed transactions.

This intentionally forces data contention in a transaction (by
updating the entity outside of the transaction).

This is to prevent regressions like googleapis#903.

system_tests/datastore.py

@@ -19,6 +19,7 @@
 from gcloud import datastore
 from gcloud.datastore import client
 from gcloud.environment_vars import TESTS_DATASET
+from gcloud.exceptions import Conflict
 # This assumes the command is being run via tox hence the
 # repository root is the current directory.
 from system_tests import populate_datastore
@@ -341,3 +342,45 @@ def test_transaction(self):
         retrieved_entity = CLIENT.get(entity.key)
         self.case_entities_to_delete.append(retrieved_entity)
         self.assertEqual(retrieved_entity, entity)
+
+    def test_failure_with_contention(self):
+        CONTENTION_KEY = 'baz'
+
+        # Insert an entity which will be retrieved in a transaction
+        # and updated outside it with a contentious value.
+        key = CLIENT.key('BreakTxn', 1234)
+        orig_entity = datastore.Entity(key=key)
+        orig_entity['foo'] = u'bar'
+        CLIENT.put(orig_entity)
+        self.case_entities_to_delete.append(orig_entity)
+
+        # Make sure we are not yet in a transaction.
+        self.assertEqual(CLIENT.current_transaction, None)
+
+        # Artificially set a transaction on the client's stack, but don't
+        # enter the context manager (we want to make an upsert outside
+        # of the transaction).
+        txn = CLIENT.transaction()
+        CLIENT._push_batch(txn)
+        self.assertEqual(CLIENT.current_transaction, txn)
+        # Since current_transaction == txn, this will occur transactionally.
+        txn.begin()
+        entity_in_txn = CLIENT.get(key)
+
+        # Update the original entity outside the transaction.
+        CLIENT._pop_batch()
+        self.assertEqual(CLIENT.current_transaction, None)
+        orig_entity[CONTENTION_KEY] = u'outside'
+        CLIENT.put(orig_entity)
+
+        # Put the transaction back in context and try to update the entity
+        # which we already updated outside the transaction.
+        CLIENT._push_batch(txn)
+        self.assertEqual(CLIENT.current_transaction, txn)
+        entity_in_txn[CONTENTION_KEY] = u'inside'
+        txn.put(entity_in_txn)
+
+        with self.assertRaises(Conflict):
+            txn.commit()
+
+        CLIENT._pop_batch()