test: add a test for unary retries of UNAVAILABLE (#1376)

Author: olavloite

tests/mockserver_tests/test_basics.py

@@ -93,6 +93,23 @@ def test_dbapi_partitioned_dml(self):
             TransactionOptions(dict(partitioned_dml={})), begin_request.options
         )
 
+    def test_batch_create_sessions_unavailable(self):
+        add_select1_result()
+        add_error(SpannerServicer.BatchCreateSessions.__name__, unavailable_status())
+        with self.database.snapshot() as snapshot:
+            results = snapshot.execute_sql("select 1")
+            result_list = []
+            for row in results:
+                result_list.append(row)
+                self.assertEqual(1, row[0])
+            self.assertEqual(1, len(result_list))
+        requests = self.spanner_service.requests
+        self.assertEqual(3, len(requests), msg=requests)
+        # The BatchCreateSessions call should be retried.
+        self.assertTrue(isinstance(requests[0], BatchCreateSessionsRequest))
+        self.assertTrue(isinstance(requests[1], BatchCreateSessionsRequest))
+        self.assertTrue(isinstance(requests[2], ExecuteSqlRequest))
+
     def test_execute_streaming_sql_unavailable(self):
         add_select1_result()
         # Add an UNAVAILABLE error that is returned the first time the