Add guard against pending transaction for multi-use snapshots.

spanner/google/cloud/spanner/snapshot.py

@@ -73,10 +73,14 @@ def read(self, table, columns, keyset, index='', limit=0,
 
         :rtype: :class:`~google.cloud.spanner.streamed.StreamedResultSet`
         :returns: a result set instance which can be used to consume rows.
-        :raises: ValueError for reuse of single-use snapshots.
+        :raises: ValueError for reuse of single-use snapshots, or if a
+                 transaction ID is pending for multiple-use snapshots.
         """
-        if not self._multi_use and self._read_request_count > 0:
-            raise ValueError("Cannot re-use single-use snapshot.")
+        if self._read_request_count > 0:
+            if not self._multi_use:
+                raise ValueError("Cannot re-use single-use snapshot.")
+            if self._transaction_id is None:
+                raise ValueError("Transaction ID pending.")
 
         database = self._session._database
         api = database.spanner_api
@@ -121,10 +125,14 @@ def execute_sql(self, sql, params=None, param_types=None, query_mode=None,
 
         :rtype: :class:`~google.cloud.spanner.streamed.StreamedResultSet`
         :returns: a result set instance which can be used to consume rows.
-        :raises: ValueError for reuse of single-use snapshots.
+        :raises: ValueError for reuse of single-use snapshots, or if a
+                 transaction ID is pending for multiple-use snapshots.
         """
-        if not self._multi_use and self._read_request_count > 0:
-            raise ValueError("Cannot re-use single-use snapshot.")
+        if self._read_request_count > 0:
+            if not self._multi_use:
+                raise ValueError("Cannot re-use single-use snapshot.")
+            if self._transaction_id is None:
+                raise ValueError("Transaction ID pending.")
 
         if params is not None:
             if param_types is None:

spanner/tests/unit/test_snapshot.py

@@ -112,7 +112,7 @@ def test_read_grpc_error(self):
         self.assertEqual(options.kwargs['metadata'],
                          [('google-cloud-resource-prefix', database.name)])
 
-    def _read_helper(self, multi_use, first=True):
+    def _read_helper(self, multi_use, first=True, count=0):
         from google.protobuf.struct_pb2 import Struct
         from google.cloud.proto.spanner.v1.result_set_pb2 import (
             PartialResultSet, ResultSetMetadata, ResultSetStats)
@@ -156,18 +156,15 @@ def _read_helper(self, multi_use, first=True):
         session = _Session(database)
         derived = self._makeDerived(session)
         derived._multi_use = multi_use
+        derived._read_request_count = count
         if not first:
             derived._transaction_id = TXN_ID
-            derived._read_request_count = 1
 
         result_set = derived.read(
             TABLE_NAME, COLUMNS, KEYSET,
             index=INDEX, limit=LIMIT, resume_token=TOKEN)
 
-        if first:
-            self.assertEqual(derived._read_request_count, 1)
-        else:
-            self.assertEqual(derived._read_request_count, 2)
+        self.assertEqual(derived._read_request_count, count + 1)
 
         if multi_use:
             self.assertIs(result_set._source, derived)
@@ -205,14 +202,21 @@ def test_read_wo_multi_use(self):
 
     def test_read_wo_multi_use_w_read_request_count_gt_0(self):
         with self.assertRaises(ValueError):
-            self._read_helper(multi_use=False, first=False)
+            self._read_helper(multi_use=False, count=1)
 
     def test_read_w_multi_use_wo_first(self):
         self._read_helper(multi_use=True, first=False)
 
+    def test_read_w_multi_use_wo_first_w_count_gt_0(self):
+        self._read_helper(multi_use=True, first=False, count=1)
+
     def test_read_w_multi_use_w_first(self):
         self._read_helper(multi_use=True, first=True)
 
+    def test_read_w_multi_use_w_first_w_count_gt_0(self):
+        with self.assertRaises(ValueError):
+            self._read_helper(multi_use=True, first=True, count=1)
+
     def test_execute_sql_grpc_error(self):
         from google.cloud.proto.spanner.v1.transaction_pb2 import (
             TransactionSelector)
@@ -249,7 +253,7 @@ def test_execute_sql_w_params_wo_param_types(self):
         with self.assertRaises(ValueError):
             derived.execute_sql(SQL_QUERY_WITH_PARAM, PARAMS)
 
-    def _execute_sql_helper(self, multi_use, first=True):
+    def _execute_sql_helper(self, multi_use, first=True, count=0):
         from google.protobuf.struct_pb2 import Struct
         from google.cloud.proto.spanner.v1.result_set_pb2 import (
             PartialResultSet, ResultSetMetadata, ResultSetStats)
@@ -291,18 +295,15 @@ def _execute_sql_helper(self, multi_use, first=True):
         session = _Session(database)
         derived = self._makeDerived(session)
         derived._multi_use = multi_use
+        derived._read_request_count = count
         if not first:
             derived._transaction_id = TXN_ID
-            derived._read_request_count = 1
 
         result_set = derived.execute_sql(
             SQL_QUERY_WITH_PARAM, PARAMS, PARAM_TYPES,
             query_mode=MODE, resume_token=TOKEN)
 
-        if first:
-            self.assertEqual(derived._read_request_count, 1)
-        else:
-            self.assertEqual(derived._read_request_count, 2)
+        self.assertEqual(derived._read_request_count, count + 1)
 
         if multi_use:
             self.assertIs(result_set._source, derived)
@@ -341,13 +342,20 @@ def test_execute_sql_wo_multi_use(self):
 
     def test_execute_sql_wo_multi_use_w_read_request_count_gt_0(self):
         with self.assertRaises(ValueError):
-            self._execute_sql_helper(multi_use=False, first=False)
+            self._execute_sql_helper(multi_use=False, count=1)
+
+    def test_execute_sql_w_multi_use_wo_first(self):
+        self._execute_sql_helper(multi_use=True, first=False)
+
+    def test_execute_sql_w_multi_use_wo_first_w_count_gt_0(self):
+        self._execute_sql_helper(multi_use=True, first=False, count=1)
 
     def test_execute_sql_w_multi_use_w_first(self):
         self._execute_sql_helper(multi_use=True, first=True)
 
-    def test_execute_sql_w_multi_use_wo_first(self):
-        self._execute_sql_helper(multi_use=True, first=False)
+    def test_execute_sql_w_multi_use_w_first_w_count_gt_0(self):
+        with self.assertRaises(ValueError):
+            self._execute_sql_helper(multi_use=True, first=True, count=1)
 
 
 class _MockCancellableIterator(object):