fix: allow setting staleness to same value in tx

Repeatedly setting the staleness property of a connection in a
transaction to the same value caused an error. This made it
harder to use this property in SQLAlchemy.

Updates googleapis/python-spanner-sqlalchemy#495

google/cloud/spanner_dbapi/connection.py

@@ -277,7 +277,7 @@ def staleness(self, value):
         Args:
             value (dict): Staleness type and value.
         """
-        if self._spanner_transaction_started:
+        if self._spanner_transaction_started and value != self._staleness:
             raise ValueError(
                 "`staleness` option can't be changed while a transaction is in progress. "
                 "Commit or rollback the current transaction and try again."

tests/unit/spanner_dbapi/test_connection.py

@@ -669,6 +669,20 @@ def test_staleness_inside_transaction(self):
         with self.assertRaises(ValueError):
             connection.staleness = {"read_timestamp": datetime.datetime(2021, 9, 21)}
 
+    def test_staleness_inside_transaction_same_value(self):
+        """
+        Verify that setting `staleness` to the same value in a transaction is allowed.
+        """
+        connection = self._make_connection()
+        connection.staleness = {"read_timestamp": datetime.datetime(2021, 9, 21)}
+        connection._spanner_transaction_started = True
+        connection._transaction = mock.Mock()
+
+        connection.staleness = {"read_timestamp": datetime.datetime(2021, 9, 21)}
+        self.assertEqual(
+            connection.staleness, {"read_timestamp": datetime.datetime(2021, 9, 21)}
+        )
+
     def test_staleness_multi_use(self):
         """
         Check that `staleness` option is correctly