Add methods for RequestContext to switch db connection

This augments the RequestContext with connection information for a
target database to query.  This will allow nova-api to issue queries to
a targeted cell database with each query.

Example usages:

    ctxt = context.RequestContext()
    cell_mapping = CellMapping()

    with context.target_cell(ctxt, cell_mapping):
        instance = objects.Instance.get_by_uuid(ctxt, uuid)

    with context.target_cell(ctxt, cell_mapping):
        with instance.obj_alternate_context(ctxt):
            instance.save()

Implements blueprint cells-db-connection-switching

Co-Authored-By: melanie witt<melwitt@yahoo-inc.com>

Change-Id: I9e2363a35b58ae55bd40194c1f8bfb89b599bf04

1 files changed, 15 insertions, 0 deletions

diff --git a/nova/tests/unit/test_context.py b/nova/tests/unit/test_context.py
index 5a4651d803..65feeb71a4 100644
--- a/nova/tests/unit/test_context.py
+++ b/nova/tests/unit/test_context.py
@@ -12,10 +12,12 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import mock
 from oslo_context import context as o_context
 from oslo_context import fixture as o_fixture
 
 from nova import context
+from nova import objects
 from nova import test
 
 
@@ -223,3 +225,16 @@ class ContextTestCase(test.NoDBTestCase):
         self.assertEqual('222', ctx.project_id)
         values2 = ctx.to_dict()
         self.assertEqual(values, values2)
+
+    @mock.patch('nova.db.create_context_manager')
+    def test_target_cell(self, mock_create_ctxt_mgr):
+        mock_create_ctxt_mgr.return_value = mock.sentinel.cm
+        ctxt = context.RequestContext('111',
+                                      '222',
+                                      roles=['admin', 'weasel'])
+        # Verify the existing db_connection, if any, is restored
+        ctxt.db_connection = mock.sentinel.db_conn
+        mapping = objects.CellMapping(database_connection='fake://')
+        with context.target_cell(ctxt, mapping):
+            self.assertEqual(ctxt.db_connection, mock.sentinel.cm)
+        self.assertEqual(mock.sentinel.db_conn, ctxt.db_connection)