Regenerate context during targeting

We are only doing a copy.copy() on context when we do target_cell(),
which may mean we're sharing more across threads than we intend to.
This makes that a full regeneration of the context (like we would
do over RPC). This will necessarily dump the targeting, if already
set, and any per-thread database context.

Closes-Bug: #1722404
Change-Id: Id24dea7465bafc1f6f58c4a121c4ffb35b7a634a
(cherry picked from commit 42b70509c6)

nova/context.py

@@ -425,7 +425,14 @@ def target_cell(context, cell_mapping):
     :param context: The RequestContext to add connection information
     :param cell_mapping: An objects.CellMapping object or None
     """
-    cctxt = copy.copy(context)
+    # Create a sanitized copy of context by serializing and deserializing it
+    # (like we would do over RPC). This help ensure that we have a clean
+    # copy of the context with all the tracked attributes, but without any
+    # of the hidden/private things we cache on a context. We do this to avoid
+    # unintentional sharing of cached thread-local data across threads.
+    # Specifically, this won't include any oslo_db-set transaction context, or
+    # any existing cell targeting.
+    cctxt = RequestContext.from_dict(context.to_dict())
     set_target_cell(cctxt, cell_mapping)
     yield cctxt
 

nova/tests/unit/test_context.py

@@ -274,6 +274,26 @@ class ContextTestCase(test.NoDBTestCase):
         self.assertEqual(mock.sentinel.db_conn, ctxt.db_connection)
         self.assertEqual(mock.sentinel.mq_conn, ctxt.mq_connection)
 
+    @mock.patch('nova.context.set_target_cell')
+    def test_target_cell_regenerates(self, mock_set):
+        ctxt = context.RequestContext('fake', 'fake')
+        # Set a non-tracked property on the context to make sure it
+        # does not make it to the targeted one (like a copy would do)
+        ctxt.sentinel = mock.sentinel.parent
+        with context.target_cell(ctxt, mock.sentinel.cm) as cctxt:
+            # Should be a different object
+            self.assertIsNot(cctxt, ctxt)
+
+            # Should not have inherited the non-tracked property
+            self.assertFalse(hasattr(cctxt, 'sentinel'),
+                             'Targeted context was copied from original')
+
+            # Set another non-tracked property
+            cctxt.sentinel = mock.sentinel.child
+
+        # Make sure we didn't pollute the original context
+        self.assertNotEqual(ctxt.sentinel, mock.sentinel.child)
+
     def test_get_context(self):
         ctxt = context.get_context()
         self.assertIsNone(ctxt.user_id)