Get rid of double-join to rbac_entries without filter

apply_filters_to_query was performing an outerjoin to rbac_entries
unconditionally when model_query could have already performed an
outerjoin (if the request was from an unprivileged user) and/or when
the join wasn't even necessary (the '?shared=False' query that uses
a subquery and not a join). This resulted in terrible performance
because of cartesian products of rbac entries with themselves.

This fixes the issue by ensuring there is only an outerjoin to the
rbac table if it's going to be used for a filter condition and it's
not already joined because of a query scope imposed due to the user
not being privileged.

Unfortunately this doesn't include tests to prevent regressions because
we don't have any methods for testing the performance of individual
queries.

Closes-Bug: #1630939
Change-Id: I4364f4a97a29041e86b2fbd8aa895578153f4cf9
(cherry picked from commit 145dbaa)

neutron/db/common_db_mixin.py

@@ -233,7 +233,6 @@ def _apply_filters_to_query(self, query, model, filters, context=None):
                 elif key == 'shared' and hasattr(model, 'rbac_entries'):
                     # translate a filter on shared into a query against the
                     # object's rbac entries
-                    query = query.outerjoin(model.rbac_entries)
                     rbac = model.rbac_entries.property.mapper.class_
                     matches = [rbac.target_tenant == '*']
                     if context:
@@ -260,6 +259,12 @@ def _apply_filters_to_query(self, query, model, filters, context=None):
                             query.session.query(rbac.object_id).
                             filter(is_shared)
                         )
+                    elif (not context or
+                          not self.model_query_scope(context, model)):
+                        # we only want to join if we aren't using the subquery
+                        # and if we aren't already joined because this is a
+                        # scoped query
+                        query = query.outerjoin(model.rbac_entries)
                     query = query.filter(is_shared)
             for _nam, hooks in six.iteritems(self._model_query_hooks.get(model,
                                                                          {})):