db: Resolve additional SAWarning warnings

Resolving the following SAWarning warnings:

  Coercing Subquery object into a select() for use in IN(); please pass
  a select() construct explicitly

  SELECT statement has a cartesian product between FROM element(s)
  "foo" and FROM element "bar". Apply join condition(s) between each
  element to resolve.

While the first of these was a trivial fix, the second one is a little
more involved. It was caused by attempting to build a query across
tables that had no relationship as part of our archive logic. For
example, consider the following queries, generated early in
'_get_fk_stmts':

  SELECT instances.uuid
  FROM instances, security_group_instance_association
  WHERE security_group_instance_association.instance_uuid = instances.uuid
    AND instances.id IN (__[POSTCOMPILE_id_1])

  SELECT security_groups.id
  FROM security_groups, security_group_instance_association, instances
  WHERE security_group_instance_association.security_group_id = security_groups.id
    AND instances.id IN (__[POSTCOMPILE_id_1])

While the first of these is fine, the second is clearly wrong: why are
we filtering on a field that is of no relevance to our join? These were
generated because we were attempting to archive one or more instances
(in this case, the instance with id=1) and needed to find related tables
to archive at the same time. A related table is any table that
references our "source" table - 'instances' here - by way of a foreign
key. For each of *these* tables, we then lookup each foreign key and
join back to the source table, filtering by matching entries in the
source table. The issue here is that we're looking up every foreign key.
What we actually want to do is lookup only the foreign keys that point
back to our source table. This flaw is why we were generating the second
SELECT above: the 'security_group_instance_association' has two foreign
keys, one pointing to our 'instances' table but also another pointing to
the 'security_groups' table. We want the first but not the second.

Resolve this by checking if the table that each foreign key points to is
actually the source table and simply skip if not. With this issue
resolved, we can enable errors on SAWarning warnings in general without
any filters.

Conflicts:
    nova/objects/cell_mapping.py
    nova/tests/fixtures/nova.py

NOTE(melwitt): The conflicts are because

  * Change Ia5304c552ce552ae3c5223a2bfb3a9cd543ec57c (db: Post
    reshuffle cleanup) is not in Wallaby

  * The WarningsFixture is in nova/tests/fixtures.py in Wallaby and
    change I07be8e16381592fc177eeecd4f0d7f2db93eb09d (tests: Enable
    SADeprecationWarning warnings) is also not in Wallaby

Change-Id: I63208c7bd5f9f4c3d5e4a40bd0f6253d0f042a37
Signed-off-by: Stephen Finucane <[email protected]>
(cherry picked from commit 8142b9d)
(cherry picked from commit ce2cc54)
(cherry picked from commit 5b1d487)

Author: stephenfin

nova/db/sqlalchemy/api.py

@@ -4133,6 +4133,12 @@ def _get_fk_stmts(metadata, conn, table, column, records):
             fk_column = fk_table.c.id
 
         for fk in fk_table.foreign_keys:
+            if table != fk.column.table:
+                # if the foreign key doesn't actually point to the table we're
+                # archiving entries from then it's not relevant; trying to
+                # resolve this would result in a cartesian product
+                continue
+
             # We need to find the records in the referring (child) table that
             # correspond to the records in our (parent) table so we can archive
             # them.
@@ -4179,6 +4185,7 @@ def _get_fk_stmts(metadata, conn, table, column, records):
                 # deque.
                 fk_delete = fk_table.delete().where(fk_column.in_(fk_records))
                 deletes.appendleft(fk_delete)
+
         # Repeat for any possible nested child tables.
         i, d = _get_fk_stmts(metadata, conn, fk_table, fk_column, fk_records)
         inserts.extendleft(i)

nova/objects/cell_mapping.py

@@ -279,11 +279,15 @@ def _get_by_project_id_from_db(context, project_id):
         # SELECT DISTINCT cell_id FROM instance_mappings \
         #   WHERE project_id = $project_id;
         cell_ids = context.session.query(
-            api_models.InstanceMapping.cell_id).filter_by(
-            project_id=project_id).distinct().subquery()
+            api_models.InstanceMapping.cell_id
+        ).filter_by(
+            project_id=project_id
+        ).distinct()
         # SELECT cell_mappings WHERE cell_id IN ($cell_ids);
-        return context.session.query(api_models.CellMapping).filter(
-            api_models.CellMapping.id.in_(cell_ids)).all()
+        return context.session.query(
+            api_models.CellMapping).filter(
+            api_models.CellMapping.id.in_(cell_ids)
+        ).all()
 
     @classmethod
     def get_by_project_id(cls, context, project_id):