Modify forum roles query for Aurora performance.

This is a fix for the performance issues in EDUCATOR-3374

AWS's Aurora backend for MySQL selects the wrong index on the
django_comment_client_role_users table, leading to performance issues.
This commit replaces that join with individual requests for permissions
for each role (of which there may be several for any given user).
It's dumber SQL, but Aurora will do the right thing.

common/djangoapps/django_comment_common/models.py

@@ -145,15 +145,19 @@ def all_permissions_for_user_in_course(user, course_id):  # pylint: disable=inva
     if course is None:
         raise ItemNotFoundError(course_id)
 
-    all_roles = {role.name for role in Role.objects.filter(users=user, course_id=course_id)}
+    roles = Role.objects.filter(users=user, course_id=course_id)
+    role_names = {role.name for role in roles}
 
-    permissions = {
-        permission.name
-        for permission
-        in Permission.objects.filter(roles__users=user, roles__course_id=course_id)
-        if not permission_blacked_out(course, all_roles, permission.name)
-    }
-    return permissions
+    permission_names = set()
+    for role in roles:
+        # Intentional n+1 query pattern to get permissions for each role because
+        # Aurora's query optimizer can't handle the join proplerly on 30M+ row
+        # tables (EDUCATOR-3374). Fortunately, there are very few forum roles.
+        for permission in role.permissions.all():
+            if not permission_blacked_out(course, role_names, permission.name):
+                permission_names.add(permission.name)
+
+    return permission_names
 
 
 class ForumsConfig(ConfigurationModel):

lms/djangoapps/courseware/tests/test_discussion_xblock.py

@@ -406,7 +406,7 @@ class TestXBlockQueryLoad(SharedModuleStoreTestCase):
         # * django_comment_client_role
         # * django_comment_client_permission
         # * lms_xblock_xblockasidesconfig
-        num_queries = 3
+        num_queries = 2
         for discussion in discussions:
             discussion_xblock = get_module_for_descriptor_internal(
                 user=user,