Fix duplicate registration for resource_extend

Description field is added to resources by inheriting
StandardAttrDescriptionMixin, and StandardAttrDescriptionMixin use the
@has_resource_extenders decorator to register resource_extend functions.
Any plugin which inherits StandardAttrDescriptionMixin will register
_extend_standard_attr_description duplicately, leading to performance
issues caused by duplicate executions. This fix ensures that resource
extend functions are executed only once.

Closes-bug: #2127835
Change-Id: Id3e36dc5475ccff69b1257ff9816201ed04257fe
Signed-off-by: Xu Qi<xuqi_yewu@cmss.chinamobile.com>

neutron_lib/db/resource_extend.py

@@ -58,9 +58,12 @@ def register_funcs(resource, funcs):
             return foo_res
 
     """
-    funcs = [helpers.make_weak_ref(f) if callable(f) else f
-             for f in funcs]
-    _resource_extend_functions.setdefault(resource, []).extend(funcs)
+    funcs = [helpers.make_weak_ref(f) if callable(f) else f for f in funcs]
+    existing_funcs = _resource_extend_functions.setdefault(resource, [])
+
+    for func in funcs:
+        if func not in existing_funcs:
+            existing_funcs.append(func)
 
 
 def get_funcs(resource):

neutron_lib/tests/unit/db/test_resource_extend.py

@@ -49,6 +49,19 @@ class TestResourceExtend(base.BaseTestCase):
         for r in resources:
             self.assertIsNotNone(resource_extend.get_funcs(r))
 
+    def test_register_funcs_no_duplicates(self):
+        resource = 'A'
+
+        def _cb(resp, db_obj):
+            pass
+
+        resource_extend.register_funcs(resource, (_cb,))
+        resource_extend.register_funcs(resource, (_cb,))
+
+        funcs = resource_extend.get_funcs(resource)
+        self.assertEqual(funcs.count(funcs[0]), 1)
+        self.assertEqual(len(funcs), 1)
+
     def test_apply_funcs(self):
         resources = ['A', 'B', 'C']
         callbacks = []

releasenotes/notes/fix-resource-extend-baf4d6418e0f1465.yaml

@@ -0,0 +1,7 @@
+---
+other:
+  - |
+    Avoid the duplicated method registration in class methods decorated with
+    ``resource_extend.has_resource_extenders``. Before registering a new
+    extended method, it is first checked if this method is already registered.
+    See bug: `2127835 <https://bugs.launchpad.net/neutron/+bug/2127835>`_.