Merge ~adam-collard/maas:middleware-speedup into maas:master

[Adam Collard]

Speed up middleware detecting disconnected controllers

By avoiding materialising all of the RackControllers, locally I see improvements on the time to run the tests:

https://paste.ubuntu.com/p/3gykFxgQMC/

[Christian Grabowski (cgrabowski)]

Nice fix!

review: Approve

[MAAS Lander (maas-lander)]

UNIT TESTS
-b middleware-speedup lp:~adam-collard/maas/+git/maas into -b master lp:~maas-committers/maas

STATUS: SUCCESS
COMMIT: f8c2e6cf32a1b46ef532f5913995f07afea3eac9

review: Approve

diff --git a/src/maasserver/middleware.py b/src/maasserver/middleware.py
index fb83acd..0db67ee 100644
--- a/src/maasserver/middleware.py
+++ b/src/maasserver/middleware.py
@@ -121,24 +121,21 @@ class ExternalComponentsMiddleware:
 
         If any rack controllers are disconnected, add a persistent error.
         """
-        controllers = RackController.objects.all()
         connected_ids = {client.ident for client in getAllClients()}
-        disconnected_controllers = {
-            controller
-            for controller in controllers
-            if controller.system_id not in connected_ids
-        }
-        if len(disconnected_controllers) == 0:
+        n_disconnected_controllers = RackController.objects.exclude(
+            system_id__in=connected_ids
+        ).count()
+        if n_disconnected_controllers == 0:
             discard_persistent_error(COMPONENT.RACK_CONTROLLERS)
         else:
-            if len(disconnected_controllers) == 1:
+            if n_disconnected_controllers == 1:
                 message = (
                     "One rack controller is not yet connected to the region"
                 )
             else:
                 message = (
                     "%d rack controllers are not yet connected to the region"
-                    % len(disconnected_controllers)
+                    % n_disconnected_controllers
                 )
             message = (
                 '%s. Visit the <a href="/MAAS/l/controllers">'