retention: Use a consistent ordering for processing realms.

This is probably a good idea for the production use case, since then
there's some consistency of behavior, and if we extend logging, one
knows exactly which realms were or were not executed before a logged
failure.

This fixes the nondeterministic test failures we've been seeing in CI:
if you use `-id` in that order_by, it happens consistently.

zerver/lib/retention.py

@@ -131,7 +131,7 @@ def clean_unused_messages() -> None:
 
 
 def archive_messages() -> None:
-    for realm in Realm.objects.filter(message_retention_days__isnull=False):
+    for realm in Realm.objects.filter(message_retention_days__isnull=False).order_by("id"):
         move_expired_messages_to_archive(realm)
         move_expired_user_messages_to_archive(realm)
         move_expired_attachments_to_archive(realm)

zerver/tests/test_retention.py

@@ -263,20 +263,10 @@ class TestRetentionLib(ZulipTestCase):
         # Get expired user messages by message ids
         expected_user_msgs_ids = list(UserMessage.objects.filter(
             message_id__in=expected_message_ids).order_by('id').values_list('id', flat=True))
-        user_messages = list(UserMessage.objects.select_related(
-            "message").filter(message_id__in=expected_message_ids).order_by('message_id'))
 
         msgs_qty = Message.objects.count()
         archive_messages()
 
-        # Temporary debugging code while we investigate CI failures
-        for user_message in user_messages:  # nocoverage
-            archived = ArchivedUserMessage.objects.filter(
-                message_id=user_message.message_id,
-                user_profile_id=user_message.user_profile_id).exists()
-            if not archived:
-                print("Missing:", user_message, user_message.message_id, flush=True)
-
         # Compare archived messages and user messages with expired messages
         self.assertEqual(ArchivedMessage.objects.count(), len(expected_message_ids))
         self.assertEqual(ArchivedUserMessage.objects.count(), len(expected_user_msgs_ids))