Fix logic in backup retention filter (#133654)

homeassistant/components/backup/config.py

@@ -467,7 +467,7 @@ async def delete_backups_exceeding_configured_count(manager: BackupManager) -> N
             sorted(
                 backups.items(),
                 key=lambda backup_item: backup_item[1].date,
-            )[: len(backups) - manager.config.data.retention.copies]
+            )[: max(len(backups) - manager.config.data.retention.copies, 0)]
         )
 
     await _delete_filtered_backups(manager, _backups_filter)

tests/components/backup/test_websocket.py

@@ -1331,6 +1331,35 @@ async def test_config_schedule_logic(
             0,
             [],
         ),
+        (
+            {
+                "type": "backup/config/update",
+                "create_backup": {"agent_ids": ["test.test-agent"]},
+                "retention": {"copies": 3, "days": None},
+                "schedule": "daily",
+            },
+            {
+                "backup-1": MagicMock(
+                    date="2024-11-10T04:45:00+01:00",
+                    with_automatic_settings=True,
+                    spec=ManagerBackup,
+                ),
+                "backup-2": MagicMock(
+                    date="2024-11-11T04:45:00+01:00",
+                    with_automatic_settings=True,
+                    spec=ManagerBackup,
+                ),
+            },
+            {},
+            {},
+            "2024-11-11T04:45:00+01:00",
+            "2024-11-12T04:45:00+01:00",
+            "2024-11-12T04:45:00+01:00",
+            1,
+            1,
+            0,
+            [],
+        ),
         (
             {
                 "type": "backup/config/update",