fix(v4l2): warn on a failed control listing and don't cache it (follow-up #3391) (#3407)

* warn when video controls could not be applied

`list_ctrls()` returning nothing leaves `vid_control_params` untouched, so the UI values are silently dropped. Only "failed to list controls of device" was logged, which does not mention that consequence.

* fix(v4l2): don't cache an empty control list

list_ctrls() cached its result unconditionally, including the empty dict a failed lookup produces. The cache has no expiry and is only cleared by list_devices(), so a briefly unreachable device kept an empty control list until that ran or motionEye restarted.

Author: Marijn0

motioneye/config.py

@@ -1131,6 +1131,12 @@ def motion_camera_ui_to_dict(ui, prev_config=None):
                 )
                 data['vid_control_params'] = ','.join(vid_control_params)
 
+            else:
+                logging.warning(
+                    f'camera {prev_config["@id"]} control listing failed, '
+                    'controls not updated'
+                )
+
     else:  # assuming netcam
         if match(
             r'^rtsp|^rtmp', data.get('netcam_url', prev_config.get('netcam_url', ''))

motioneye/controls/v4l2ctl.py

@@ -205,6 +205,8 @@ def list_ctrls(device):
 
         controls[control] = properties
 
-    _ctrls_cache[device] = controls
+    # don't cache an empty result, the cache has no expiry
+    if controls:
+        _ctrls_cache[device] = controls
 
     return controls