fix(display): a failed on-demand request drops the session it ended

CHANGELOG.md

@@ -19,6 +19,13 @@ accepts both, but the store flags the old spelling as deprecated
 
 ## Unreleased
 
+### Fixes
+
+- A failed on-demand request no longer comes back after a restart as the
+  session it ended. A failed request ends any running session, but the
+  saved copy of that session (`display_on_demand_config`) was left behind,
+  so the next restart of the display resumed it.
+
 ## 3.8.2
 
 The display hands freed memory back to the OS (#774), and sports consolidation

src/display_controller.py

@@ -668,7 +668,6 @@ def load_single_plugin(plugin_id):
             except Exception:  # pylint: disable=broad-except
                 cached_session = None
             if self.on_demand_active or cached_session:
-                self.cache_manager.clear_cache('display_on_demand_config')
                 self._set_on_demand_error('restore-failed')
             # Its state machine no longer describes what runs; let the last
             # snapshot go stale (readers then say unknown) rather than keep
@@ -1817,8 +1816,14 @@ def _publish_on_demand_state(self) -> None:
             logger.error("Failed to publish on-demand state: %s", err, exc_info=True)
 
     def _set_on_demand_error(self, message: str) -> None:
-        """Set on-demand state to error and publish."""
+        """Set on-demand state to error and publish.
+
+        Ends any running session, so its saved copy goes too: a failed
+        request that replaced a session left display_on_demand_config
+        behind, and the next restart resumed the session that had ended.
+        """
         self._reset_on_demand_fields()
+        self.cache_manager.clear_cache('display_on_demand_config')
         self.on_demand_status = 'error'
         self.on_demand_last_error = message
         self.on_demand_last_event = None
@@ -2756,7 +2761,6 @@ def _populate_on_demand_modes_from_plugin(self) -> None:
             logger.error("On-demand session for plugin '%s' cannot resume after the "
                          "restart: the plugin has no loaded display modes (did it "
                          "fail to load?); ending it", plugin_id)
-            self.cache_manager.clear_cache('display_on_demand_config')
             self._set_on_demand_error('restore-failed')
             return
 

test/test_on_demand_disabled_plugin.py

@@ -217,6 +217,15 @@ def test_a_failed_request_that_ends_the_session_unloads_it(self, controller):
         assert controller.current_display_mode == 'clock'
         assert controller.force_change is True
 
+    def test_a_failed_request_that_ends_the_session_drops_its_saved_copy(self, controller):
+        """Otherwise the next restart resumes the session that just ended."""
+        _start(controller, plugin_id='clock')
+        controller.cache_manager.clear_cache.reset_mock()
+
+        _start(controller, plugin_id='uninstalled')
+
+        controller.cache_manager.clear_cache.assert_called_once_with('display_on_demand_config')
+
     def test_a_plugin_enabled_during_the_session_stays_loaded(self, controller):
         _start(controller)
         controller.test_config['preview-me'] = {'enabled': True}