Eager-load and expunge Exercise.config in the build thread

The build thread detaches the Exercise plus its forward relationships so
attribute access during the long-running Docker build does not trigger a
lazy load that would reopen a transaction and hold the build advisory
lock. Extends that pattern to the config relationship added with
ExerciseConfig, otherwise touching exercise.config in __run_build raises
DetachedInstanceError.

Author: Nils Bars

webapp/ref/core/image.py

@@ -448,6 +448,7 @@ def __run_build_by_id(app, exercise_id: int):
                 exercise = Exercise.query.options(
                     joinedload(Exercise.entry_service),
                     joinedload(Exercise.services),
+                    joinedload(Exercise.config),
                 ).get(exercise_id)
                 if exercise is None:
                     _log_build(
@@ -470,13 +471,15 @@ def __run_build_by_id(app, exercise_id: int):
                 # only populates forward relationships, not reverse ones.
                 entry_service = exercise.entry_service
                 services = list(exercise.services)
+                config = exercise.config
                 app.db.session.expunge(exercise)
                 if entry_service:
                     app.db.session.expunge(entry_service)
                     entry_service.exercise = exercise
                 for svc in services:
                     app.db.session.expunge(svc)
                     svc.exercise = exercise
+                app.db.session.expunge(config)
                 app.db.session.commit()
                 ExerciseImageManager.__run_build(app, exercise)
             _log_build(f"[BUILD] Build thread finished for exercise_id={exercise_id}")