Handle database lock timeout errors gracefully

- Change statement_timeout reset from finally to else block to avoid
  executing SQL on an already-failed transaction
- Roll back the DB session in the 500 error handler when the cause
  is a lock timeout, so template rendering can still query the DB

Author: Nils Bars

webapp/ref/core/util.py

@@ -131,8 +131,10 @@ def lock_db(connection: sqlalchemy.engine.Connection, readonly=False):
                 "Another request may be holding the lock for too long."
             ) from e
         raise
-    finally:
-        # Reset statement timeout to default (0 = no limit)
+    else:
+        # Reset statement timeout to default (0 = no limit).
+        # Only do this on success — if the lock timed out, the transaction is
+        # in a failed state and any further SQL would raise InFailedSqlTransaction.
         connection.execute(sqlalchemy.text("SET LOCAL statement_timeout = 0;"))
 
     elapsed = time.monotonic() - start_time

webapp/ref/error.py

@@ -13,6 +13,7 @@
 )
 
 from ref.core import InconsistentStateError, failsafe
+from ref.core.util import DatabaseLockTimeoutError
 
 error_handlers = []
 
@@ -102,5 +103,20 @@ def internal_error(_, e):
     if isinstance(e, (AssertionError, InconsistentStateError)):
         failsafe()
 
+    # Roll back the session if it's in a failed state (e.g., after a database
+    # lock timeout). Without this, rendering the error template would fail
+    # because base.html queries the DB for settings like COURSE_NAME.
+    orig_exception = e
+    while orig_exception is not None:
+        if isinstance(orig_exception, DatabaseLockTimeoutError):
+            try:
+                from ref import db
+
+                db.session.rollback()
+            except Exception:
+                pass
+            break
+        orig_exception = getattr(orig_exception, "__cause__", None)
+
     text = f"Internal Error: If the problem persists, please contact the server administrator and provide the following error code {code}"
     return render_error_template(text, InternalServerError.code)