Apache Airflow version

apache-airflow==3.1.8
apache-airflow-providers-fab==3.4.0
apache-airflow-providers-standard==1.12.0

What happened

POST /auth/token always returns 500 Internal Server Error.

The FAB auth manager calls get_application_builder() on every token request, which initializes Flask-AppBuilder and calls register_views(). During form initialization, HookMetaService.MockOptional instances are passed as WTForms validators. Since MockOptional has no __call__ method, WTForms raises TypeError.

Root Cause

HookMetaService._get_hooks_with_mocked_fab() patches wtforms.validators.Optional with MockOptional class:

with mock.patch("wtforms.validators.Optional", HookMetaService.MockOptional):
    pm = ProvidersManager()  # providers call Optional() → creates MockOptional() instances
    return pm.hooks, pm.connection_form_widgets, ...

After the with block exits, MockOptional() instances remain stored inside provider form widget definitions. When FAB later initializes real WTForms forms using those definitions, check_validators requires validators to be callable — but MockOptional instances are not.

Traceback

File "airflow/providers/fab/auth_manager/api_fastapi/routes/login.py", line 42, in create_token
    with get_application_builder():
File "airflow/providers/fab/auth_manager/cli_commands/utils.py", line 69, in get_application_builder
File "airflow/providers/fab/www/extensions/init_appbuilder.py", line 156, in __init__
File "airflow/providers/fab/auth_manager/fab_auth_manager.py", line 624, in register_views
File "airflow/providers/fab/auth_manager/security_manager/override.py", line 472, in register_views
File "flask_appbuilder/baseviews.py", line 904, in _init_forms
File "wtforms/fields/core.py", line 171, in check_validators
TypeError: <airflow.api_fastapi.core_api.services.ui.connections.HookMetaService.MockOptional object at 0x...>
          is not a valid validator because it is not callable

Expected behavior

POST /auth/token should return a valid JWT token.

Suggested fix

Add __call__ to MockOptional so it is usable as a WTForms validator:

class MockOptional:
    """Mock for wtforms.validators.Optional."""

    def __init__(self, *args, **kwargs):
        pass

    def __call__(self, form, field):
        pass  # no-op

How to reproduce

1. Deploy Airflow 3.1.8 with apache-airflow-providers-fab==3.4.0
2. POST http://<airflow-api-server>/auth/token with valid credentials
3. Observe 500 Internal Server Error