🐛 [#4824] Schedule task to create FormVariables after saving FormStep

this ensures that the Form cannot be left in a broken state with regards to FormVariables, because every component will always have a FormVariable

this is a bandaid fix, because ideally we would have a single endpoint and the FormStep wouldn't be updated if there are errors with variables

src/openforms/forms/api/viewsets.py

@@ -21,6 +21,7 @@
 
 from openforms.api.pagination import PageNumberPagination
 from openforms.api.serializers import ExceptionSerializer, ValidationErrorSerializer
+from openforms.forms.tasks import create_form_variables_for_components
 from openforms.translations.utils import set_language_cookie
 from openforms.utils.patches.rest_framework_nested.viewsets import NestedViewSetMixin
 from openforms.utils.urls import is_admin_request
@@ -102,6 +103,34 @@ def get_serializer_context(self):
             )
         return context
 
+    @transaction.atomic()
+    def create(self, request, *args, **kwargs):
+        response = super().create(request, *args, **kwargs)
+
+        form = get_object_or_404(Form, uuid=self.kwargs["form_uuid_or_slug"])
+        transaction.on_commit(
+            lambda: create_form_variables_for_components.apply_async(
+                args=(form.id,),
+                countdown=60,
+            )
+        )
+
+        return response
+
+    @transaction.atomic()
+    def update(self, request, *args, **kwargs):
+        response = super().update(request, *args, **kwargs)
+
+        form = get_object_or_404(Form, uuid=self.kwargs["form_uuid_or_slug"])
+        transaction.on_commit(
+            lambda: create_form_variables_for_components.apply_async(
+                args=(form.id,),
+                countdown=60,
+            )
+        )
+
+        return response
+
 
 _FORMSTEP_ADMIN_FIELDS_MARKDOWN = get_admin_fields_markdown(FormStepSerializer)
 FormStepViewSet.__doc__ = inspect.getdoc(FormStepViewSet).format(

src/openforms/forms/tasks.py

@@ -96,6 +96,29 @@ def repopulate_reusable_definition_variables_to_form_variables(form_id: int) ->
         FormVariable.objects.create_for_form(form)
 
 
+@app.task(ignore_result=True)
+@transaction.atomic()
+def create_form_variables_for_components(form_id: int) -> None:
+    """Create FormVariables for each component in the Form
+
+    This task is scheduled after creating/updating a FormStep, to ensure that the saved
+    Form has the appropriate FormVariables, even if errors occur in the variables update
+    endpoint. This is done to avoid leaving the Form in a broken state.
+
+    See also: https://github.com/open-formulieren/open-forms/issues/4824#issuecomment-2514913073
+    """
+    from .models import Form, FormVariable  # due to circular import
+
+    form = Form.objects.get(id=form_id)
+
+    # delete the existing form variables, we will re-create them
+    FormVariable.objects.filter(
+        form=form_id, source=FormVariableSources.component
+    ).delete()
+
+    FormVariable.objects.create_for_form(form)
+
+
 @app.task()
 def activate_forms():
     """Activate all the forms that should be activated by the specific date and time."""