🐛 [#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/serializers/form_step.py

@@ -1,3 +1,6 @@
+from functools import partial
+
+from django.db import transaction
 from django.db.models import Q
 
 from rest_framework import serializers
@@ -10,6 +13,7 @@
 )
 
 from ...models import FormDefinition, FormStep
+from ...tasks import on_formstep_save_event
 from ...validators import validate_no_duplicate_keys_across_steps
 from ..validators import FormStepIsApplicableIfFirstValidator
 from .button_text import ButtonTextSerializer
@@ -170,3 +174,19 @@ def validate_form_definition(self, current_form_definition):
         )
 
         return current_form_definition
+
+    @transaction.atomic()
+    def save(self, **kwargs):
+        """
+        Bandaid fix for #4824
+
+        Ensure that the FormVariables are in line with the state of the FormDefinitions
+        after saving
+        """
+        instance = super().save(**kwargs)
+
+        transaction.on_commit(
+            partial(on_formstep_save_event, instance.form.id, countdown=60)
+        )
+
+        return instance

src/openforms/forms/tasks.py

@@ -34,6 +34,28 @@ def on_variables_bulk_update_event(form_id: int) -> None:
     actions_chain.delay()
 
 
+def on_formstep_save_event(form_id: int, countdown: int) -> None:
+    """
+    Celery chain of tasks to execute after saving a FormStep.
+    """
+    create_form_variables_for_components_task = create_form_variables_for_components.si(
+        form_id
+    )
+    repopulate_reusable_definition_variables_task = (
+        repopulate_reusable_definition_variables_to_form_variables.si(form_id)
+    )
+    recouple_submission_variables_task = (
+        recouple_submission_variables_to_form_variables.si(form_id)
+    )
+
+    actions_chain = chain(
+        create_form_variables_for_components_task,
+        repopulate_reusable_definition_variables_task,
+        recouple_submission_variables_task,
+    )
+    actions_chain.apply_async(countdown=countdown)
+
+
 @app.task(ignore_result=True)
 def recouple_submission_variables_to_form_variables(form_id: int) -> None:
     """Recouple SubmissionValueVariable to FormVariable
@@ -96,6 +118,29 @@ 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."""