**Description**

For a DAG that looks like this:

```
A >> B
```

If A fails, B goes into "upstream_failed" state. If a user then marks A "success", B will remain in "upstream_failed" state. It will not automatically start running. This scenario often happens if the failure of A is dealt with outside of Airflow and the user does not want Airflow to run A again. But he usually expect Airflow to run B after A is marked "success".

**Use case / motivation**

After A is marked "success", its downstream tasks B should be cleared automatically and get ready to be scheduled. To avoid changing this behaviour completely, there can be a toggle button next to "Success" that lets the user decide if he wants Airflow to automatically clear downstream tasks that are in "upstream_failed" state.
airflow/www/views.py
"""Triggers DAG Run."""
        dag_id = request.values.get('dag_id')
        origin = get_safe_url(request.values.get('origin'))
        request_conf = request.values.get('conf')
        if request.method == 'GET':
            
            default_conf = ''
            dag = current_app.dag_bag.get_dag(dag_id)
            doc_md = wwwutils.wrapped_markdown(getattr(dag, 'doc_md', None))
            if request_conf:
                default_conf = request_conf
            else:
                try:
                    default_conf = json.dumps(dag.params, indent=4)
                except TypeError:
                    flash("Could not pre-populate conf field due to non-JSON-serializable data-types")
            return self.render_template(
                'airflow/trigger.html', dag_id=dag_id, origin=origin, conf=default_conf, doc_md=doc_md
            )