**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
'airflow/chart.html',
            dag=dag,
            chart=Markup(chart.htmlcontent),
            height=str(chart_height + 100) + "px",
            root=root,
            form=form,
            tab_title='Landing times',
        )
    @expose('/paused', methods=['POST'])
    @auth.has_access(
        [
            (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_DAG),
        ]
    )
    @action_logging
    def paused(self):
        """Toggle paused."""
        dag_id = request.args.get('dag_id')
        is_paused = request.args.get('is_paused') == 'false'
        models.DagModel.get_dagmodel(dag_id).set_is_paused(is_paused=is_paused)
        return "OK"
    @expose('/refresh', methods=['POST'])
    @auth.has_access(
        [
            (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_DAG),
        ]
    )
    @action_logging
    @provide_session
    def refresh(self, session=None):