**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/models/dag.py
"""Returns dttm + interval unless dttm is first interval then it returns dttm"""
        following = self.following_schedule(dttm)
        
        if not following:
            return dttm
        if self.previous_schedule(following) != dttm:
            return following
        return dttm
    @provide_session
    def get_last_dagrun(self, session=None, include_externally_triggered=False):