**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
self.timezone = settings.TIMEZONE
        
        if 'end_date' in self.default_args and self.default_args['end_date']:
            if isinstance(self.default_args['end_date'], str):
                self.default_args['end_date'] = timezone.parse(
                    self.default_args['end_date'], timezone=self.timezone
                )
        self.start_date = timezone.convert_to_utc(start_date)
        self.end_date = timezone.convert_to_utc(end_date)
        
        if 'start_date' in self.default_args:
            self.default_args['start_date'] = timezone.convert_to_utc(self.default_args['start_date'])
        if 'end_date' in self.default_args:
            self.default_args['end_date'] = timezone.convert_to_utc(self.default_args['end_date'])
        self.schedule_interval = schedule_interval
        if isinstance(template_searchpath, str):
            template_searchpath = [template_searchpath]
        self.template_searchpath = template_searchpath
        self.template_undefined = template_undefined
        self.parent_dag: Optional[DAG] = None  
        self.last_loaded = timezone.utcnow()
        self.safe_dag_id = dag_id.replace('.', '__dot__')
        self.max_active_runs = max_active_runs
        self.dagrun_timeout = dagrun_timeout
        self.sla_miss_callback = sla_miss_callback
        if default_view in DEFAULT_VIEW_PRESETS:
            self._default_view: str = default_view
        else:
            raise AirflowException(
                f'Invalid values of dag.default_view: only support '