Airflow v1 REST List task instances api can not get `no_status` task instance

[li1234yun]

Apache Airflow version:
2.0

Environment:

• OS ubuntu 18.04
• Kernel 5.4.0-47-generic

What happened:
When I use list task instances REST api, I can not get the instances that status is no_status.

### Get Task Instances
POST  {{baseUrl}}/dags/~/dagRuns/~/taskInstances/list
Authorization: Basic admin:xxx
Content-Type: application/json

{
  "dag_ids": ["stop_dags"]
}

or 

{
    "dag_ids": ["stop_dags"],
    "state": ["null"]
}

What you expected to happen:
include all state task instances when I don't have specific states.

How to reproduce it:
use REST test tools like postman to visit the api.

Anything else we need to know:
I can not find the REST api that I get all the dag runs instances with specific state, maybe should extend the REST api.

Thanks!

[boring-cyborg]

Thanks for opening your first issue here! Be sure to follow the issue template!

[ephraimbuddy]

I have not been able to reproduce this. @mik-laj can you take a look?

[github-actions]

This issue has been automatically marked as stale because it has been open for 30 days with no response from the author. It will be closed in next 7 days if no further activity occurs from the issue author.

[mik-laj]

@ephraimbuddy
The field state is nullable

airflow/airflow/models/taskinstance.py

Line 333 in 26ad55b

state = Column(String(20))

In API, we use IN statement to filter elemenets.

airflow/airflow/api_connexion/endpoints/task_instance_endpoint.py

Line 144 in 26ad55b

base_query = _apply_array_filter(base_query, key=TI.state, values=state)

airflow/airflow/api_connexion/endpoints/task_instance_endpoint.py

Lines 76 to 79 in 26ad55b

	def _apply_array_filter(query, key, values):
	if values is not None:
	query = query.filter(key.in_(values))
	return query

Unfortunately, the IN expression never finds an element that is NULL. We need to handle this case and detect "NULL" / "NO_STATUS"/"NONE"(preferred) in the expression and build a more complex query that will contain the IS NULL expression.
When I looked at this field, I also noticed another problem with this field. We miss enum values validation.
https://github.com/apache/airflow/pull/19105/files#r744343850