Alt: Add a session backend to store session data in the database

[jedcunningham]

This is an alternate approach to #21167, the primary difference being we don't need to vendor in Flask-Session.

The downside, however, is the weirdness of having our own session model that isn't actually used other than to remove the table during db reset. Is there a better way to accomplish that?

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[blag]

LGTM

[ashb]

https://github.com/apache/airflow/blob/ee20a03908a2f3ad37dbfcec71a19f37f1446088/airflow/utils/db.py#L1057

We can drop the table like this without needing a model

[ephraimbuddy]

I think we should also add an ORM model for this table so that it's not only through migration file that the table can be created.
I'm working on a change that would have new DBs created through the ORM rather than going through the migration files. #21462

[ashb]

We can do it like this I think

from flask.session import SqlAlchemySessionInterface
from flask_appbuilder import SQLA
...

    db = SQLA()
    db.session = settings.Session
    SessionModel = SqlAlchemySessionInterface(db=db).sql_session_model

[ashb]

(If we should or not is the next question)

[jedcunningham]

This is what it ends up looking like, when considering #21462's changes:

--- a/airflow/utils/db.py
+++ b/airflow/utils/db.py
@@ -606,12 +606,21 @@ def initdb(session: Session = NEW_SESSION):
     from airflow.models.base import Base
     from airflow.www.app import create_app
 
+    from airflow.www.session import AirflowDatabaseSessionInterface
+    from flask_sqlalchemy import SQLAlchemy
+
     Base.metadata.create_all(settings.engine)
     # Stamp migration head with alembic
     config = _get_alembic_config()
     command.stamp(config, "head")
     # sets up the db; Sync permissions etc
-    create_app(config={'UPDATE_FAB_PERMS': True})
+    app = create_app(config={'UPDATE_FAB_PERMS': True})
+
+    # Create `session` table
+    db = SQLAlchemy(app)
+    AirflowDatabaseSessionInterface(app=app, db=db, table='session', key_prefix='')
+    db.create_all()
+
     if conf.getboolean('core', 'LOAD_DEFAULT_CONNECTIONS'):
         create_default_connections(session=session)