Create FernetEnabledRule to ease upgrade to Airflow 2.0

[turbaszek]

This issue is part of #8765

Rule

Create FernetEnabledRule which corresponds to

Fernet is enabled by default

entry in UPDATING.md. This rule should allow users to check if their current configuration needs any adjusting
before migration to Airflow 2.0.

How to guide

To implement a new rule, create a class that inherits from airflow.upgrade.rules.base_rule.BaseRule.
It will be auto-registered and used by airflow upgrade-check command. The custom rule class has to have title,
description properties and should implement check method which returns a list of error messages in case of
incompatibility.

For example:

airflow/airflow/upgrade/rules/conn_type_is_not_nullable.py

Lines 25 to 42 in ea36166

	class ConnTypeIsNotNullableRule(BaseRule):
	title = "Connection.conn_type is not nullable"
	description = """\
	The `conn_type` column in the `connection` table must contain content. Previously, this rule was \
	enforced by application logic, but was not enforced by the database schema.
	If you made any modifications to the table directly, make sure you don't have null in the conn_type column.\
	"""
	@provide_session
	def check(self, session=None):
	invalid_connections = session.query(Connection).filter(Connection.conn_type.is_(None))
	return (
	'Connection<id={}", conn_id={}> have empty conn_type field.'.format(conn.id, conn.conn_id)
	for conn in invalid_connections
	)

Remember to open the PR against v1-10-test branch.

[Sangarshanan]

Can pick this up : )

[turbaszek]

@Sangarshanan I assigned you 👌

[Sangarshanan]

Heya : ) just wanted to confirm what needs to be done

Check if cryptography is not importable or if fernetkey is empty

Return a message saying that fernet will be enabled by default when upgrading and
an empty key has to be explicitly set in the fernet_key option of
section [core] in the airflow.cfg file to retain the current behaviour

Or am I going about this the wrong way?

[turbaszek]

Return a message saying that fernet will be enabled by default when upgrading and
an empty key has to be explicitly set in the fernet_key option of
section [core] in the airflow.cfg file to retain the current behaviour

as per https://github.com/apache/airflow/blob/master/UPDATING.md#fernet-is-enabled-by-default

So we should check if fernet_key key in config is empty. If yes we should inform users than when migrating they will have to change it explicitly to empty. If someone has something in this field - they are all good.

Also we should inform about this requirement:

At the same time, this means that the apache-airflow[crypto] extra-packages are always installed. However, this requires that your operating system has libffi-dev installed.

[turbaszek]

Closed via #11116