charm becomes blocked with status "Failed to connect to MySQL" after adding relation "shared-db-cell"

[Marcin Wilk]

In a fully deployed focal-ussuri model after adding "shared-db-cell" to the "nova-cloud-controller" charm, the "mysql-router" reaches "blocked" status with message "Failed to connect to MySQL".

"juju debug-log" shows following error message:
unit-nova-cell-controller-cell2-mysql-router-0: 10:54:09 ERROR unit.nova-cell-controller-cell2-mysql-router/0.juju-log Failed to connect to database due to '(1045, "Access denied for user 'mysqlrouteruser'@'10.5.2.140' (using password: YES)")'

The same charm, when related with other principle charm, ie "nova-cell-controller" via "shared-db" relation, successfully provides connectivity to the mysql innodb cluster.

Steps to recreate:
1. deploy focal-ussuri base bundle [1] and wait for the deployment to settle
2. add following apps and relations to the model:
    juju deploy -n 3 ch:mysql-innodb-cluster mysql-innodb-cluster-cell2
    juju deploy ch:mysql-router nova-cell-controller-cell2-mysql-router
    juju add-relation mysql-innodb-cluster-cell2:certificates vault:certificates
    juju add-relation nova-cell-controller-cell2-mysql-router:db-router mysql-innodb-cluster-cell2:db-router
    juju add-relation nova-cloud-controller:shared-db-cell nova-cell-controller-cell2-mysql-router:shared-db
3. wait for the deployment to settle and check status for the "nova-cell-controller-cell2-mysql-router" app. I will be in the "blocked" status with message: "Failed to connect to MySQL"

[1] - https://github.com/openstack-charmers/openstack-bundles/blob/master/stable/openstack-base/bundle.yaml