If the user connects to Scylla via the maintenance socket, it may happen
that `auth_integration` has not been registered in the service level
controller yet. One example is maintenance mode when that will never
happen; another when the connection occurs before Scylla is fully
initialized.
To avoid unnecessary crashes, we add new branches if the passed user is
absent or if it corresponds to the anonymous role. Since the role
corresponding to a connection via the maintenance socket is the anonymous
role, that solves the problem.
In those cases, we completely circumvent any calls to `auth_integration`
and handle them separately. The modified methods are:
* `get_user_scheduling_group`,
* `with_user_service_level`,
* `describe_service_levels`.
For the first two, the new behavior is in line with the previous
implementation of those functions. The last behaves differently now,
but since it's a soft error, crashing the node is not necessary anyway.
We throw an exception instead, whose error message should give the user
a hint of what might be wrong.
The other uses of `auth_integration` within the service level controller
are not problematic:
* `find_effective_service_level`,
* `find_cached_effective_service_level`.
They take the name of a role as their argument. Since the anonymous role
doesn't have a name, it's not possible to call them with it.
Fixesscylladb/scylladb#26816
c0f7622d12