The functions password_authenticator::start and
standard_role_manager::start have a similar structure: they spawn a
fiber which invokes a callback that performs some migration until that
migration succeeds. Both handlers set a shared promise called
_superuser_created_promise (those are actually two promises, one for the
password authenticator and the other for the role manager).
The handlers are similar in both cases. They check if auth is in legacy
mode, and behave differently depending on that. If in legacy mode, the
promise is set (if it was not set before), and some legacy migration
actions follow. In auth-on-raft mode, the superuser is attempted to be
created, and if it succeeds then the promise is _unconditionally_ set.
While it makes sense at a glance to set the promise unconditionally,
there is a non-obvious corner case during upgrade to topology on raft.
During the upgrade, auth switches from the legacy mode to auth on raft
mode. Thus, if the callback didn't succeed in legacy mode and then tries
to run in auth-on-raft mode and succeds, it will unconditionally set a
promise that was already set - this is a bug and triggers an assertion
in seastar.
Fix the issue by surrounding the `shared_promise::set_value` call with
an `if` - like it is already done for the legacy case.
Fixes: scylladb/scylladb#24975Closesscylladb/scylladb#24976
a14b7f71fe