Only Task objects pass through get_task_id() and never AsyncResult, thus get_task_id()
is now useless and is equivalent to task.freeze().id
I think an old version of attach_task() was recursively called with either Task
or AsyncResult thus the handling of both cases, but I cannot find the relevant
changeset (maybe this code was never used ? ...)
I'm pretty confident on this because all known use-cases are covered by tests.