[Maas-devel] Cluster download tasks completely block other jobs :(

[Raphaël Badin]

[...]
> I've filed a critical bug about this.
> https://bugs.launchpad.net/maas/+bug/1328351

Even if other tasks might take a long time to execute (powering up a 
machine might for instance), the import task is a bit of a special case 
in the sense that it can take a really long time to execute *and* it 
doesn't make any sense to have two instances of this task running at the 
same time.  Plus it can be triggered both by a user and by a cron-like 
mechanism.

We could change the import task so that it grabs a file-based lock when 
it starts.  Any import task started before the release of the lock would 
just exit silently.  Celery has provision to help us deal gracefully 
with failure modes (task crashing without releasing the lock, etc.).