when `--reload` is sent to the dispatcher, it sends a special QUIT
message to each worker in the pool so that it will exit gracefully at
the next opportunity
when a worker process exits unexpectedly, the dispatcher attempts to
recover its queued messages and sends them to another worker in the
pool; in this scenario, we should _never_ re-enqueue these special
QUIT messages (because the process doesn't need to quit, it's already
gone)
To reproduce this race condition:
1. Launch an adhoc that does `sleep 60`
2. Run `awx-manage run_dispatcher --reload` to enqueue a `QUIT` message
into the worker's queue
3. Find the pid of the worker running the `sleep 60` and `SIGKILL` it.
4. Observe that dispatcher attempts to requeue the `QUIT` message and
logs a confusing error.
this commit implements the bulk of `awx-manage run_dispatcher`, a new
command that binds to RabbitMQ via kombu and balances messages across
a pool of workers that are similar to celeryd workers in spirit.
Specifically, this includes:
- a new decorator, `awx.main.dispatch.task`, which can be used to
decorate functions or classes so that they can be designated as
"Tasks"
- support for fanout/broadcast tasks (at this point in time, only
`conf.Setting` memcached flushes use this functionality)
- support for job reaping
- support for success/failure hooks for job runs (i.e.,
`handle_work_success` and `handle_work_error`)
- support for auto scaling worker pool that scale processes up and down
on demand
- minimal support for RPC, such as status checks and pool recycle/reload
