Enable graceful shutdown of rq workers (#5214)

* Enable graceful shutdown of rq workers

* Use `exec` in the `worker` command of the entrypoint to propagate
  the `TERM` signal
* Allow rq processes managed by supervisor to exit without restart on
  expected status codes
* Allow supervisorctl to contact the running supervisor
* Add a `shutdown_worker` command that will send `TERM` to all running
  worker processes and then sleep. This allows orchestration systems
  to initiate a graceful shutdown before sending `SIGTERM` to
  supervisord

* Use Heroku worker as the BaseWorker

This implements a graceful shutdown on SIGTERM, which simplifies
external shutdown procedures.

* Fix imports based upon review

* Remove supervisorctl config

bin/docker-entrypoint

@@ -19,7 +19,7 @@ worker() {
   export WORKERS_COUNT=${WORKERS_COUNT:-2}
   export QUEUES=${QUEUES:-}
 
-  supervisord -c worker.conf
+  exec supervisord -c worker.conf
 }
 
 dev_worker() {

redash/tasks/worker.py

@@ -3,7 +3,8 @@ import os
 import signal
 import time
 from redash import statsd_client
-from rq import Worker as BaseWorker, Queue as BaseQueue, get_current_job
+from rq import Queue as BaseQueue, get_current_job
+from rq.worker import HerokuWorker # HerokuWorker implements graceful shutdown on SIGTERM
 from rq.utils import utcnow
 from rq.timeouts import UnixSignalDeathPenalty, HorseMonitorTimeoutException
 from rq.job import Job as BaseJob, JobStatus
@@ -40,7 +41,7 @@ class RedashQueue(StatsdRecordingQueue, CancellableQueue):
     pass
 
 
-class StatsdRecordingWorker(BaseWorker):
+class StatsdRecordingWorker(HerokuWorker):
     """
     RQ Worker Mixin that overrides `execute_job` to increment/modify metrics via Statsd
     """
@@ -58,7 +59,7 @@ class StatsdRecordingWorker(BaseWorker):
                 statsd_client.incr("rq.jobs.failed.{}".format(queue.name))
 
 
-class HardLimitingWorker(BaseWorker):
+class HardLimitingWorker(HerokuWorker):
     """
     RQ's work horses enforce time limits by setting a timed alarm and stopping jobs
     when they reach their time limits. However, the work horse may be entirely blocked