Added Celery for background tasks.

The implementation is still rather simple, using hard-coded configuration
values. This will change in subsequent commits.

The worker can be started with "manage.py operations worker". Celery
Worker CLI options can be passed after a double dash, like this:

    ./manage.py operations worker -- -C -E

pillar/celery/celery_cfg.py

@@ -0,0 +1,19 @@
+from celery import Celery
+
+task_modules = [
+    'pillar.celery.tasks',
+]
+
+celery_cfg = Celery('proj',
+                    backend='redis://redis/1',
+                    broker='amqp://guest:guest@rabbit//',
+                    include=task_modules,
+                    task_track_started=True)
+
+# Optional configuration, see the application user guide.
+celery_cfg.conf.update(
+    result_expires=3600,
+)
+
+if __name__ == '__main__':
+    celery_cfg.start()

pillar/celery/tasks.py

@@ -0,0 +1,21 @@
+import logging
+import typing
+
+from .celery_cfg import celery_cfg
+
+log = logging.getLogger(__name__)
+
+
+@celery_cfg.task(track_started=True)
+def long_task(numbers: typing.List[int]):
+    _log = log.getChild('long_task')
+    _log.info('Computing sum of %i items', len(numbers))
+
+    import time
+    time.sleep(6)
+    thesum = sum(numbers)
+
+    _log.info('Computed sum of %i items', len(numbers))
+
+    return thesum
+

pillar/cli/operations.py

@@ -159,3 +159,25 @@ def index_users_update_settings():
             'unordered(roles)'
         ]
     })
+
+
+@manager_operations.option('args', nargs='*')
+def worker(args):
+    """Runs a Celery worker."""
+
+    import sys
+
+    argv0 = f'{sys.argv[0]} operations worker'
+    argvother = [
+        '-E',
+        '-l', 'INFO',
+        '--concurrency', '1',
+        '--pool', 'solo',  # No preforking, as PyMongo can't handle connect-before-fork.
+                           # We might get rid of this and go for the default Celery worker
+                           # preforking concurrency model, *if* we can somehow reset the
+                           # PyMongo client and reconnect after forking.
+    ] + list(args)
+
+    from pillar.celery.celery_cfg import celery_cfg
+
+    celery_cfg.worker_main([argv0] + argvother)

requirements.txt

@@ -7,6 +7,7 @@ bcrypt==3.1.3
 blinker==1.4
 bugsnag==2.3.1
 bleach==1.4.3
+celery[redis]==4.0.2
 commonmark==0.7.2
 Eve==0.7.3
 Flask==0.12.2
@@ -29,6 +30,8 @@ wheel==0.29.0
 zencoder==0.6.5
 
 # Secondary requirements
+amqp==2.1.4
+billiard==3.5.0.2
 Flask-PyMongo==0.4.1
 CommonMark==0.7.2
 cerberus==0.9.2
@@ -38,15 +41,18 @@ html5lib==0.9999999
 googleapis-common-protos==1.1.0
 itsdangerous==0.24
 jinja2==2.9.6
+kombu==4.0.2
 oauth2client==2.0.2
 oauthlib==2.0.1
 protobuf==3.0.0b2.post2
 protorpc==0.11.1
 pyasn1-modules==0.0.8
 pymongo==3.4.0
+pytz==2017.2
 requests-oauthlib==0.7.0
 rsa==3.4.2
 simplejson==3.10.0
 six==1.10.0
+vine==1.1.3
 WTForms==2.1
 werkzeug==0.11.15