Refactor canceling to work through messaging and signals, not database

If canceled attempted before, still allow attempting another cancel
in this case, attempt to send the sigterm signal again.
Keep clicking, you might help!

Replace other cancel_callbacks with sigterm watcher
  adapt special inventory mechanism for this too

Get rid of the cancel_watcher method with exception in main thread

Handle academic case of sigterm race condition

Process cancelation as control signal

Fully connect cancel method and run_dispatcher to control

Never transition workflows directly to canceled, add logs

awx/main/management/commands/run_dispatcher.py

@@ -1,6 +1,7 @@
 # Copyright (c) 2015 Ansible, Inc.
 # All Rights Reserved.
 import logging
+import yaml
 
 from django.conf import settings
 from django.core.cache import cache as django_cache
@@ -30,7 +31,16 @@ class Command(BaseCommand):
             '--reload',
             dest='reload',
             action='store_true',
-            help=('cause the dispatcher to recycle all of its worker processes;' 'running jobs will run to completion first'),
+            help=('cause the dispatcher to recycle all of its worker processes; running jobs will run to completion first'),
+        )
+        parser.add_argument(
+            '--cancel',
+            dest='cancel',
+            help=(
+                'Cancel a particular task id. Takes either a single id string, or a JSON list of multiple ids. '
+                'Can take in output from the --running argument as input to cancel all tasks. '
+                'Only running tasks can be canceled, queued tasks must be started before they can be canceled.'
+            ),
         )
 
     def handle(self, *arg, **options):
@@ -42,6 +52,16 @@ class Command(BaseCommand):
             return
         if options.get('reload'):
             return Control('dispatcher').control({'control': 'reload'})
+        if options.get('cancel'):
+            cancel_str = options.get('cancel')
+            try:
+                cancel_data = yaml.safe_load(cancel_str)
+            except Exception:
+                cancel_data = [cancel_str]
+            if not isinstance(cancel_data, list):
+                cancel_data = [cancel_str]
+            print(Control('dispatcher').cancel(cancel_data))
+            return
 
         # It's important to close these because we're _about_ to fork, and we
         # don't want the forked processes to inherit the open sockets