Bug 1470886 - only produce tasks for recognized tasks_for r=tomprince

This also updates actions to "see through" the conditional.  Soon we won't be
using kind=task, so this hack will be less important.

MozReview-Commit-ID: Aa6g9ZqoPMa

--HG--
extra : rebase_source : 7434f2047c48ff0d1fa6de9e3419fb4e0bf0bb72

.taskcluster.yml

@@ -4,6 +4,8 @@
 # - action tasks - taskcluster/taskgraph/actions/registry.py
 version: 1
 tasks:
+  $if: 'tasks_for in ["hg-push", "action", "cron"]'
+  then:
   - $let:
       # sometimes the push user is just `ffxbld` or the like, but we want an email-like field..
       ownerEmail: {$if: '"@" in push.owner', then: '${push.owner}', else: '${push.owner}@noreply.mozilla.org'}
@@ -197,3 +199,5 @@ tasks:
             then:
               cron: {$json: {$eval: 'cron'}}
           - tasks_for: '${tasks_for}'
+  # no tasks for unrecognized tasks_for
+  else: []

taskcluster/taskgraph/actions/registry.py

@@ -193,7 +193,13 @@ def register_callback_action(name, title, symbol, description, order=10000,
                 if taskcluster_yml['version'] != 1:
                     raise Exception(
                         'actions.json must be updated to work with .taskcluster.yml')
-                if not isinstance(taskcluster_yml['tasks'], list):
+
+                # allow a top-level conditional (bug 1470886)
+                tasks = taskcluster_yml['tasks']
+                if isinstance(tasks, dict) and '$if' in tasks:
+                    tasks = tasks['then']
+
+                if not isinstance(tasks, list):
                     raise Exception(
                         '.taskcluster.yml "tasks" must be a list for action tasks')
 
@@ -206,7 +212,7 @@ def register_callback_action(name, title, symbol, description, order=10000,
                             'push': push,
                             'action': action,
                         },
-                        'in': taskcluster_yml['tasks'][0],
+                        'in': tasks[0],
                     },
                 })