Bug 1534283 - filter out tasks when DONT-BUILD is in the message r=aki,tomprince

(without the dash, because I want *this* push to build)

This filters out all tasks, but that means that several things will still run:

 * docker images and tasks they depend on (debian packages)
 * always_run tasks (various python-y things)

Differential Revision: https://phabricator.services.mozilla.com/D23020

--HG--
extra : moz-landing-system : lando

taskcluster/taskgraph/decision.py

@@ -222,6 +222,8 @@ def get_decision_parameters(config, options):
         if n in options and options[n] is not None:
             parameters[n] = options[n]
 
+    commit_message = get_hg_commit_message(os.path.join(GECKO, product_dir))
+
     # Define default filter list, as most configurations shouldn't need
     # custom filters.
     parameters['filters'] = [
@@ -233,8 +235,7 @@ def get_decision_parameters(config, options):
     parameters['build_number'] = 1
     parameters['version'] = get_version(product_dir)
     parameters['app_version'] = get_app_version(product_dir)
-    parameters['message'] = try_syntax_from_message(
-            get_hg_commit_message(os.path.join(GECKO, product_dir)))
+    parameters['message'] = try_syntax_from_message(commit_message)
     parameters['hg_branch'] = get_hg_revision_branch(GECKO, revision=parameters['head_rev'])
     parameters['next_version'] = None
     parameters['phabricator_diff'] = None
@@ -276,6 +277,12 @@ def get_decision_parameters(config, options):
     if options.get('target_tasks_method'):
         parameters['target_tasks_method'] = options['target_tasks_method']
 
+    # ..but can be overridden by the commit message: if it contains the special
+    # string "DONTBUILD" and this is an on-push decision task, then use the
+    # special 'nothing' target task method.
+    if 'DONTBUILD' in commit_message and options['tasks_for'] == 'hg-push':
+        parameters['target_tasks_method'] = 'nothing'
+
     # If the target method is nightly, we should build partials. This means
     # knowing what has been released previously.
     # An empty release_history is fine, it just means no partials will be built

taskcluster/taskgraph/target_tasks.py

@@ -682,3 +682,9 @@ def target_tasks_codereview(full_task_graph, parameters, graph_config):
         return False
 
     return [l for l, t in full_task_graph.tasks.iteritems() if filter(t)]
+
+
+@_target_task('nothing')
+def target_tasks_nothing(full_task_graph, parameters, graph_config):
+    """Select nothing, for DONTBUILD pushes"""
+    return []