Bug 1624763 - Enable custom actions to be defined in a repo. r=tomprince

This is a port of https://hg.mozilla.org/ci/taskgraph/rev/a61ab06c25222 to allow
for custom actions in a project repo. Previously, registration of actions worked
but did not run when triggered.

Differential Revision: https://phabricator.services.mozilla.com/D68891

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

taskcluster/taskgraph/actions/registry.py

@@ -305,6 +305,7 @@ def trigger_action_callback(task_group_id, task_id, input, callback, parameters,
     the action callback in testing mode, without actually creating tasks.
     """
     graph_config = load_graph_config(root)
+    graph_config.register()
     callbacks = _get_callbacks(graph_config)
     cb = callbacks.get(callback, None)
     if not cb:

taskcluster/taskgraph/config.py

@@ -138,10 +138,16 @@ class GraphConfig(object):
         Add the project's taskgraph directory to the python path, and register
         any extensions present.
         """
+        modify_path = os.path.dirname(self.root_dir)
         if GraphConfig._PATH_MODIFIED:
+            if GraphConfig._PATH_MODIFIED == modify_path:
+                # Already modified path with the same root_dir.
+                # We currently need to do this to enable actions to call
+                # taskgraph_decision, e.g. relpro.
+                return
             raise Exception("Can't register multiple directories on python path.")
-        GraphConfig._PATH_MODIFIED = True
-        sys.path.insert(0, os.path.dirname(self.root_dir))
+        GraphConfig._PATH_MODIFIED = modify_path
+        sys.path.insert(0, modify_path)
         register_path = self['taskgraph'].get('register')
         if register_path:
             find_object(register_path)(self)