Bug 1542242 - [taskgraph] Hack required mozbase module onto PYTHONPATH rather than run 'mach python' for Windows builds, r=marco

This is a hack to get around Windows ccov build hangs caused by bug 1195299.
Bug 1543149 will track the investigation of the hang and removal of this hack.

Differential Revision: https://phabricator.services.mozilla.com/D26750

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

taskcluster/taskgraph/transforms/job/mozharness.py

@@ -300,12 +300,23 @@ def mozharness_on_generic_worker(config, job, taskdesc):
             "Task generation for mozharness build jobs currently only supported on Windows"
         )
 
+    # TODO We should run the mozharness script with `mach python` so these
+    # modules are automatically available, but doing so somehow caused hangs in
+    # Windows ccov builds (see bug 1543149).
     gecko = env['GECKO_PATH'].replace('.', '%cd%')
+    mozbase_dir = "{}/testing/mozbase".format(gecko)
+    env['PYTHONPATH'] = ';'.join([
+        "{}/manifestparser".format(mozbase_dir),
+        "{}/mozinfo".format(mozbase_dir),
+        "{}/mozfile".format(mozbase_dir),
+        "{}/mozprocess".format(mozbase_dir),
+        "{}/third_party/python/six".format(gecko),
+    ])
+
     mh_command = [
             'c:/mozilla-build/python/python.exe',
-            '{}/mach'.format(gecko), 'python',
+            '{}/testing/{}'.format(gecko, run['script']),
     ]
-    mh_command.append('/'.join([gecko, 'testing', run['script']]))
 
     if 'config-paths' in run:
         for path in run['config-paths']: