Bug 1244189: add 'mach taskcluster-load-image'; r=garndt

The command helpfully downloads a docker image created by a taskcluster task,
which can be helpful when trying to replicate subtle bugs that only occur
in-tree.

This also fixes a bug in hashing Dockerfile directories, where the full
pathname was taken into account.  While this pathname is consistent from
decision task to decision task, it is not consistent with developers' home
directories.  This change omits the directory prefix, which will cause a
one-time shift in all directory hashes.

MozReview-Commit-ID: EamQzUGG5qY

testing/taskcluster/mach_commands.py

@@ -226,6 +226,40 @@ class DecisionTask(object):
         task = templates.load(params['task'], parameters)
         print(json.dumps(task, indent=4))
 
+@CommandProvider
+class LoadImage(object):
+    @Command('taskcluster-load-image', category="ci",
+        description="Load a pre-built Docker image")
+    @CommandArgument('--task-id',
+        help="Load the image at public/image.tar in this task, rather than "
+             "searching the index")
+    @CommandArgument('image_name', nargs='?',
+        help="Load the image of this name based on the current contents of the tree "
+             "(as built for mozilla-central or mozilla-inbound)")
+    def load_image(self, image_name, task_id):
+        from taskcluster_graph.image_builder import (
+            task_id_for_image,
+            docker_load_from_url
+        )
+
+        if not image_name and not task_id:
+            print("Specify either IMAGE-NAME or TASK-ID")
+            sys.exit(1)
+
+        if not task_id:
+            task_id = task_id_for_image({}, 'mozilla-inbound', image_name, create=False)
+            if not task_id:
+                print("No task found in the TaskCluster index for {}".format(image_name))
+                sys.exit(1)
+
+        print("Task ID: {}".format(task_id))
+
+        ARTIFACT_URL = 'https://queue.taskcluster.net/v1/task/{}/artifacts/{}'
+        image_name = docker_load_from_url(ARTIFACT_URL.format(task_id, 'public/image.tar'))
+
+        print("Loaded image is named {}".format(image_name))
+
+
 @CommandProvider
 class Graph(object):
     @Command('taskcluster-graph', category="ci",