Bug 1229178: modify --json output to contain a single object r=mshal

MozReview-Commit-ID: DNlxPfQh3o0

taskcluster/docs/taskgraph.rst

@@ -163,8 +163,12 @@ context.
 Taskgraph JSON Format
 ---------------------
 
-Each task in the graph is represented as a JSON object.  The output is suitable
-for processing with the `jq <https://stedolan.github.io/jq/>`_ utility.
+Task graphs -- both the graph artifacts produced by the decision task and those
+output by the ``--json`` option to the ``mach taskgraph`` commands -- are JSON
+objects, keyed by label, or for optimized task graphs, by taskId.  For
+convenience, the decision task also writes out ``label-to-taskid.json``
+containing a mapping from label to taskId.  Each task in the graph is
+represented as a JSON object.
 
 Each task has the following properties:
 
@@ -202,7 +206,11 @@ in the content:
   array is populated with the full list of dependency taskIds.  All task
   references are resolved in the optimized graph.
 
-The graph artifacts produced by the decision task are JSON objects, keyed by
-label (``full-task-graph.json`` and ``target-tasks``) or by taskId
-(``task-graph.json``).  For convenience, the decision task also writes out
-``label-to-taskid.json`` containing a mapping from label to taskId.
+The output of the ``mach taskgraph`` commands are suitable for processing with
+the `jq <https://stedolan.github.io/jq/>`_ utility.  For example, to extract all
+tasks' labels and their dependencies:
+
+.. code-block:: shell
+
+    jq 'to_entries | map({label: .value.label, dependencies: .value.dependencies})'
+

taskcluster/mach_commands.py

@@ -37,7 +37,7 @@ class ShowTaskGraphSubCommand(SubCommand):
                             help="include debug-level logging output"),
             CommandArgument('--json', '-J', action="store_const",
                             dest="format", const="json",
-                            help="Output each task in the task graph as a JSON object"),
+                            help="Output task graph as a JSON object"),
             CommandArgument('--labels', '-L', action="store_const",
                             dest="format", const="labels",
                             help="Output the label for each task in the task graph (default)"),
@@ -199,10 +199,8 @@ class MachCommands(MachCommandBase):
             print(label)
 
     def show_taskgraph_json(self, taskgraph):
-        # JSON output is a sequence of JSON objects, rather than a single object, so
-        # disassemble the dictionary
-        for task in taskgraph.to_json().itervalues():
-            print(json.dumps(task))
+        print(json.dumps(taskgraph.to_json(),
+              sort_keys=True, indent=2, separators=(',', ': ')))
 
 
 @CommandProvider