Bug 1296842: make target-task related parameters a fixed set; r=jmaher

The `from_parameters` method was never used, and let do confusion over the role
of these parameters.  Now there are only two, and they are always required.

MozReview-Commit-ID: AbPqijXucu5

--HG--
extra : rebase_source : 85affd063a543c549afaaa36ce7ee31ed1f943d5

taskcluster/docs/parameters.rst

@@ -89,20 +89,9 @@ those in the target set, recursively.  In a decision task, this set can be
 specified programmatically using one of a variety of methods (e.g., parsing try
 syntax or reading a project-specific configuration file).
 
-The decision task writes its task set to the ``target_tasks.json`` artifact,
-and this can be copied into ``parameters.target_tasks`` and
-``parameters.target_tasks_method`` set to ``"from_parameters"`` for debugging
-with other ``mach taskgraph`` commands.
-
 ``target_tasks_method``
-   (optional) The method to use to determine the target task set.  This is the
-   suffix of one of the functions in ``tascluster/taskgraph/target_tasks.py``.
-   If omitted, all tasks are targeted.
-
-``target_tasks``
-   (optional) The target set method ``from_parameters`` reads the target set, as
-   a list of task labels, from this parameter.
+    The method to use to determine the target task set.  This is the suffix of
+    one of the functions in ``tascluster/taskgraph/target_tasks.py``.
 
 ``optimize_target_tasks``
-   (optional; default True) If true, then target tasks are eligible for
-   optimization.
+   If true, then target tasks are eligible for optimization.

taskcluster/taskgraph/target_tasks.py

@@ -34,14 +34,6 @@ def get_method(method):
     return _target_task_methods[method]
 
 
-@_target_task('from_parameters')
-def target_tasks_from_parameters(full_task_graph, parameters):
-    """Get the target task set from parameters['target_tasks'].  This is
-    useful for re-running a decision task with the same target set as in an
-    earlier run, by copying `target_tasks.json` into `parameters.yml`."""
-    return parameters['target_tasks']
-
-
 @_target_task('try_option_syntax')
 def target_tasks_try_option_syntax(full_task_graph, parameters):
     """Generate a list of target tasks based on try syntax in

taskcluster/taskgraph/test/test_target_tasks.py

@@ -26,11 +26,6 @@ class FakeTryOptionSyntax(object):
 
 class TestTargetTasks(unittest.TestCase):
 
-    def test_from_parameters(self):
-        method = target_tasks.get_method('from_parameters')
-        self.assertEqual(method(None, {'target_tasks': ['a', 'b']}),
-                         ['a', 'b'])
-
     def default_matches(self, run_on_projects, project):
         method = target_tasks.get_method('default')
         graph = TaskGraph(tasks={