Bug 1383880: allow only one optimization per task; r=ahal

It is not at *all* clear how multiple optimizations for a single task should
interact. No simple logical operation is right in all cases, and in fact in
most imaginable cases the desired behavior turns out to be independent of all
but one of the optimizations. For example, given both `seta` and
`skip-unless-files-changed` optimizations, if SETA says to skip a test, it is
low value and should be skipped regardless of what files have changed. But if
SETA says to run a test, then it has likely been skipped in previous pushes, so
it should be run regardless of what has changed in this push.

This also adds a bit more output about optimization, that may be useful for
anyone wondering why a particular job didn't run.

MozReview-Commit-ID: 3OsvRnWjai4

taskcluster/taskgraph/task.py

@@ -13,7 +13,7 @@ class Task(object):
     - label; the label for this task
     - attributes: a dictionary of attributes for this task (used for filtering)
     - task: the task definition (JSON-able dictionary)
-    - optimizations: optimizations to apply to the task (see taskgraph.optimize)
+    - optimization: optimization to apply to the task (see taskgraph.optimize)
     - dependencies: tasks this one depends on, in the form {name: label}, for example
       {'build': 'build-linux64/opt', 'docker-image': 'build-docker-image-desktop-test'}
 
@@ -26,7 +26,7 @@ class Task(object):
     display, comparison, serialization, etc. It has no functionality of its own.
     """
     def __init__(self, kind, label, attributes, task,
-                 optimizations=None, dependencies=None):
+                 optimization=None, dependencies=None):
         self.kind = kind
         self.label = label
         self.attributes = attributes
@@ -37,7 +37,7 @@ class Task(object):
 
         self.attributes['kind'] = kind
 
-        self.optimizations = optimizations or []
+        self.optimization = optimization
         self.dependencies = dependencies or {}
 
     def __eq__(self, other):
@@ -46,12 +46,12 @@ class Task(object):
             self.attributes == other.attributes and \
             self.task == other.task and \
             self.task_id == other.task_id and \
-            self.optimizations == other.optimizations and \
+            self.optimization == other.optimization and \
             self.dependencies == other.dependencies
 
     def __repr__(self):
         return ('Task({kind!r}, {label!r}, {attributes!r}, {task!r}, '
-                'optimizations={optimizations!r}, '
+                'optimization={optimization!r}, '
                 'dependencies={dependencies!r})'.format(**self.__dict__))
 
     def to_json(self):
@@ -60,7 +60,7 @@ class Task(object):
             'label': self.label,
             'attributes': self.attributes,
             'dependencies': self.dependencies,
-            'optimizations': self.optimizations,
+            'optimization': self.optimization,
             'task': self.task,
         }
         if self.task_id:
@@ -79,7 +79,7 @@ class Task(object):
             label=task_dict['label'],
             attributes=task_dict['attributes'],
             task=task_dict['task'],
-            optimizations=task_dict['optimizations'],
+            optimization=task_dict['optimization'],
             dependencies=task_dict.get('dependencies'))
         if 'task_id' in task_dict:
             rv.task_id = task_dict['task_id']