Bug 1656465 - [taskgraph.optimize] Implement a 'Not' composite strategy, r=marco

It turns out that 'Not' is needed to negate "backstops". E.g, we normally
we want to use a pattern like so:

    All("skip-unless-backstop", "test")

Since 'skip-unless-backstop' returns False on backstop pushes, it disables
the test strategy there.

However, suppose we wanted to run a special optimization, *only* on backstop
pushes. I.e, the opposite of the above example. Then we need to use:

    All(Not("skip-unless-backstop"), "test-backstop")

Depends on D89500

Differential Revision: https://phabricator.services.mozilla.com/D89734

taskcluster/taskgraph/optimize/__init__.py

@@ -473,6 +473,19 @@ class Alias(CompositeStrategy):
         return next(results)
 
 
+class Not(CompositeStrategy):
+    """Given a strategy, returns the opposite."""
+    def __init__(self, strategy):
+        super(Not, self).__init__(strategy)
+
+    @property
+    def description(self):
+        return "not-" + self.substrategies[0].description
+
+    def reduce(self, results):
+        return not next(results)
+
+
 def split_bugbug_arg(arg, substrategies):
     """Split args for bugbug based strategies.
 

taskcluster/taskgraph/test/test_optimize.py

@@ -8,7 +8,7 @@ from functools import partial
 
 import pytest
 from taskgraph import graph, optimize
-from taskgraph.optimize import OptimizationStrategy, All, Any
+from taskgraph.optimize import OptimizationStrategy, All, Any, Not
 from taskgraph.taskgraph import TaskGraph
 from taskgraph.task import Task
 from mozunit import main
@@ -146,6 +146,18 @@ def make_triangle(deps=True, **opts):
             id="composite_strategies_all",
         ),
 
+        # Tasks with the 'not' composite strategy are removed when the substrategy says not to
+        pytest.param(
+            make_graph(
+                make_task('t1', {'not-never': None}),
+                make_task('t2', {'not-remove': None}),
+            ),
+            {"strategies": lambda: {"not-never": Not("never"), "not-remove": Not("remove")}},
+            # expectations
+            {"t1"},
+            id="composite_strategies_not",
+        ),
+
         # Removable tasks that are depended on by non-removable tasks are not removed
         pytest.param(
             make_triangle(
@@ -272,9 +284,11 @@ def test_remove_tasks(monkeypatch, graph, kwargs, exp_removed):
     # set up strategies
     strategies = default_strategies()
     monkeypatch.setattr(optimize, "registry", strategies)
-    strategies = kwargs.pop('strategies', None) or strategies
-    if callable(strategies):
-        strategies = strategies()
+    extra = kwargs.pop('strategies', None)
+    if extra:
+        if callable(extra):
+            extra = extra()
+        strategies.update(extra)
 
     kwargs.setdefault("params", {})
     kwargs.setdefault("do_not_optimize", set())