Bug 1624902 - [tryselect] Validate '--strategy' argument to ./mach try auto, r=marco

This ensures we fail with invalid module paths early. Otherwise users wouldn't
find out until the decision task fails.

Differential Revision: https://phabricator.services.mozilla.com/D68464

--HG--
extra : moz-landing-system : lando

tools/tryselect/task_config.py

@@ -20,6 +20,8 @@ from textwrap import dedent
 
 import mozpack.path as mozpath
 from mozbuild.base import BuildEnvironmentNotFoundException, MozbuildObject
+from taskgraph.util.python_path import find_object
+
 from .tasks import resolve_tests_by_suite
 
 here = os.path.abspath(os.path.dirname(__file__))
@@ -296,6 +298,16 @@ class OptimizeStrategies(TryConfig):
             if ':' not in strategy:
                 strategy = "taskgraph.optimize:experimental.{}".format(strategy)
 
+            try:
+                obj = find_object(strategy)
+            except (ImportError, AttributeError):
+                print("error: invalid module path '{}'".format(strategy))
+                sys.exit(1)
+
+            if not isinstance(obj, dict):
+                print("error: object at '{}' must be a dict".format(strategy))
+                sys.exit(1)
+
             return {
                 'optimize-strategies': strategy,
             }

tools/tryselect/test/test_task_configs.py

@@ -42,8 +42,11 @@ TASK_CONFIG_TESTS = {
     ],
     'strategy': [
         ([], None),
-        (['--strategy', 'foo'], {'optimize-strategies': 'taskgraph.optimize:experimental.foo'}),
-        (['--strategy', 'foo:bar'], {'optimize-strategies': 'foo:bar'}),
+        (['--strategy', 'seta'], {'optimize-strategies': 'taskgraph.optimize:experimental.seta'}),
+        (['--strategy', 'taskgraph.optimize:experimental.seta'], {'optimize-strategies': 'taskgraph.optimize:experimental.seta'}),  # noqa
+        (['--strategy', 'taskgraph.optimize:experimental'], SystemExit),
+        (['--strategy', 'foo'], SystemExit),
+        (['--strategy', 'foo:bar'], SystemExit),
     ],
     'worker-overrides': [
         ([], None),