config: Throw if justknobs value is not a boolean (#139488)

This helps avoid an issue, where someone uses a mutable type that
justknobs does not support within the code. And then it gets overriden
to a different type
Pull Request resolved: https://github.com/pytorch/pytorch/pull/139488
Approved by: https://github.com/ezyang

test/test_utils_config_module.py

@@ -11,7 +11,7 @@ from typing import Optional
 
 from torch.testing._internal import fake_config_module as config
 from torch.testing._internal.common_utils import run_tests, TestCase
-from torch.utils._config_module import _UNSET_SENTINEL
+from torch.utils._config_module import _UNSET_SENTINEL, Config
 
 
 class TestConfigModule(TestCase):
@@ -329,6 +329,13 @@ torch.testing._internal.fake_config_module._save_config_ignore = ['e_ignored']""
             self.assertFalse(config.e_bool)
         self.assertTrue(config.e_bool)
 
+    def test_bad_jk_type(self):
+        with self.assertRaises(
+            AssertionError,
+            msg="AssertionError: justknobs only support booleans, thisisnotvalid is not a boolean",
+        ):
+            Config(default="bad", justknob="fake_knob")
+
 
 if __name__ == "__main__":
     run_tests()

torch/utils/_config_module.py

@@ -71,6 +71,10 @@ class Config:
         self.env_name_default = env_name_default
         self.env_name_force = env_name_force
         self.value_type = value_type
+        if self.justknob is not None:
+            assert isinstance(
+                self.default, bool
+            ), f"justknobs only support booleans, {self.default} is not a boolean"
 
 
 # Types saved/loaded in configs