fix TypeError cause failure in singal_connector teardown (#10961)

Author: four4fish

CHANGELOG.md

@@ -22,6 +22,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed an issue with `DDPSpawnPlugin` and related plugins leaving a temporary checkpoint behind ([#10934](https://github.com/PyTorchLightning/pytorch-lightning/pull/10934))
 
 
+- Fixed TypeError cause failure in `singal_connector` `teardown` method by adding None check ([#10961](https://github.com/PyTorchLightning/pytorch-lightning/pull/10961))
+
+
 ## [1.5.4] - 2021-11-30
 
 ### Fixed

pytorch_lightning/trainer/connectors/signal_connector.py

@@ -144,7 +144,7 @@ def _is_on_slurm(self) -> bool:
 
     @staticmethod
     def _has_already_handler(signum: Signals) -> bool:
-        return signal.getsignal(signum) is not signal.SIG_DFL
+        return signal.getsignal(signum) not in (None, signal.SIG_DFL)
 
     @staticmethod
     def _register_signal(signum: Signals, handlers: HandlersCompose) -> None:

tests/trainer/connectors/test_signal_connector.py

@@ -102,6 +102,7 @@ def signal_handler(self):
 @pytest.mark.parametrize(
     ["handler", "expected_return"],
     [
+        (None, False),
         (signal.Handlers.SIG_IGN, True),
         (signal.Handlers.SIG_DFL, False),
         (signal_handler, True),