[CLI] Fix `SaveConfigCallback` with DDP spawn (#12011)

pytorch_lightning/utilities/cli.py

@@ -415,8 +415,6 @@ class SaveConfigCallback(Callback):
         self.multifile = multifile
 
     def setup(self, trainer: Trainer, pl_module: LightningModule, stage: Optional[str] = None) -> None:
-        # save the config in `setup` because (1) we want it to save regardless of the trainer function run
-        # and we want to save before processes are spawned
         log_dir = trainer.log_dir  # this broadcasts the directory
         assert log_dir is not None
         config_path = os.path.join(log_dir, self.config_filename)
@@ -437,7 +435,7 @@ class SaveConfigCallback(Callback):
 
         # save the file on rank 0
         if trainer.is_global_zero:
-            # save only on rank zero to avoid race conditions on DDP.
+            # save only on rank zero to avoid race conditions.
             # the `log_dir` needs to be created as we rely on the logger to do it usually
             # but it hasn't logged anything at this point
             fs.makedirs(log_dir, exist_ok=True)
@@ -445,10 +443,6 @@ class SaveConfigCallback(Callback):
                 self.config, config_path, skip_none=False, overwrite=self.overwrite, multifile=self.multifile
             )
 
-    def __reduce__(self) -> Tuple[Type["SaveConfigCallback"], Tuple, Dict]:
-        # `ArgumentParser` is un-pickleable. Drop it
-        return self.__class__, (None, self.config, self.config_filename), {}
-
 
 class LightningCLI:
     """Implementation of a configurable command line tool for pytorch-lightning."""

tests/utilities/test_cli.py

@@ -50,7 +50,7 @@ from pytorch_lightning.utilities.cli import (
     SaveConfigCallback,
 )
 from pytorch_lightning.utilities.exceptions import MisconfigurationException
-from pytorch_lightning.utilities.imports import _TORCHVISION_AVAILABLE
+from pytorch_lightning.utilities.imports import _TORCH_GREATER_EQUAL_1_8, _TORCHVISION_AVAILABLE
 from tests.helpers import BoringDataModule, BoringModel
 from tests.helpers.runif import RunIf
 from tests.helpers.utils import no_warning_call
@@ -576,21 +576,17 @@ class EarlyExitTestModel(BoringModel):
         raise MisconfigurationException("Error on fit start")
 
 
+@RunIf(skip_windows=True)
 @pytest.mark.parametrize("logger", (False, True))
-@pytest.mark.parametrize(
-    "trainer_kwargs",
-    (
-        # dict(strategy="ddp_spawn")
-        # dict(strategy="ddp")
-        # the previous accl_conn will choose singleDeviceStrategy for both strategy=ddp/ddp_spawn
-        # TODO revisit this test as it never worked with DDP or DDPSpawn
-        dict(strategy="single_device"),
-        pytest.param({"tpu_cores": 1}, marks=RunIf(tpu=True)),
-    ),
-)
-def test_cli_distributed_save_config_callback(tmpdir, logger, trainer_kwargs):
+@pytest.mark.parametrize("strategy", ("ddp_spawn", "ddp"))
+def test_cli_distributed_save_config_callback(tmpdir, logger, strategy):
+    if _TORCH_GREATER_EQUAL_1_8:
+        from torch.multiprocessing import ProcessRaisedException
+    else:
+        ProcessRaisedException = Exception
+
     with mock.patch("sys.argv", ["any.py", "fit"]), pytest.raises(
-        MisconfigurationException, match=r"Error on fit start"
+        (MisconfigurationException, ProcessRaisedException), match=r"Error on fit start"
     ):
         LightningCLI(
             EarlyExitTestModel,
@@ -599,7 +595,9 @@ def test_cli_distributed_save_config_callback(tmpdir, logger, trainer_kwargs):
                 "logger": logger,
                 "max_steps": 1,
                 "max_epochs": 1,
-                **trainer_kwargs,
+                "strategy": strategy,
+                "accelerator": "auto",
+                "devices": 1,
             },
         )
     if logger: