Fix CLI race condition saving the config

CHANGELOG.md

@@ -354,6 +354,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed double evaluation bug with fault-tolerance enabled where the second call was completely skipped ([#11119](https://github.com/PyTorchLightning/pytorch-lightning/pull/11119))
 
 
+- Fixed `LightningCLI` race condition while saving the config ([#11199](https://github.com/PyTorchLightning/pytorch-lightning/pull/11199))
+
+
 - Fixed an issue with the `TPUSpawnPlugin` handling the `XLA_USE_BF16` environment variable incorrectly ([#10990](https://github.com/PyTorchLightning/pytorch-lightning/pull/10990))
 
 

pytorch_lightning/utilities/cli.py

@@ -404,21 +404,30 @@ def __init__(
     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
+        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)
-        if not self.overwrite and os.path.isfile(config_path):
-            raise RuntimeError(
-                f"{self.__class__.__name__} expected {config_path} to NOT exist. Aborting to avoid overwriting"
-                " results of a previous run. You can delete the previous config file,"
-                " set `LightningCLI(save_config_callback=None)` to disable config saving,"
-                " or set `LightningCLI(save_config_overwrite=True)` to overwrite the config file."
-            )
+        fs = get_filesystem(log_dir)
+
+        if not self.overwrite:
+            # check if the file exists on rank 0
+            file_exists = fs.isfile(config_path) if trainer.is_global_zero else False
+            # broadcast whether to fail to all ranks
+            file_exists = trainer.strategy.broadcast(file_exists)
+            if file_exists:
+                raise RuntimeError(
+                    f"{self.__class__.__name__} expected {config_path} to NOT exist. Aborting to avoid overwriting"
+                    " results of a previous run. You can delete the previous config file,"
+                    " set `LightningCLI(save_config_callback=None)` to disable config saving,"
+                    " or set `LightningCLI(save_config_overwrite=True)` to overwrite the config file."
+                )
+
+        # save the file on rank 0
         if trainer.is_global_zero:
             # save only on rank zero to avoid race conditions on DDP.
             # 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
-            get_filesystem(log_dir).makedirs(log_dir, exist_ok=True)
+            fs.makedirs(log_dir, exist_ok=True)
             self.parser.save(
                 self.config, config_path, skip_none=False, overwrite=self.overwrite, multifile=self.multifile
             )