Fix symlink permission error for "last" checkpoint on Windows (#18942)

Author: awaelchli

src/lightning/pytorch/CHANGELOG.md

@@ -46,6 +46,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed an issue parsing the version from folders that don't include a version number in `TensorBoardLogger` and `CSVLogger` ([#18897](https://github.com/Lightning-AI/lightning/issues/18897))
 
 
+- Fixed an issue causing permission errors on Windows when attempting to create a symlink for the "last" checkpoint ([#18942](https://github.com/Lightning-AI/lightning/issues/18942))
+
+
 ## [2.1.0] - 2023-10-11
 
 ### Added

src/lightning/pytorch/callbacks/model_checkpoint.py

@@ -389,7 +389,12 @@ def _link_checkpoint(trainer: "pl.Trainer", filepath: str, linkpath: str) -> Non
                 os.remove(linkpath)
             elif os.path.isdir(linkpath):
                 shutil.rmtree(linkpath)
-            os.symlink(filepath, linkpath)
+            try:
+                os.symlink(filepath, linkpath)
+            except OSError:
+                # on Windows, special permissions are required to create symbolic links as a regular user
+                # fall back to copying the file
+                shutil.copy(filepath, linkpath)
         trainer.strategy.barrier()
 
     def _should_skip_saving_checkpoint(self, trainer: "pl.Trainer") -> bool:

tests/tests_pytorch/checkpointing/test_model_checkpoint.py

@@ -556,6 +556,15 @@ def test_model_checkpoint_link_checkpoint(tmp_path):
     assert os.path.islink(folder_link)
     assert os.path.realpath(folder_link) == str(new_folder)
 
+    # simulate permission error on Windows (creation of symbolic links requires privileges)
+    file = tmp_path / "win_file"
+    file.touch()
+    link = tmp_path / "win_link"
+    with mock.patch("lightning.pytorch.callbacks.model_checkpoint.os.symlink", Mock(side_effect=OSError)):
+        ModelCheckpoint._link_checkpoint(trainer, filepath=str(file), linkpath=str(link))
+    assert not os.path.islink(link)
+    assert os.path.isfile(link)  # fall back to copying instead of linking
+
 
 def test_invalid_top_k(tmpdir):
     """Make sure that a MisconfigurationException is raised for a negative save_top_k argument."""