Make saving 'last' checkpoint as symbolic link opt-in

src/lightning/pytorch/CHANGELOG.md

@@ -21,6 +21,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Added `TransformerEnginePrecision(fallback_compute_dtype=)` to control the dtype of operations that don't support fp8 ([#19082](https://github.com/Lightning-AI/lightning/pull/19082))
 
 
+- Added the option `ModelCheckpoint(save_last='link')` to create a symbolic link for the 'last.ckpt' file ([#19191](https://github.com/Lightning-AI/lightning/pull/19191))
+
+
 ### Changed
 
 - `seed_everything()` without passing in a seed no longer randomly selects a seed, and now defaults to `0` ([#18846](https://github.com/Lightning-AI/lightning/pull/18846))
@@ -47,6 +50,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - The columns in the `metrics.csv` file produced by `CSVLogger` are now sorted alphabetically ([#19159](https://github.com/Lightning-AI/lightning/pull/19159))
 
 
+- Reverted back to creating a checkpoint copy when `ModelCheckpoint(save_last=True)` instead of creating a symbolic link ([#19191](https://github.com/Lightning-AI/lightning/pull/19191))
+
+
 ### Deprecated
 
 - Deprecated all precision plugin classes under `lightning.pytorch.plugins` with the suffix `Plugin` in the name ([#18840](https://github.com/Lightning-AI/lightning/pull/18840))

src/lightning/pytorch/callbacks/model_checkpoint.py

@@ -26,7 +26,7 @@
 from copy import deepcopy
 from datetime import timedelta
 from pathlib import Path
-from typing import Any, Dict, Optional, Set
+from typing import Any, Dict, Literal, Optional, Set
 from weakref import proxy
 
 import torch
@@ -83,9 +83,9 @@ class ModelCheckpoint(Checkpoint):
             the number of finished epoch and optimizer steps respectively.
         monitor: quantity to monitor. By default it is ``None`` which saves a checkpoint only for the last epoch.
         verbose: verbosity mode. Default: ``False``.
-        save_last: When ``True``, saves a `last.ckpt` whenever a checkpoint file gets saved. On a local filesystem,
-            this will be a symbolic link, and otherwise a copy of the checkpoint file. This allows accessing the latest
-            checkpoint in a deterministic manner. Default: ``None``.
+        save_last: When ``True``, saves a `last.ckpt` copy whenever a checkpoint file gets saved. Can be set to
+            ``'link'`` on a local filesystem to create a symbolic link. This allows accessing the latest checkpoint
+            in a deterministic manner. Default: ``None``.
         save_top_k: if ``save_top_k == k``,
             the best k models according to the quantity monitored will be saved.
             if ``save_top_k == 0``, no models are saved.
@@ -216,7 +216,7 @@ def __init__(
         filename: Optional[str] = None,
         monitor: Optional[str] = None,
         verbose: bool = False,
-        save_last: Optional[bool] = None,
+        save_last: Optional[Literal[True, False, "link"]] = None,
         save_top_k: int = 1,
         save_weights_only: bool = False,
         mode: str = "min",
@@ -272,6 +272,10 @@ def setup(self, trainer: "pl.Trainer", pl_module: "pl.LightningModule", stage: s
         self._fs = get_filesystem(self.dirpath or "")
         if trainer.is_global_zero and stage == "fit":
             self.__warn_if_dir_not_empty(self.dirpath)
+        if self.save_last == "link" and not _is_local_file_protocol(self.dirpath):
+            raise ValueError(
+                f"`ModelCheckpoint(save_last='link')` is only supported for local file paths, got `dirpath={dirpath}`."
+            )
 
     @override
     def on_train_start(self, trainer: "pl.Trainer", pl_module: "pl.LightningModule") -> None:
@@ -684,7 +688,7 @@ def _save_last_checkpoint(self, trainer: "pl.Trainer", monitor_candidates: Dict[
 
         # set the last model path before saving because it will be part of the state.
         previous, self.last_model_path = self.last_model_path, filepath
-        if _is_local_file_protocol(filepath) and self._last_checkpoint_saved and self.save_top_k != 0:
+        if self.save_last == "link" and self._last_checkpoint_saved and self.save_top_k != 0:
             self._link_checkpoint(trainer, self._last_checkpoint_saved, filepath)
         else:
             self._save_checkpoint(trainer, filepath)

tests/tests_pytorch/checkpointing/test_model_checkpoint.py

@@ -485,13 +485,14 @@ def test_model_checkpoint_file_extension(tmpdir):
     assert set(expected) == set(os.listdir(tmpdir))
 
 
-def test_model_checkpoint_save_last(tmpdir, monkeypatch):
+@pytest.mark.parametrize("save_last", [True, "link"])
+def test_model_checkpoint_save_last(save_last, tmpdir, monkeypatch):
     """Tests that save_last produces only one last checkpoint."""
     seed_everything()
     model = LogInTwoMethods()
     epochs = 3
     monkeypatch.setattr(ModelCheckpoint, "CHECKPOINT_NAME_LAST", "last-{epoch}")
-    model_checkpoint = ModelCheckpoint(monitor="early_stop_on", dirpath=tmpdir, save_top_k=-1, save_last=True)
+    model_checkpoint = ModelCheckpoint(monitor="early_stop_on", dirpath=tmpdir, save_top_k=-1, save_last=save_last)
     trainer = Trainer(
         default_root_dir=tmpdir,
         callbacks=[model_checkpoint],
@@ -509,10 +510,19 @@ def test_model_checkpoint_save_last(tmpdir, monkeypatch):
     assert set(os.listdir(tmpdir)) == set(
         [f"epoch={i}-step={j}.ckpt" for i, j in zip(range(epochs), [10, 20, 30])] + [last_filename]
     )
-    assert os.path.islink(tmpdir / last_filename)
+    if save_last == "link":
+        assert os.path.islink(tmpdir / last_filename)
+    else:
+        assert os.path.isfile(tmpdir / last_filename)
     assert os.path.realpath(tmpdir / last_filename) == model_checkpoint._last_checkpoint_saved
 
 
+def test_model_checkpoint_save_last_as_link_not_local(tmp_path):
+    callback = ModelCheckpoint(dirpath="memory://not-a-filesystem-path", save_last="link")
+    with pytest.raises(ValueError, match="save_last='link'.* is only supported for local file paths"):
+        callback.setup(trainer=Trainer(), pl_module=BoringModel(), stage="fit")
+
+
 def test_model_checkpoint_link_checkpoint(tmp_path):
     """Test that linking a checkpoint works and overwrites an existing link if present."""
     trainer = Mock()
@@ -676,7 +686,7 @@ def test_model_checkpoint_save_last_none_monitor(tmpdir, caplog):
     expected = [f"epoch={i}-step={j}.ckpt" for i, j in zip(range(epochs), [10, 20])]
     expected.append("last.ckpt")
     assert set(os.listdir(tmpdir)) == set(expected)
-    assert os.path.islink(tmpdir / "last.ckpt")
+    assert os.path.isfile(tmpdir / "last.ckpt")
 
 
 @pytest.mark.parametrize("every_n_epochs", list(range(4)))
@@ -887,7 +897,7 @@ def test_model_checkpoint_save_last_checkpoint_contents(tmpdir):
     path_last = str(tmpdir / "last.ckpt")
     assert path_last == model_checkpoint.last_model_path
     assert os.path.isfile(path_last_epoch)
-    assert os.path.islink(path_last)
+    assert os.path.isfile(path_last)
 
     ckpt_last_epoch = torch.load(path_last_epoch)
     ckpt_last = torch.load(path_last)