Prevent last checkpoint being deleted after resumed training with changed `dirpath` (#12225)

Co-authored-by: Carlos Mocholi <carlossmocholi@gmail.com>
Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>
Co-authored-by: Rohit Gupta <rohitgr1998@gmail.com>

CHANGELOG.md

@@ -819,6 +819,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Fixed
 
+- Fixed an issue where `ModelCheckpoint` could delete last checkpoint from the old directory when `dirpath` has changed during resumed training ([#12225](https://github.com/PyTorchLightning/pytorch-lightning/pull/12225))
+
+
 - Fixed an issue where `ModelCheckpoint` could delete older checkpoints when `dirpath` has changed during resumed training ([#12045](https://github.com/PyTorchLightning/pytorch-lightning/pull/12045))
 
 

pytorch_lightning/callbacks/model_checkpoint.py

@@ -147,7 +147,7 @@ class ModelCheckpoint(Callback):
         then you should create multiple ``ModelCheckpoint`` callbacks.
 
         If the checkpoint's ``dirpath`` changed from what it was before while resuming the training,
-        only ``last_model_path`` and ``best_model_path`` will be reloaded and a warning will be issued.
+        only ``best_model_path`` will be reloaded and a warning will be issued.
 
     Raises:
         MisconfigurationException:
@@ -337,13 +337,14 @@ class ModelCheckpoint(Callback):
             self.kth_best_model_path = state_dict.get("kth_best_model_path", self.kth_best_model_path)
             self.kth_value = state_dict.get("kth_value", self.kth_value)
             self.best_k_models = state_dict.get("best_k_models", self.best_k_models)
+            self.last_model_path = state_dict.get("last_model_path", self.last_model_path)
         else:
             warnings.warn(
                 f"The dirpath has changed from {dirpath_from_ckpt!r} to {self.dirpath!r},"
-                " therefore `best_model_score`, `kth_best_model_path`, `kth_value` and `best_k_models`"
-                " won't be reloaded. Only `last_model_path` and `best_model_path` will be reloaded."
+                " therefore `best_model_score`, `kth_best_model_path`, `kth_value`, `last_model_path` and"
+                " `best_k_models` won't be reloaded. Only `best_model_path` will be reloaded."
             )
-        self.last_model_path = state_dict.get("last_model_path", self.last_model_path)
+
         self.best_model_path = state_dict["best_model_path"]
 
     def save_checkpoint(self, trainer: "pl.Trainer") -> None:  # pragma: no-cover

tests/checkpointing/test_model_checkpoint.py

@@ -1192,8 +1192,8 @@ def test_model_checkpoint_saveload_ckpt(tmpdir):
             "kth_best_model_path": False,
             "kth_value": False,
             "best_k_models": False,
+            "last_model_path": False,
             "best_model_path": True,
-            "last_model_path": True,
         }
         for key, should_match in expected_keys.items():
             if should_match:
@@ -1245,6 +1245,40 @@ def test_model_checkpoint_saveload_ckpt(tmpdir):
     make_assertions(cb_restore, written_ckpt)
 
 
+def test_resume_training_preserves_old_ckpt_last(tmpdir):
+    """Ensures that the last saved checkpoint is not deleted from the previous folder when training is resumed from
+    the old checkpoint."""
+    model = BoringModel()
+    trainer_kwargs = {
+        "default_root_dir": tmpdir,
+        "max_epochs": 1,
+        "limit_train_batches": 3,
+        "limit_val_batches": 0,
+        "enable_model_summary": False,
+        "logger": False,
+    }
+    mc_kwargs = {
+        "filename": "{step}",
+        "monitor": "step",
+        "mode": "max",
+        "save_last": True,
+        "save_top_k": 2,
+        "every_n_train_steps": 1,
+    }
+    trainer = Trainer(**trainer_kwargs, callbacks=ModelCheckpoint(**mc_kwargs))
+    trainer.fit(model)
+    # Make sure that the last checkpoint file exists in the dirpath passed (`tmpdir`)
+    assert set(os.listdir(tmpdir / "checkpoints")) == {"last.ckpt", "step=2.ckpt", "step=3.ckpt"}
+
+    # Training it for 2 epochs for extra surety, that nothing gets deleted after multiple epochs
+    trainer_kwargs["max_epochs"] += 1
+    mc_kwargs["dirpath"] = f"{tmpdir}/new"
+    trainer = Trainer(**trainer_kwargs, callbacks=ModelCheckpoint(**mc_kwargs))
+    trainer.fit(model, ckpt_path=f"{tmpdir}/checkpoints/step=2.ckpt")
+    # Ensure that the file is not deleted from the old folder
+    assert os.path.isfile(f"{tmpdir}/checkpoints/last.ckpt")
+
+
 def test_save_last_saves_correct_last_model_path(tmpdir):
     mc = ModelCheckpoint(dirpath=tmpdir, save_last=True)
     mc.CHECKPOINT_NAME_LAST = "{foo}-last"