Use symlinks for best epochs (#1123)

* utils: add symlink_or_copyfile

* pruned_transducer_stateless7: use symlinks (when possible) to output best epochs

* Rename function

---------

Co-authored-by: Yifan Yang <64255737+yfyeung@users.noreply.github.com>

egs/librispeech/ASR/pruned_transducer_stateless7/train.py

@@ -50,7 +50,6 @@ import copy
 import logging
 import warnings
 from pathlib import Path
-from shutil import copyfile
 from typing import Any, Dict, Optional, Tuple, Union
 
 import k2
@@ -89,6 +88,7 @@ from icefall.utils import (
     filter_uneven_sized_batch,
     setup_logger,
     str2bool,
+    symlink_or_copy,
 )
 
 LRSchedulerType = Union[torch.optim.lr_scheduler._LRScheduler, optim.LRScheduler]
@@ -601,7 +601,8 @@ def save_checkpoint(
     """
     if rank != 0:
         return
-    filename = params.exp_dir / f"epoch-{params.cur_epoch}.pt"
+    epoch_basename = f"epoch-{params.cur_epoch}.pt"
+    filename = params.exp_dir / epoch_basename
     save_checkpoint_impl(
         filename=filename,
         model=model,
@@ -615,12 +616,14 @@ def save_checkpoint(
     )
 
     if params.best_train_epoch == params.cur_epoch:
-        best_train_filename = params.exp_dir / "best-train-loss.pt"
-        copyfile(src=filename, dst=best_train_filename)
+        symlink_or_copy(
+            exp_dir=params.exp_dir, src=epoch_basename, dst="best-train-loss.pt"
+        )
 
     if params.best_valid_epoch == params.cur_epoch:
-        best_valid_filename = params.exp_dir / "best-valid-loss.pt"
-        copyfile(src=filename, dst=best_valid_filename)
+        symlink_or_copy(
+            exp_dir=params.exp_dir, src=epoch_basename, dst="best-valid-loss.pt"
+        )
 
 
 def compute_loss(

icefall/utils.py

@@ -28,6 +28,7 @@ from contextlib import contextmanager
 from dataclasses import dataclass
 from datetime import datetime
 from pathlib import Path
+from shutil import copyfile
 from typing import Dict, Iterable, List, Optional, TextIO, Tuple, Union
 
 import k2
@@ -1881,3 +1882,20 @@ def is_cjk(character):
             ]
         ]
     )
+
+
+def symlink_or_copy(exp_dir: Path, src: str, dst: str):
+    """
+    In the experiment directory, create a symlink pointing to src named dst.
+    If symlink creation fails (Windows?), fall back to copyfile."""
+
+    dir_fd = os.open(exp_dir, os.O_RDONLY)
+    try:
+        os.remove(dst, dir_fd=dir_fd)
+    except FileNotFoundError:
+        pass
+    try:
+        os.symlink(src=src, dst=dst, dir_fd=dir_fd)
+    except OSError:
+        copyfile(src=exp_dir / src, dst=exp_dir / dst)
+    os.close(dir_fd)