[Download] Probe umask next to incomplete file instead of two levels above destination (#4305)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Wauplin

src/huggingface_hub/file_download.py

@@ -1907,8 +1907,12 @@ def _chmod_and_move(src: Path, dst: Path) -> None:
     - Fix issue: https://github.com/huggingface/huggingface_hub/issues/1141
     - Fix issue: https://github.com/huggingface/huggingface_hub/issues/1215
     """
-    # Get umask by creating a temporary file in the cached repo folder.
-    tmp_file = dst.parent.parent / f"tmp_{uuid.uuid4()}"
+    # Get umask by creating a temporary file in the folder containing the incomplete file.
+    # We know this folder is writable since the incomplete file has just been written there.
+    # Probing next to `dst` is not always possible: when downloading to a local dir, `dst` is the
+    # final file location and its parents might not be writable (e.g. read-only root filesystem).
+    # See https://github.com/huggingface/huggingface_hub/issues/4304.
+    tmp_file = src.parent / f"tmp_{uuid.uuid4()}"
     try:
         tmp_file.touch()
         cache_dir_mode = Path(tmp_file).stat().st_mode