Fix temp filename max size (#912)

Fixes #891

shelfmark/download/fs.py

@@ -101,6 +101,8 @@ def run_blocking_io[T](func: Callable[..., T], *args: Any, **kwargs: Any) -> T:
 
 _VERIFY_IO_WAIT_SECONDS = 3.0
 _PUBLISH_VERIFY_RETRY_SECONDS = 0.25
+_TEMPFILE_PREFIX = ".shelfmark."
+_TEMPFILE_SUFFIX = ".tmp"
 
 
 def _verify_transfer_size(
@@ -339,10 +341,16 @@ def _hardlink_not_supported(error: OSError) -> bool:
 
 
 def _create_temp_path(dest_path: Path) -> Path:
+    """Create a destination-adjacent temp file without inheriting the full basename.
+
+    Reusing the entire destination filename in the temp prefix can push otherwise
+    valid long names over the filesystem component limit once `tempfile` adds its
+    random suffix.
+    """
     fd, temp_path = run_blocking_io(
         tempfile.mkstemp,
-        prefix=f".{dest_path.name}.",
-        suffix=".tmp",
+        prefix=_TEMPFILE_PREFIX,
+        suffix=_TEMPFILE_SUFFIX,
         dir=str(dest_path.parent),
     )
     run_blocking_io(os.close, fd)

tests/core/test_download_processing.py

@@ -147,6 +147,20 @@ class TestAtomicCopy:
 
         assert result == tmp_path / "dest_3.txt"
 
+    def test_long_destination_name(self, tmp_path):
+        """Copies long-but-valid destination names without overflowing temp filenames."""
+        from shelfmark.download.fs import atomic_copy as _atomic_copy
+
+        source = tmp_path / "source.epub"
+        source.write_bytes(b"epub content")
+        dest = tmp_path / f"{'A' * 240}.epub"
+
+        result = _atomic_copy(source, dest)
+
+        assert result == dest
+        assert result.exists()
+        assert result.read_bytes() == b"epub content"
+
     def test_preserves_extension(self, tmp_path):
         """Keeps extension when adding counter suffix."""
         from shelfmark.download.fs import atomic_copy as _atomic_copy

tests/core/test_hardlink.py

@@ -350,6 +350,27 @@ class TestAtomicMove:
             assert not source.exists()
             assert result.read_text() == "content"
 
+    def test_cross_filesystem_fallback_handles_long_destination_name(self, tmp_path, monkeypatch):
+        """Cross-filesystem fallback handles long destination names safely."""
+        import errno
+
+        from shelfmark.download.fs import atomic_move as _atomic_move
+
+        source = tmp_path / "source.epub"
+        source.write_text("content")
+        dest = tmp_path / f"{'A' * 240}.epub"
+
+        def _raise_exdev(*_args, **_kwargs):
+            raise OSError(errno.EXDEV, "Cross-device link")
+
+        monkeypatch.setattr(os, "rename", _raise_exdev)
+
+        result = _atomic_move(source, dest)
+
+        assert result == dest
+        assert not source.exists()
+        assert result.read_text() == "content"
+
     def test_cross_filesystem_permission_fallback(self, tmp_path, monkeypatch):
         """Falls back to copy when cross-filesystem move hits permission error."""
         import errno