Give upload temp files a fixed-length name

root/app/backend/file_manager.py

@@ -83,8 +83,10 @@ async def save_uploaded_file(upload_file: UploadFile, relative_path: str = "", o
     if target_file.exists() and not overwrite:
         raise HTTPException(status_code=409, detail=f"{filename} already exists")
 
-    # Write to a temp file first so a failed upload never leaves a truncated file behind
-    tmp_file = target_dir / f".{filename}.{uuid.uuid4().hex}.part"
+    # Write to a temp file first so a failed upload never leaves a truncated file behind.
+    # Its name is fixed-length: built from the upload's name, a long but valid name went
+    # over the filesystem's 255-byte limit.
+    tmp_file = target_dir / f".upload-{uuid.uuid4().hex}.part"
     try:
         async with aiofiles.open(tmp_file, "wb") as f:
             while chunk := await upload_file.read(1024 * 1024): # 1MB chunks

tests/test_file_manager.py

@@ -200,3 +200,37 @@ def test_parent_traversal_is_still_rejected_on_delete():
     with pytest.raises(HTTPException) as exc:
         fm.delete_path("../etc")
     assert exc.value.status_code == 403
+
+
+# ----------------- Uploads -----------------
+
+def upload(name, data=b"hello", path="", overwrite=False):
+    import asyncio
+    import io
+    from fastapi import UploadFile
+    return asyncio.run(fm.save_uploaded_file(UploadFile(io.BytesIO(data), filename=name), path, overwrite))
+
+
+@pytest.mark.parametrize("name", [
+    pytest.param("a" * 250 + ".txt", id="254-chars",  # a valid name on Linux and NTFS
+                 marks=pytest.mark.skipif(os.name == "nt", reason="exceeds Windows' 260-character path limit here")),
+    pytest.param("é" * 110 + ".txt", id="224-bytes"),  # over the old limit in UTF-8 bytes
+])
+def test_a_long_but_valid_name_can_be_uploaded(shared, name):
+    result = upload(name)
+    assert result["filename"] == name
+    assert (shared / name).read_bytes() == b"hello"
+
+
+def test_no_temp_file_is_left_behind(shared):
+    upload("report.txt")
+    assert sorted(p.name for p in shared.iterdir()) == ["report.txt"]
+
+
+def test_an_existing_file_is_only_replaced_with_overwrite(shared):
+    upload("report.txt", b"one")
+    with pytest.raises(HTTPException) as exc:
+        upload("report.txt", b"two")
+    assert exc.value.status_code == 409
+    upload("report.txt", b"two", overwrite=True)
+    assert (shared / "report.txt").read_bytes() == b"two"