langflow-ai/langflow · error · UserComponentError

Failed to write {target.name}: {exc.strerror or exc}

Error message

Failed to write {target.name}: {exc.strerror or exc}

What it means

Raised when the atomic write of the component file (tmp file + os.replace into <components_dir>/<ClassName>.py) fails with an OSError. The tmp file is unlinked before raising so no stray .py.tmp artifacts accumulate. As with the mkdir failure, the OS strerror is included — this is a host/filesystem problem, not a code problem.

Source

Thrown at src/backend/base/langflow/agentic/services/user_components.py:297

    accumulates stray ``.tmp`` artifacts. ``os.replace`` is atomic on
    POSIX and on Windows (since Python 3.3) when source and dest are on
    the same filesystem — which they are here, both inside the user's
    sandbox.
    """
    # Use tempfile inside the SAME directory so os.replace stays
    # cross-device-safe (replace requires source+dest on the same FS).
    tmp_fd, tmp_name = tempfile.mkstemp(prefix=f"{target.stem}.", suffix=".py.tmp", dir=str(target.parent))
    tmp_path = Path(tmp_name)
    try:
        with os.fdopen(tmp_fd, "w", encoding="utf-8") as f:
            f.write(text)
        tmp_path.replace(target)
    except OSError as exc:
        # Clean up the tmp file (it might have data but is unreachable).
        with contextlib.suppress(OSError):
            tmp_path.unlink(missing_ok=True)
        msg = f"Failed to write {target.name}: {exc.strerror or exc}"
        raise UserComponentError(msg) from exc

View on GitHub (pinned to 976ec789d2)

Solutions

1. Read the strerror to identify the host-level cause (space, permissions, lock).
2. Retry the registration once — the write is atomic and a transient lock/space issue often clears.
3. On Windows, exclude the sandbox directory from antivirus scanning if 'file in use' errors recur.
4. Ensure only one process registers the same class_name at a time.

for attempt in range(2):
    try:
        return register_user_component(user_id=uid, class_name=name, code=src)
    except UserComponentError as e:
        if str(e).startswith("Failed to write") and attempt == 0:
            continue  # transient lock / space: tmp file already cleaned up, safe to retry
        raise

When it happens

Trigger: Disk fills up between mkdir and write; the target file is held open with a mandatory lock (Windows); the directory was removed concurrently; permission revoked mid-write.

Common situations: Transient disk-full on shared hosts; antivirus/indexer briefly locking freshly created files on Windows; concurrent registrations racing on the same class name.

Related errors

• Cannot create .components directory: {exc.strerror or exc}
• Multiple files are not allowed
• provider directory not found: {src}
• destination already exists (already consolidated?): {dst}
• In-tree provider directory not found: {in_tree}

AI-assisted analysis of langflow-ai/langflow@976ec789d2 (2026-08-14). Data as JSON: /api/errors/05cd2e405d12e1c2. Report an issue: GitHub.