fs

def available_bytes(path: Path) -> int:
    """The bytes free to an unprivileged writer on ``path``'s filesystem."""
    stats = os.statvfs(path)
    return stats.f_bavail * stats.f_frsize

def promote(source: Path, destination: Path) -> bool:
    """Move the tree ``source`` to ``destination`` atomically, unless one is already there.

    Returns:
        Whether ``source`` became ``destination``; ``False`` means an equal tree
        was already present and ``source`` was discarded.
    """
    if destination.exists():
        remove_tree(source)
        return False
    try:
        os.rename(source, destination)
    except OSError as error:
        if error.errno in (errno.EEXIST, errno.ENOTEMPTY):
            remove_tree(source)
            return False
        raise
    return True

def remove_tree(path: Path) -> None:
    """Delete ``path`` and everything under it; an absent path is fine."""
    shutil.rmtree(path, ignore_errors=True)

def tree_size(path: Path) -> int:
    """The bytes under ``path`` (a file's own size, a directory's recursive total)."""
    status = os.lstat(path)
    if not os.path.isdir(path):
        return status.st_size
    total = 0
    for directory, _, names in os.walk(path):
        for name in names:
            total += os.lstat(os.path.join(directory, name)).st_size
    return total