import select
import socket
import sys
import typing


def is_socket_readable(sock: typing.Optional[socket.socket]) -> bool:
    """
    Return whether a socket, as identifed by its file descriptor, is readable.
    "A socket is readable" means that the read buffer isn't empty, i.e. that calling
    .recv() on it would immediately return some data.
    """
    # NOTE: we want check for readability without actually attempting to read, because
    # we don't want to block forever if it's not readable.

    # In the case that the socket no longer exists, or cannot return a file
    # descriptor, we treat it as being readable, as if it the next read operation
    # on it is ready to return the terminating `b""`.
    sock_fd = None if sock is None else sock.fileno()
    if sock_fd is None or sock_fd < 0:  # pragma: nocover
        return True

    # The implementation below was stolen from:
    # https://github.com/python-trio/trio/blob/20ee2b1b7376db637435d80e266212a35837ddcc/trio/_socket.py#L471-L478
    # See also: https://github.com/encode/httpcore/pull/193#issuecomment-703129316

    # Use select.select on Windows, and when poll is unavailable and select.poll
    # everywhere else. (E.g. When eventlet is in use. See #327)
    if (
        sys.platform == "win32" or getattr(select, "poll", None) is None
    ):  # pragma: nocover
        rready, _, _ = select.select([sock_fd], [], [], 0)
        return bool(rready)
    p = select.poll()
    p.register(sock_fd, select.POLLIN)
    return bool(p.poll(0))