why pymongo 3.7 SocketChecker set select.POLLIN in _EVENT_MASK

Question

In pymongo 3.7 SocketChecker, why the _EVENT_MASK had a select.POLLIN | select.POLLPRI?

From My Understanding, pool use the SocketChecker.socket_closed to check the socket obj status, reconnect this socket obj if it is select.POLLERR，select.POLLHUP， select.POLLNVAL.

but the socket obj had select.POLLIN, it will be reconnect?

pymongo/pool.py

class Pool:
    def __init__(self, address, options, handshake=True):
        ……
        self.socket_checker = SocketChecker()
   def _check(self, sock_info):
        """This side-effecty function checks if this socket has been idle for
        for longer than the max idle time, or if the socket has been closed by
        some external network error, and if so, attempts to create a new
        socket. If this connection attempt fails we raise the
        ConnectionFailure.

        Checking sockets lets us avoid seeing *some*
        :class:`~pymongo.errors.AutoReconnect` exceptions on server
        hiccups, etc. We only check if the socket was closed by an external
        error if it has been > 1 second since the socket was checked into the
        pool, to keep performance reasonable - we can't avoid AutoReconnects
        completely anyway.
        """
        idle_time_seconds = sock_info.idle_time_seconds()
        # If socket is idle, open a new one.
        if (self.opts.max_idle_time_seconds is not None and
                idle_time_seconds > self.opts.max_idle_time_seconds):
            sock_info.close()
            return self.connect()

        if (self._check_interval_seconds is not None and (
                0 == self._check_interval_seconds or
                idle_time_seconds > self._check_interval_seconds)):
            if self.socket_checker.socket_closed(sock_info.sock):
                sock_info.close()
                return self.connect()

        return sock_info

pymongo/network.py

try:
    from select import poll
    _EVENT_MASK = (
        select.POLLIN | select.POLLPRI | select.POLLERR | select.POLLHUP)
except ImportError:
    _HAS_POLL = False

class SocketChecker(object):

    def __init__(self):
        if _HAS_POLL:
            self._lock = threading.Lock()
            self._poller = poll()
        else:
            self._lock = None
            self._poller = None

    def socket_closed(self, sock):
        """Return True if we know socket has been closed, False otherwise.
        """
        while True:
            try:
                if self._poller:
                    with self._lock:
                        self._poller.register(sock, _EVENT_MASK)
                        try:
                            rd = self._poller.poll(0)
                        finally:
                            self._poller.unregister(sock)
                else:
                    rd, _, _ = select.select([sock], [], [], 0)
            except (RuntimeError, KeyError):
                # RuntimeError is raised during a concurrent poll. KeyError
                # is raised by unregister if the socket is not in the poller.
                # These errors should not be possible since we protect the
                # poller with a mutex.
                raise
            except ValueError:
                # ValueError is raised by register/unregister/select if the
                # socket file descriptor is negative or outside the range for
                # select (> 1023).
                return True
            except (_SELECT_ERROR, IOError) as exc:
                if _errno_from_exception(exc) in (errno.EINTR, errno.EAGAIN):
                    continue
                return True
            except Exception:
                # Any other exceptions should be attributed to a closed
                # or invalid socket.
                return True
            return len(rd) > 0

I try to print the rd result, I found some socket obj had select.POLLIN event and it return True, socket obj will be reconnect.