cpython/Lib/asyncio/selector_events.py

"""Event loop using a selector and related classes.

A selector is a "notify-when-ready" multiplexer.  For a subclass which
also includes support for signal handling, see the unix_events sub-module.
"""

__all__ = ['BaseSelectorEventLoop']

import collections
import errno
import functools
import socket
import sys
import warnings
try:
    import ssl
except ImportError:  # pragma: no cover
    ssl = None

from . import base_events
from . import constants
from . import events
from . import futures
from . import selectors
from . import transports
from . import sslproto
from .coroutines import coroutine
from .log import logger


def _test_selector_event(selector, fd, event):
    # Test if the selector is monitoring 'event' events
    # for the file descriptor 'fd'.
    try:
        key = selector.get_key(fd)
    except KeyError:
        return False
    else:
        return bool(key.events & event)


class BaseSelectorEventLoop(base_events.BaseEventLoop):
    """Selector event loop.

    See events.EventLoop for API specification.
    """

    def __init__(self, selector=None):
        super().__init__()

        if selector is None:
            selector = selectors.DefaultSelector()
        logger.debug('Using selector: %s', selector.__class__.__name__)
        self._selector = selector
        self._make_self_pipe()

    def _make_socket_transport(self, sock, protocol, waiter=None, *,
                               extra=None, server=None):
        return _SelectorSocketTransport(self, sock, protocol, waiter,
                                        extra, server)

    def _make_ssl_transport(self, rawsock, protocol, sslcontext, waiter=None,
                            *, server_side=False, server_hostname=None,
                            extra=None, server=None):
        if not sslproto._is_sslproto_available():
            return self._make_legacy_ssl_transport(
                rawsock, protocol, sslcontext, waiter,
                server_side=server_side, server_hostname=server_hostname,
                extra=extra, server=server)

        ssl_protocol = sslproto.SSLProtocol(self, protocol, sslcontext, waiter,
                                            server_side, server_hostname)
        _SelectorSocketTransport(self, rawsock, ssl_protocol,
                                 extra=extra, server=server)
        return ssl_protocol._app_transport

    def _make_legacy_ssl_transport(self, rawsock, protocol, sslcontext,
                                   waiter, *,
                                   server_side=False, server_hostname=None,
                                   extra=None, server=None):
        # Use the legacy API: SSL_write, SSL_read, etc. The legacy API is used
        # on Python 3.4 and older, when ssl.MemoryBIO is not available.
        return _SelectorSslTransport(
            self, rawsock, protocol, sslcontext, waiter,
            server_side, server_hostname, extra, server)

    def _make_datagram_transport(self, sock, protocol,
                                 address=None, waiter=None, extra=None):
        return _SelectorDatagramTransport(self, sock, protocol,
                                          address, waiter, extra)

    def close(self):
        if self.is_running():
            raise RuntimeError("Cannot close a running event loop")
        if self.is_closed():
            return
        self._close_self_pipe()
        super().close()
        if self._selector is not None:
            self._selector.close()
            self._selector = None

    def _socketpair(self):
        raise NotImplementedError

    def _close_self_pipe(self):
        self.remove_reader(self._ssock.fileno())
        self._ssock.close()
        self._ssock = None
        self._csock.close()
        self._csock = None
        self._internal_fds -= 1

    def _make_self_pipe(self):
        # A self-socket, really. :-)
        self._ssock, self._csock = self._socketpair()
        self._ssock.setblocking(False)
        self._csock.setblocking(False)
        self._internal_fds += 1
        self.add_reader(self._ssock.fileno(), self._read_from_self)

    def _process_self_data(self, data):
        pass

    def _read_from_self(self):
        while True:
            try:
                data = self._ssock.recv(4096)
                if not data:
                    break
                self._process_self_data(data)
            except InterruptedError:
                continue
            except BlockingIOError:
                break

    def _write_to_self(self):
        # This may be called from a different thread, possibly after
        # _close_self_pipe() has been called or even while it is
        # running.  Guard for self._csock being None or closed.  When
        # a socket is closed, send() raises OSError (with errno set to
        # EBADF, but let's not rely on the exact error code).
        csock = self._csock
        if csock is not None:
            try:
                csock.send(b'\0')
            except OSError:
                if self._debug:
                    logger.debug("Fail to write a null byte into the "
                                 "self-pipe socket",
                                 exc_info=True)

    def _start_serving(self, protocol_factory, sock,
                       sslcontext=None, server=None):
        self.add_reader(sock.fileno(), self._accept_connection,
                        protocol_factory, sock, sslcontext, server)

    def _accept_connection(self, protocol_factory, sock,
                           sslcontext=None, server=None):
        try:
            conn, addr = sock.accept()
            if self._debug:
                logger.debug("%r got a new connection from %r: %r",
                             server, addr, conn)
            conn.setblocking(False)
        except (BlockingIOError, InterruptedError, ConnectionAbortedError):
            pass  # False alarm.
        except OSError as exc:
            # There's nowhere to send the error, so just log it.
            if exc.errno in (errno.EMFILE, errno.ENFILE,
                             errno.ENOBUFS, errno.ENOMEM):
                # Some platforms (e.g. Linux keep reporting the FD as
                # ready, so we remove the read handler temporarily.
                # We'll try again in a while.
                self.call_exception_handler({
                    'message': 'socket.accept() out of system resource',
                    'exception': exc,
                    'socket': sock,
                })
                self.remove_reader(sock.fileno())
                self.call_later(constants.ACCEPT_RETRY_DELAY,
                                self._start_serving,
                                protocol_factory, sock, sslcontext, server)
            else:
                raise  # The event loop will catch, log and ignore it.
        else:
            extra = {'peername': addr}
            accept = self._accept_connection2(protocol_factory, conn, extra,
                                              sslcontext, server)
            self.create_task(accept)

    @coroutine
    def _accept_connection2(self, protocol_factory, conn, extra,
                            sslcontext=None, server=None):
        protocol = None
        transport = None
        try:
            protocol = protocol_factory()
            waiter = futures.Future(loop=self)
            if sslcontext:
                transport = self._make_ssl_transport(
                    conn, protocol, sslcontext, waiter=waiter,
                    server_side=True, extra=extra, server=server)
            else:
                transport = self._make_socket_transport(
                    conn, protocol, waiter=waiter, extra=extra,
                    server=server)

            try:
                yield from waiter
            except:
                transport.close()
                raise

            # It's now up to the protocol to handle the connection.
        except Exception as exc:
            if self._debug:
                context = {
                    'message': ('Error on transport creation '
                                'for incoming connection'),
                    'exception': exc,
                }
                if protocol is not None:
                    context['protocol'] = protocol
                if transport is not None:
                    context['transport'] = transport
                self.call_exception_handler(context)

    def add_reader(self, fd, callback, *args):
        """Add a reader callback."""
        self._check_closed()
        handle = events.Handle(callback, args, self)
        try:
            key = self._selector.get_key(fd)
        except KeyError:
            self._selector.register(fd, selectors.EVENT_READ,
                                    (handle, None))
        else:
            mask, (reader, writer) = key.events, key.data
            self._selector.modify(fd, mask | selectors.EVENT_READ,
                                  (handle, writer))
            if reader is not None:
                reader.cancel()

    def remove_reader(self, fd):
        """Remove a reader callback."""
        if self.is_closed():
            return False
        try:
            key = self._selector.get_key(fd)
        except KeyError:
            return False
        else:
            mask, (reader, writer) = key.events, key.data
            mask &= ~selectors.EVENT_READ
            if not mask:
                self._selector.unregister(fd)
            else:
                self._selector.modify(fd, mask, (None, writer))

            if reader is not None:
                reader.cancel()
                return True
            else:
                return False

    def add_writer(self, fd, callback, *args):
        """Add a writer callback.."""
        self._check_closed()
        handle = events.Handle(callback, args, self)
        try:
            key = self._selector.get_key(fd)
        except KeyError:
            self._selector.register(fd, selectors.EVENT_WRITE,
                                    (None, handle))
        else:
            mask, (reader, writer) = key.events, key.data
            self._selector.modify(fd, mask | selectors.EVENT_WRITE,
                                  (reader, handle))
            if writer is not None:
                writer.cancel()

    def remove_writer(self, fd):
        """Remove a writer callback."""
        if self.is_closed():
            return False
        try:
            key = self._selector.get_key(fd)
        except KeyError:
            return False
        else:
            mask, (reader, writer) = key.events, key.data
            # Remove both writer and connector.
            mask &= ~selectors.EVENT_WRITE
            if not mask:
                self._selector.unregister(fd)
            else:
                self._selector.modify(fd, mask, (reader, None))

            if writer is not None:
                writer.cancel()
                return True
            else:
                return False

    def sock_recv(self, sock, n):
        """Receive data from the socket.

        The return value is a bytes object representing the data received.
        The maximum amount of data to be received at once is specified by
        nbytes.

        This method is a coroutine.
        """
        if self._debug and sock.gettimeout() != 0:
            raise ValueError("the socket must be non-blocking")
        fut = futures.Future(loop=self)
        self._sock_recv(fut, False, sock, n)
        return fut

    def _sock_recv(self, fut, registered, sock, n):
        # _sock_recv() can add itself as an I/O callback if the operation can't
        # be done immediately. Don't use it directly, call sock_recv().
        fd = sock.fileno()
        if registered:
            # Remove the callback early.  It should be rare that the
            # selector says the fd is ready but the call still returns
            # EAGAIN, and I am willing to take a hit in that case in
            # order to simplify the common case.
            self.remove_reader(fd)
        if fut.cancelled():
            return
        try:
            data = sock.recv(n)
        except (BlockingIOError, InterruptedError):
            self.add_reader(fd, self._sock_recv, fut, True, sock, n)
        except Exception as exc:
            fut.set_exception(exc)
        else:
            fut.set_result(data)

    def sock_sendall(self, sock, data):
        """Send data to the socket.

        The socket must be connected to a remote socket. This method continues
        to send data from data until either all data has been sent or an
        error occurs. None is returned on success. On error, an exception is
        raised, and there is no way to determine how much data, if any, was
        successfully processed by the receiving end of the connection.

        This method is a coroutine.
        """
        if self._debug and sock.gettimeout() != 0:
            raise ValueError("the socket must be non-blocking")
        fut = futures.Future(loop=self)
        if data:
            self._sock_sendall(fut, False, sock, data)
        else:
            fut.set_result(None)
        return fut

    def _sock_sendall(self, fut, registered, sock, data):
        fd = sock.fileno()

        if registered:
            self.remove_writer(fd)
        if fut.cancelled():
            return

        try:
            n = sock.send(data)
        except (BlockingIOError, InterruptedError):
            n = 0
        except Exception as exc:
            fut.set_exception(exc)
            return

        if n == len(data):
            fut.set_result(None)
        else:
            if n:
                data = data[n:]
            self.add_writer(fd, self._sock_sendall, fut, True, sock, data)

    def sock_connect(self, sock, address):
        """Connect to a remote socket at address.

        The address must be already resolved to avoid the trap of hanging the
        entire event loop when the address requires doing a DNS lookup. For
        example, it must be an IP address, not an hostname, for AF_INET and
        AF_INET6 address families. Use getaddrinfo() to resolve the hostname
        asynchronously.

        This method is a coroutine.
        """
        if self._debug and sock.gettimeout() != 0:
            raise ValueError("the socket must be non-blocking")
        fut = futures.Future(loop=self)
        try:
            if self._debug:
                base_events._check_resolved_address(sock, address)
        except ValueError as err:
            fut.set_exception(err)
        else:
            self._sock_connect(fut, sock, address)
        return fut

    def _sock_connect(self, fut, sock, address):
        fd = sock.fileno()
        try:
            while True:
                try:
                    sock.connect(address)
                except InterruptedError:
                    continue
                else:
                    break
        except BlockingIOError:
            fut.add_done_callback(functools.partial(self._sock_connect_done,
                                                    fd))
            self.add_writer(fd, self._sock_connect_cb, fut, sock, address)
        except Exception as exc:
            fut.set_exception(exc)
        else:
            fut.set_result(None)

    def _sock_connect_done(self, fd, fut):
        self.remove_writer(fd)

    def _sock_connect_cb(self, fut, sock, address):
        if fut.cancelled():
            return

        try:
            err = sock.getsockopt(socket.SOL_SOCKET, socket.SO_ERROR)
            if err != 0:
                # Jump to any except clause below.
                raise OSError(err, 'Connect call failed %s' % (address,))
        except (BlockingIOError, InterruptedError):
            # socket is still registered, the callback will be retried later
            pass
        except Exception as exc:
            fut.set_exception(exc)
        else:
            fut.set_result(None)

    def sock_accept(self, sock):
        """Accept a connection.

        The socket must be bound to an address and listening for connections.
        The return value is a pair (conn, address) where conn is a new socket
        object usable to send and receive data on the connection, and address
        is the address bound to the socket on the other end of the connection.

        This method is a coroutine.
        """
        if self._debug and sock.gettimeout() != 0:
            raise ValueError("the socket must be non-blocking")
        fut = futures.Future(loop=self)
        self._sock_accept(fut, False, sock)
        return fut

    def _sock_accept(self, fut, registered, sock):
        fd = sock.fileno()
        if registered:
            self.remove_reader(fd)
        if fut.cancelled():
            return
        try:
            conn, address = sock.accept()
            conn.setblocking(False)
        except (BlockingIOError, InterruptedError):
            self.add_reader(fd, self._sock_accept, fut, True, sock)
        except Exception as exc:
            fut.set_exception(exc)
        else:
            fut.set_result((conn, address))

    def _process_events(self, event_list):
        for key, mask in event_list:
            fileobj, (reader, writer) = key.fileobj, key.data
            if mask & selectors.EVENT_READ and reader is not None:
                if reader._cancelled:
                    self.remove_reader(fileobj)
                else:
                    self._add_callback(reader)
            if mask & selectors.EVENT_WRITE and writer is not None:
                if writer._cancelled:
                    self.remove_writer(fileobj)
                else:
                    self._add_callback(writer)

    def _stop_serving(self, sock):
        self.remove_reader(sock.fileno())
        sock.close()


class _SelectorTransport(transports._FlowControlMixin,
                         transports.Transport):

    max_size = 256 * 1024  # Buffer size passed to recv().

    _buffer_factory = bytearray  # Constructs initial value for self._buffer.

    # Attribute used in the destructor: it must be set even if the constructor
    # is not called (see _SelectorSslTransport which may start by raising an
    # exception)
    _sock = None

    def __init__(self, loop, sock, protocol, extra=None, server=None):
        super().__init__(extra, loop)
        self._extra['socket'] = sock
        self._extra['sockname'] = sock.getsockname()
        if 'peername' not in self._extra:
            try:
                self._extra['peername'] = sock.getpeername()
            except socket.error:
                self._extra['peername'] = None
        self._sock = sock
        self._sock_fd = sock.fileno()
        self._protocol = protocol
        self._protocol_connected = True
        self._server = server
        self._buffer = self._buffer_factory()
        self._conn_lost = 0  # Set when call to connection_lost scheduled.
        self._closing = False  # Set when close() called.
        if self._server is not None:
            self._server._attach()

    def __repr__(self):
        info = [self.__class__.__name__]
        if self._sock is None:
            info.append('closed')
        elif self._closing:
            info.append('closing')
        info.append('fd=%s' % self._sock_fd)
        # test if the transport was closed
        if self._loop is not None:
            polling = _test_selector_event(self._loop._selector,
                                           self._sock_fd, selectors.EVENT_READ)
            if polling:
                info.append('read=polling')
            else:
                info.append('read=idle')

            polling = _test_selector_event(self._loop._selector,
                                           self._sock_fd,
                                           selectors.EVENT_WRITE)
            if polling:
                state = 'polling'
            else:
                state = 'idle'

            bufsize = self.get_write_buffer_size()
            info.append('write=<%s, bufsize=%s>' % (state, bufsize))
        return '<%s>' % ' '.join(info)

    def abort(self):
        self._force_close(None)

    def close(self):
        if self._closing:
            return
        self._closing = True
        self._loop.remove_reader(self._sock_fd)
        if not self._buffer:
            self._conn_lost += 1
            self._loop.call_soon(self._call_connection_lost, None)

    # On Python 3.3 and older, objects with a destructor part of a reference
    # cycle are never destroyed. It's not more the case on Python 3.4 thanks
    # to the PEP 442.
    if sys.version_info >= (3, 4):
        def __del__(self):
            if self._sock is not None:
                warnings.warn("unclosed transport %r" % self, ResourceWarning)
                self._sock.close()

    def _fatal_error(self, exc, message='Fatal error on transport'):
        # Should be called from exception handler only.
        if isinstance(exc, (BrokenPipeError,
                            ConnectionResetError, ConnectionAbortedError)):
            if self._loop.get_debug():
                logger.debug("%r: %s", self, message, exc_info=True)
        else:
            self._loop.call_exception_handler({
                'message': message,
                'exception': exc,
                'transport': self,
                'protocol': self._protocol,
            })
        self._force_close(exc)

    def _force_close(self, exc):
        if self._conn_lost:
            return
        if self._buffer:
            self._buffer.clear()
            self._loop.remove_writer(self._sock_fd)
        if not self._closing:
            self._closing = True
            self._loop.remove_reader(self._sock_fd)
        self._conn_lost += 1
        self._loop.call_soon(self._call_connection_lost, exc)

    def _call_connection_lost(self, exc):
        try:
            if self._protocol_connected:
                self._protocol.connection_lost(exc)
        finally:
            self._sock.close()
            self._sock = None
            self._protocol = None
            self._loop = None
            server = self._server
            if server is not None:
                server._detach()
                self._server = None

    def get_write_buffer_size(self):
        return len(self._buffer)


class _SelectorSocketTransport(_SelectorTransport):

    def __init__(self, loop, sock, protocol, waiter=None,
                 extra=None, server=None):
        super().__init__(loop, sock, protocol, extra, server)
        self._eof = False
        self._paused = False

        self._loop.call_soon(self._protocol.connection_made, self)
        # only start reading when connection_made() has been called
        self._loop.call_soon(self._loop.add_reader,
                             self._sock_fd, self._read_ready)
        if waiter is not None:
            # only wake up the waiter when connection_made() has been called
            self._loop.call_soon(waiter._set_result_unless_cancelled, None)

    def pause_reading(self):
        if self._closing:
            raise RuntimeError('Cannot pause_reading() when closing')
        if self._paused:
            raise RuntimeError('Already paused')
        self._paused = True
        self._loop.remove_reader(self._sock_fd)
        if self._loop.get_debug():
            logger.debug("%r pauses reading", self)

    def resume_reading(self):
        if not self._paused:
            raise RuntimeError('Not paused')
        self._paused = False
        if self._closing:
            return
        self._loop.add_reader(self._sock_fd, self._read_ready)
        if self._loop.get_debug():
            logger.debug("%r resumes reading", self)

    def _read_ready(self):
        try:
            data = self._sock.recv(self.max_size)
        except (BlockingIOError, InterruptedError):
            pass
        except Exception as exc:
            self._fatal_error(exc, 'Fatal read error on socket transport')
        else:
            if data:
                self._protocol.data_received(data)
            else:
                if self._loop.get_debug():
                    logger.debug("%r received EOF", self)
                keep_open = self._protocol.eof_received()
                if keep_open:
                    # We're keeping the connection open so the
                    # protocol can write more, but we still can't
                    # receive more, so remove the reader callback.
                    self._loop.remove_reader(self._sock_fd)
                else:
                    self.close()

    def write(self, data):
        if not isinstance(data, (bytes, bytearray, memoryview)):
            raise TypeError('data argument must be byte-ish (%r)',
                            type(data))
        if self._eof:
            raise RuntimeError('Cannot call write() after write_eof()')
        if not data:
            return

        if self._conn_lost:
            if self._conn_lost >= constants.LOG_THRESHOLD_FOR_CONNLOST_WRITES:
                logger.warning('socket.send() raised exception.')
            self._conn_lost += 1
            return

        if not self._buffer:
            # Optimization: try to send now.
            try:
                n = self._sock.send(data)
            except (BlockingIOError, InterruptedError):
                pass
            except Exception as exc:
                self._fatal_error(exc, 'Fatal write error on socket transport')
                return
            else:
                data = data[n:]
                if not data:
                    return
            # Not all was written; register write handler.
            self._loop.add_writer(self._sock_fd, self._write_ready)

        # Add it to the buffer.
        self._buffer.extend(data)
        self._maybe_pause_protocol()

    def _write_ready(self):
        assert self._buffer, 'Data should not be empty'

        try:
            n = self._sock.send(self._buffer)
        except (BlockingIOError, InterruptedError):
            pass
        except Exception as exc:
            self._loop.remove_writer(self._sock_fd)
            self._buffer.clear()
            self._fatal_error(exc, 'Fatal write error on socket transport')
        else:
            if n:
                del self._buffer[:n]
            self._maybe_resume_protocol()  # May append to buffer.
            if not self._buffer:
                self._loop.remove_writer(self._sock_fd)
                if self._closing:
                    self._call_connection_lost(None)
                elif self._eof:
                    self._sock.shutdown(socket.SHUT_WR)

    def write_eof(self):
        if self._eof:
            return
        self._eof = True
        if not self._buffer:
            self._sock.shutdown(socket.SHUT_WR)

    def can_write_eof(self):
        return True


class _SelectorSslTransport(_SelectorTransport):

    _buffer_factory = bytearray

    def __init__(self, loop, rawsock, protocol, sslcontext, waiter=None,
                 server_side=False, server_hostname=None,
                 extra=None, server=None):
        if ssl is None:
            raise RuntimeError('stdlib ssl module not available')

        if not sslcontext:
            sslcontext = sslproto._create_transport_context(server_side, server_hostname)

        wrap_kwargs = {
            'server_side': server_side,
            'do_handshake_on_connect': False,
        }
        if server_hostname and not server_side:
            wrap_kwargs['server_hostname'] = server_hostname
        sslsock = sslcontext.wrap_socket(rawsock, **wrap_kwargs)

        super().__init__(loop, sslsock, protocol, extra, server)
        # the protocol connection is only made after the SSL handshake
        self._protocol_connected = False

        self._server_hostname = server_hostname
        self._waiter = waiter
        self._sslcontext = sslcontext
        self._paused = False

        # SSL-specific extra info.  (peercert is set later)
        self._extra.update(sslcontext=sslcontext)

        if self._loop.get_debug():
            logger.debug("%r starts SSL handshake", self)
            start_time = self._loop.time()
        else:
            start_time = None
        self._on_handshake(start_time)

    def _wakeup_waiter(self, exc=None):
        if self._waiter is None:
            return
        if not self._waiter.cancelled():
            if exc is not None:
                self._waiter.set_exception(exc)
            else:
                self._waiter.set_result(None)
        self._waiter = None

    def _on_handshake(self, start_time):
        try:
            self._sock.do_handshake()
        except ssl.SSLWantReadError:
            self._loop.add_reader(self._sock_fd,
                                  self._on_handshake, start_time)
            return
        except ssl.SSLWantWriteError:
            self._loop.add_writer(self._sock_fd,
                                  self._on_handshake, start_time)
            return
        except BaseException as exc:
            if self._loop.get_debug():
                logger.warning("%r: SSL handshake failed",
                               self, exc_info=True)
            self._loop.remove_reader(self._sock_fd)
            self._loop.remove_writer(self._sock_fd)
            self._sock.close()
            self._wakeup_waiter(exc)
            if isinstance(exc, Exception):
                return
            else:
                raise

        self._loop.remove_reader(self._sock_fd)
        self._loop.remove_writer(self._sock_fd)

        peercert = self._sock.getpeercert()
        if not hasattr(self._sslcontext, 'check_hostname'):
            # Verify hostname if requested, Python 3.4+ uses check_hostname
            # and checks the hostname in do_handshake()
            if (self._server_hostname and
                self._sslcontext.verify_mode != ssl.CERT_NONE):
                try:
                    ssl.match_hostname(peercert, self._server_hostname)
                except Exception as exc:
                    if self._loop.get_debug():
                        logger.warning("%r: SSL handshake failed "
                                       "on matching the hostname",
                                       self, exc_info=True)
                    self._sock.close()
                    self._wakeup_waiter(exc)
                    return

        # Add extra info that becomes available after handshake.
        self._extra.update(peercert=peercert,
                           cipher=self._sock.cipher(),
                           compression=self._sock.compression(),
                           )

        self._read_wants_write = False
        self._write_wants_read = False
        self._loop.add_reader(self._sock_fd, self._read_ready)
        self._protocol_connected = True
        self._loop.call_soon(self._protocol.connection_made, self)
        # only wake up the waiter when connection_made() has been called
        self._loop.call_soon(self._wakeup_waiter)

        if self._loop.get_debug():
            dt = self._loop.time() - start_time
            logger.debug("%r: SSL handshake took %.1f ms", self, dt * 1e3)

    def pause_reading(self):
        # XXX This is a bit icky, given the comment at the top of
        # _read_ready().  Is it possible to evoke a deadlock?  I don't
        # know, although it doesn't look like it; write() will still
        # accept more data for the buffer and eventually the app will
        # call resume_reading() again, and things will flow again.

        if self._closing:
            raise RuntimeError('Cannot pause_reading() when closing')
        if self._paused:
            raise RuntimeError('Already paused')
        self._paused = True
        self._loop.remove_reader(self._sock_fd)
        if self._loop.get_debug():
            logger.debug("%r pauses reading", self)

    def resume_reading(self):
        if not self._paused:
            raise RuntimeError('Not paused')
        self._paused = False
        if self._closing:
            return
        self._loop.add_reader(self._sock_fd, self._read_ready)
        if self._loop.get_debug():
            logger.debug("%r resumes reading", self)

    def _read_ready(self):
        if self._write_wants_read:
            self._write_wants_read = False
            self._write_ready()

            if self._buffer:
                self._loop.add_writer(self._sock_fd, self._write_ready)

        try:
            data = self._sock.recv(self.max_size)
        except (BlockingIOError, InterruptedError, ssl.SSLWantReadError):
            pass
        except ssl.SSLWantWriteError:
            self._read_wants_write = True
            self._loop.remove_reader(self._sock_fd)
            self._loop.add_writer(self._sock_fd, self._write_ready)
        except Exception as exc:
            self._fatal_error(exc, 'Fatal read error on SSL transport')
        else:
            if data:
                self._protocol.data_received(data)
            else:
                try:
                    if self._loop.get_debug():
                        logger.debug("%r received EOF", self)
                    keep_open = self._protocol.eof_received()
                    if keep_open:
                        logger.warning('returning true from eof_received() '
                                       'has no effect when using ssl')
                finally:
                    self.close()

    def _write_ready(self):
        if self._read_wants_write:
            self._read_wants_write = False
            self._read_ready()

            if not (self._paused or self._closing):
                self._loop.add_reader(self._sock_fd, self._read_ready)

        if self._buffer:
            try:
                n = self._sock.send(self._buffer)
            except (BlockingIOError, InterruptedError, ssl.SSLWantWriteError):
                n = 0
            except ssl.SSLWantReadError:
                n = 0
                self._loop.remove_writer(self._sock_fd)
                self._write_wants_read = True
            except Exception as exc:
                self._loop.remove_writer(self._sock_fd)
                self._buffer.clear()
                self._fatal_error(exc, 'Fatal write error on SSL transport')
                return

            if n:
                del self._buffer[:n]

        self._maybe_resume_protocol()  # May append to buffer.

        if not self._buffer:
            self._loop.remove_writer(self._sock_fd)
            if self._closing:
                self._call_connection_lost(None)

    def write(self, data):
        if not isinstance(data, (bytes, bytearray, memoryview)):
            raise TypeError('data argument must be byte-ish (%r)',
                            type(data))
        if not data:
            return

        if self._conn_lost:
            if self._conn_lost >= constants.LOG_THRESHOLD_FOR_CONNLOST_WRITES:
                logger.warning('socket.send() raised exception.')
            self._conn_lost += 1
            return

        if not self._buffer:
            self._loop.add_writer(self._sock_fd, self._write_ready)

        # Add it to the buffer.
        self._buffer.extend(data)
        self._maybe_pause_protocol()

    def can_write_eof(self):
        return False


class _SelectorDatagramTransport(_SelectorTransport):

    _buffer_factory = collections.deque

    def __init__(self, loop, sock, protocol, address=None,
                 waiter=None, extra=None):
        super().__init__(loop, sock, protocol, extra)
        self._address = address
        self._loop.call_soon(self._protocol.connection_made, self)
        # only start reading when connection_made() has been called
        self._loop.call_soon(self._loop.add_reader,
                             self._sock_fd, self._read_ready)
        if waiter is not None:
            # only wake up the waiter when connection_made() has been called
            self._loop.call_soon(waiter._set_result_unless_cancelled, None)

    def get_write_buffer_size(self):
        return sum(len(data) for data, _ in self._buffer)

    def _read_ready(self):
        try:
            data, addr = self._sock.recvfrom(self.max_size)
        except (BlockingIOError, InterruptedError):
            pass
        except OSError as exc:
            self._protocol.error_received(exc)
        except Exception as exc:
            self._fatal_error(exc, 'Fatal read error on datagram transport')
        else:
            self._protocol.datagram_received(data, addr)

    def sendto(self, data, addr=None):
        if not isinstance(data, (bytes, bytearray, memoryview)):
            raise TypeError('data argument must be byte-ish (%r)',
                            type(data))
        if not data:
            return

        if self._address and addr not in (None, self._address):
            raise ValueError('Invalid address: must be None or %s' %
                             (self._address,))

        if self._conn_lost and self._address:
            if self._conn_lost >= constants.LOG_THRESHOLD_FOR_CONNLOST_WRITES:
                logger.warning('socket.send() raised exception.')
            self._conn_lost += 1
            return

        if not self._buffer:
            # Attempt to send it right away first.
            try:
                if self._address:
                    self._sock.send(data)
                else:
                    self._sock.sendto(data, addr)
                return
            except (BlockingIOError, InterruptedError):
                self._loop.add_writer(self._sock_fd, self._sendto_ready)
            except OSError as exc:
                self._protocol.error_received(exc)
                return
            except Exception as exc:
                self._fatal_error(exc,
                                  'Fatal write error on datagram transport')
                return

        # Ensure that what we buffer is immutable.
        self._buffer.append((bytes(data), addr))
        self._maybe_pause_protocol()

    def _sendto_ready(self):
        while self._buffer:
            data, addr = self._buffer.popleft()
            try:
                if self._address:
                    self._sock.send(data)
                else:
                    self._sock.sendto(data, addr)
            except (BlockingIOError, InterruptedError):
                self._buffer.appendleft((data, addr))  # Try again later.
                break
            except OSError as exc:
                self._protocol.error_received(exc)
                return
            except Exception as exc:
                self._fatal_error(exc,
                                  'Fatal write error on datagram transport')
                return

        self._maybe_resume_protocol()  # May append to buffer.
        if not self._buffer:
            self._loop.remove_writer(self._sock_fd)
            if self._closing:
                self._call_connection_lost(None)