Accept multiple connections on listener EVENT_READ

When a large number of connections are made at once the event loop
struggles to accept them because it only accepts and begins
handling one new connection on each pass of the loop in which the
EVENT_READ is emitted for the listening socket. This causes
incoming connections to time out and sever.

This patch uses the backlog setting of the listening socket to
allow for accepting a bounded number of new incoming connections
rather than a single one.

Patch by Kevin Conway <kevinjacobconway@gmail.com>.

Author: 1st1

asyncio/base_events.py

@@ -1034,7 +1034,7 @@ def create_server(self, protocol_factory, host=None, port=None,
         for sock in sockets:
             sock.listen(backlog)
             sock.setblocking(False)
-            self._start_serving(protocol_factory, sock, ssl, server)
+            self._start_serving(protocol_factory, sock, ssl, server, backlog)
         if self._debug:
             logger.info("%r is serving", server)
         return server

asyncio/selector_events.py

@@ -162,43 +162,50 @@ def _write_to_self(self):
                                  exc_info=True)
 
     def _start_serving(self, protocol_factory, sock,
-                       sslcontext=None, server=None):
+                       sslcontext=None, server=None, backlog=100):
         self.add_reader(sock.fileno(), self._accept_connection,
-                        protocol_factory, sock, sslcontext, server)
+                        protocol_factory, sock, sslcontext, server, backlog)
 
     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)
+                           sslcontext=None, server=None, backlog=100):
+        # This method is only called once for each event loop tick where the
+        # listening socket has triggered an EVENT_READ. There may be multiple
+        # connections waiting for an .accept() so it is called in a loop.
+        # See https://bugs.python.org/issue27906 for more details.
+        for _ in range(backlog):
+            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):
+                # Early exit because the socket accept buffer is empty.
+                return None
+            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,
+                                    backlog)
+                else:
+                    raise  # The event loop will catch, log and ignore it.
             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)
+                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,

tests/test_base_events.py

@@ -1634,7 +1634,7 @@ def test_accept_connection_exception(self, m_log):
         self.loop.call_later.assert_called_with(constants.ACCEPT_RETRY_DELAY,
                                                 # self.loop._start_serving
                                                 mock.ANY,
-                                                MyProto, sock, None, None)
+                                                MyProto, sock, None, None, mock.ANY)
 
     def test_call_coroutine(self):
         @asyncio.coroutine

tests/test_selector_events.py

@@ -687,6 +687,20 @@ def test_process_events_write_cancelled(self):
               selectors.EVENT_WRITE)])
         self.loop.remove_writer.assert_called_with(1)
 
+    def test_accept_connection_multiple(self):
+        sock = mock.Mock()
+        sock.accept.return_value = (mock.Mock(), mock.Mock())
+        backlog = 100
+        # Mock the coroutine generation for a connection to prevent
+        # warnings related to un-awaited coroutines.
+        mock_obj = mock.patch.object
+        with mock_obj(self.loop, '_accept_connection2') as accept2_mock:
+            accept2_mock.return_value = None
+            with mock_obj(self.loop, 'create_task') as task_mock:
+                task_mock.return_value = None
+                self.loop._accept_connection(mock.Mock(), sock, backlog=backlog)
+        self.assertEqual(sock.accept.call_count, backlog)
+
 
 class SelectorTransportTests(test_utils.TestCase):