Don't cancel web handler on disconnection

CHANGES/4080.feature

@@ -0,0 +1 @@
+Don't cancel web handler on peer disconnection, raise `OSError` on reading/writing instead.

aiohttp/http_websocket.py

@@ -13,7 +13,6 @@
 
 from .base_protocol import BaseProtocol
 from .helpers import NO_EXTENSIONS
-from .log import ws_logger
 from .streams import DataQueue
 
 __all__ = ('WS_CLOSED_MESSAGE', 'WS_CLOSING_MESSAGE', 'WS_KEY',
@@ -558,8 +557,8 @@ def __init__(self, protocol: BaseProtocol, transport: asyncio.Transport, *,
     async def _send_frame(self, message: bytes, opcode: int,
                           compress: Optional[int]=None) -> None:
         """Send a frame over the websocket with message as its payload."""
-        if self._closing:
-            ws_logger.warning('websocket connection is closing.')
+        if self._closing and not (opcode & WSMsgType.CLOSE):
+            raise ConnectionResetError('Cannot write to closing transport')
 
         rsv = 0
 
@@ -601,21 +600,26 @@ async def _send_frame(self, message: bytes, opcode: int,
             mask = mask.to_bytes(4, 'big')
             message = bytearray(message)
             _websocket_mask(mask, message)
-            self.transport.write(header + mask + message)
+            self._write(header + mask + message)
             self._output_size += len(header) + len(mask) + len(message)
         else:
             if len(message) > MSG_SIZE:
-                self.transport.write(header)
-                self.transport.write(message)
+                self._write(header)
+                self._write(message)
             else:
-                self.transport.write(header + message)
+                self._write(header + message)
 
             self._output_size += len(header) + len(message)
 
         if self._output_size > self._limit:
             self._output_size = 0
             await self.protocol._drain_helper()
 
+    def _write(self, data: bytes) -> None:
+        if self.transport is None or self.transport.is_closing():
+            raise ConnectionResetError('Cannot write to closing transport')
+        self.transport.write(data)
+
     async def pong(self, message: bytes=b'') -> None:
         """Send pong message."""
         if isinstance(message, str):

aiohttp/web_protocol.py

@@ -137,7 +137,8 @@ class RequestHandler(BaseProtocol):
                  '_waiter', '_error_handler', '_task_handler',
                  '_upgrade', '_payload_parser', '_request_parser',
                  '_reading_paused', 'logger', 'access_log',
-                 'access_logger', '_close', '_force_close')
+                 'access_logger', '_close', '_force_close',
+                 '_current_request')
 
     def __init__(self, manager: 'Server', *,
                  loop: asyncio.AbstractEventLoop,
@@ -156,6 +157,7 @@ def __init__(self, manager: 'Server', *,
 
         self._request_count = 0
         self._keepalive = False
+        self._current_request = None  # type: Optional[BaseRequest]
         self._manager = manager  # type: Optional[Server]
         self._request_handler = manager.request_handler  # type: Optional[_RequestHandler]  # noqa
         self._request_factory = manager.request_factory  # type: Optional[_RequestFactory]  # noqa
@@ -225,6 +227,9 @@ async def shutdown(self, timeout: Optional[float]=15.0) -> None:
                         not self._error_handler.done()):
                     await self._error_handler
 
+                if self._current_request is not None:
+                    self._current_request._cancel(asyncio.CancelledError())
+
                 if (self._task_handler is not None and
                         not self._task_handler.done()):
                     await self._task_handler
@@ -264,8 +269,10 @@ def connection_lost(self, exc: Optional[BaseException]) -> None:
         if self._keepalive_handle is not None:
             self._keepalive_handle.cancel()
 
-        if self._task_handler is not None:
-            self._task_handler.cancel()
+        if self._current_request is not None:
+            if exc is None:
+                exc = ConnectionResetError("Connetion lost")
+            self._current_request._cancel(exc)
 
         if self._error_handler is not None:
             self._error_handler.cancel()
@@ -402,7 +409,11 @@ async def _handle_request(self,
                               ) -> Tuple[StreamResponse, bool]:
         assert self._request_handler is not None
         try:
-            resp = await self._request_handler(request)
+            try:
+                self._current_request = request
+                resp = await self._request_handler(request)
+            finally:
+                self._current_request = None
         except HTTPException as exc:
             resp = Response(status=exc.status,
                             reason=exc.reason,

aiohttp/web_request.py

@@ -685,6 +685,9 @@ def __eq__(self, other: object) -> bool:
     async def _prepare_hook(self, response: StreamResponse) -> None:
         return
 
+    def _cancel(self, exc: BaseException) -> None:
+        self._payload.set_exception(exc)
+
 
 class Request(BaseRequest):
 

aiohttp/web_ws.py

@@ -458,3 +458,7 @@ async def __anext__(self) -> WSMessage:
                         WSMsgType.CLOSED):
             raise StopAsyncIteration  # NOQA
         return msg
+
+    def _cancel(self, exc: BaseException) -> None:
+        if self._reader is not None:
+            self._reader.set_exception(exc)

docs/web_advanced.rst

@@ -19,103 +19,22 @@ But in case of custom regular expressions for
 *percent encoded*: if you pass Unicode patterns they don't match to
 *requoted* path.
 
+Peer disconnection
+------------------
 
-Web Handler Cancellation
-------------------------
-
-.. warning::
-
-   :term:`web-handler` execution could be canceled on every ``await``
-   if client drops connection without reading entire response's BODY.
-
-   The behavior is very different from classic WSGI frameworks like
-   Flask and Django.
-
-Sometimes it is a desirable behavior: on processing ``GET`` request the
-code might fetch data from database or other web resource, the
-fetching is potentially slow.
-
-Canceling this fetch is very good: the peer dropped connection
-already, there is no reason to waste time and resources (memory etc) by
-getting data from DB without any chance to send it back to peer.
-
-But sometimes the cancellation is bad: on ``POST`` request very often
-is needed to save data to DB regardless to peer closing.
-
-Cancellation prevention could be implemented in several ways:
-
-* Applying :func:`asyncio.shield` to coroutine that saves data into DB.
-* Spawning a new task for DB saving
-* Using aiojobs_ or other third party library.
-
-:func:`asyncio.shield` works pretty good. The only disadvantage is you
-need to split web handler into exactly two async functions: one
-for handler itself and other for protected code.
-
-For example the following snippet is not safe::
-
-   async def handler(request):
-       await asyncio.shield(write_to_redis(request))
-       await asyncio.shield(write_to_postgres(request))
-       return web.Response(text='OK')
-
-Cancellation might be occurred just after saving data in REDIS,
-``write_to_postgres`` will be not called.
-
-Spawning a new task is much worse: there is no place to ``await``
-spawned tasks::
-
-   async def handler(request):
-       request.loop.create_task(write_to_redis(request))
-       return web.Response(text='OK')
-
-In this case errors from ``write_to_redis`` are not awaited, it leads
-to many asyncio log messages *Future exception was never retrieved*
-and *Task was destroyed but it is pending!*.
-
-Moreover on :ref:`aiohttp-web-graceful-shutdown` phase *aiohttp* don't
-wait for these tasks, you have a great chance to loose very important
-data.
-
-On other hand aiojobs_ provides an API for spawning new jobs and
-awaiting their results etc. It stores all scheduled activity in
-internal data structures and could terminate them gracefully::
-
-   from aiojobs.aiohttp import setup, spawn
-
-   async def coro(timeout):
-       await asyncio.sleep(timeout)  # do something in background
-
-   async def handler(request):
-       await spawn(request, coro())
-       return web.Response()
-
-   app = web.Application()
-   setup(app)
-   app.router.add_get('/', handler)
-
-All not finished jobs will be terminated on
-:attr:`Application.on_cleanup` signal.
+When a client peer is gone a subsequent reading or writing raises :exc:`OSError`
+or more specific exception like :exc:`ConnectionResetError`.
 
-To prevent cancellation of the whole :term:`web-handler` use
-``@atomic`` decorator::
+The reason for disconnection is vary; it can be a network issue or explicit
+socket closing on the peer side without reading the whole server response.
 
-   from aiojobs.aiohttp import atomic
+*aiohttp* handles disconnection properly but you can handle it explicitly, e.g.::
 
-   @atomic
    async def handler(request):
-       await write_to_db()
-       return web.Response()
-
-   app = web.Application()
-   setup(app)
-   app.router.add_post('/', handler)
-
-It prevents all ``handler`` async function from cancellation,
-``write_to_db`` will be never interrupted.
-
-.. _aiojobs: http://aiojobs.readthedocs.io/en/latest/
-
+       try:
+           text = await request.text()
+       except OSError:
+           # disconnected
 
 Passing a coroutine into run_app and Gunicorn
 ---------------------------------------------

tests/test_client_ws.py

@@ -9,7 +9,6 @@
 import aiohttp
 from aiohttp import client, hdrs
 from aiohttp.http import WS_KEY
-from aiohttp.log import ws_logger
 from aiohttp.streams import EofStream
 from aiohttp.test_utils import make_mocked_coro
 
@@ -364,7 +363,7 @@ async def test_close_exc2(loop, ws_key, key_data) -> None:
                     await resp.close()
 
 
-async def test_send_data_after_close(ws_key, key_data, loop, mocker) -> None:
+async def test_send_data_after_close(ws_key, key_data, loop) -> None:
     resp = mock.Mock()
     resp.status = 101
     resp.headers = {
@@ -382,16 +381,13 @@ async def test_send_data_after_close(ws_key, key_data, loop, mocker) -> None:
                 'http://test.org')
             resp._writer._closing = True
 
-            mocker.spy(ws_logger, 'warning')
-
             for meth, args in ((resp.ping, ()),
                                (resp.pong, ()),
                                (resp.send_str, ('s',)),
                                (resp.send_bytes, (b'b',)),
                                (resp.send_json, ({},))):
-                await meth(*args)
-                assert ws_logger.warning.called
-                ws_logger.warning.reset_mock()
+                with pytest.raises(ConnectionResetError):
+                    await meth(*args)
 
 
 async def test_send_data_type_errors(ws_key, key_data, loop) -> None:

tests/test_web_protocol.py

@@ -841,7 +841,11 @@ async def test_two_data_received_without_waking_up_start_task(srv) -> None:
 async def test_client_disconnect(aiohttp_server) -> None:
 
     async def handler(request):
-        await request.content.read(10)
+        buf = b""
+        with pytest.raises(ConnectionError):
+            while len(buf) < 10:
+                buf += await request.content.read(10)
+        # return with closed transport means premature client disconnection
         return web.Response()
 
     loop = asyncio.get_event_loop()