Raise ConnectionResetError if transport is None

When a client disconnects immediately after connecting, the transport
might become None. Replace the assert with a ConnectionResetError,
which is consistent with the existing aiohttp exception hierarchy and
the documented guidance to handle OSError for peer disconnection.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: agners

CHANGES/11761.bugfix.rst

@@ -0,0 +1,4 @@
+Fixed ``AssertionError`` when the transport is ``None`` during WebSocket
+preparation or file response sending (e.g. when a client disconnects
+immediately after connecting). A ``ConnectionResetError`` is now raised
+instead -- by :user:`agners`.

aiohttp/web_fileresponse.py

@@ -128,7 +128,8 @@ async def _sendfile(
 
         loop = request._loop
         transport = request.transport
-        assert transport is not None
+        if transport is None:
+            raise ConnectionResetError("Transport is not available")
 
         try:
             await loop.sendfile(transport, fobj, offset, count)

aiohttp/web_ws.py

@@ -361,7 +361,8 @@ def _pre_start(self, request: BaseRequest) -> tuple[str | None, WebSocketWriter]
         self.force_close()
         self._compress = compress
         transport = request._protocol.transport
-        assert transport is not None
+        if transport is None:
+            raise ConnectionResetError("Transport is not available")
         writer = WebSocketWriter(
             request._protocol,
             transport,

tests/test_web_websocket.py

@@ -772,3 +772,12 @@ async def test_get_extra_info(
     ws._writer = ws_transport
 
     assert expected_result == ws.get_extra_info(valid_key, default_value)
+
+
+async def test_prepare_transport_not_available(make_request: _RequestMaker) -> None:
+    req = make_request("GET", "/")
+    ws = web.WebSocketResponse()
+    # Simulate transport being None
+    req._protocol.transport = None
+    with pytest.raises(ConnectionResetError, match="Transport is not available"):
+        await ws.prepare(req)

tests/test_web_websocket_functional.py

@@ -11,7 +11,7 @@
 import pytest
 
 import aiohttp
-from aiohttp import WSServerHandshakeError, web
+from aiohttp import WSServerHandshakeError, hdrs, web
 from aiohttp.http import WSCloseCode, WSMsgType
 from aiohttp.pytest_plugin import AiohttpClient, AiohttpServer
 
@@ -1659,3 +1659,56 @@ async def websocket_handler(
     assert msg.type is aiohttp.WSMsgType.TEXT
     assert msg.data == "success"
     await ws.close()
+
+
+async def test_prepare_after_client_disconnect(aiohttp_client: AiohttpClient) -> None:
+    """Test ConnectionResetError when client disconnects before ws.prepare().
+
+    Reproduces the race condition where:
+    - Client connects and sends a WebSocket upgrade request
+    - Handler starts async work (e.g. authentication) before calling ws.prepare()
+    - Client disconnects while the handler is busy
+    - Handler then calls ws.prepare() → ConnectionResetError (not AssertionError)
+    """
+    handler_started = asyncio.Event()
+    captured_protocol = None
+
+    async def handler(request: web.Request) -> web.Response:
+        nonlocal captured_protocol
+        ws = web.WebSocketResponse()
+        captured_protocol = request._protocol
+        handler_started.set()
+        # Simulate async work (e.g., auth check) during which client disconnects.
+        await asyncio.sleep(0)
+        with pytest.raises(ConnectionResetError, match="Transport is not available"):
+            await ws.prepare(request)
+        return web.Response(status=503)
+
+    app = web.Application()
+    app.router.add_route("GET", "/", handler)
+    client = await aiohttp_client(app)
+
+    request_task = asyncio.create_task(
+        client.session.get(
+            client.make_url("/"),
+            headers={
+                hdrs.UPGRADE: "websocket",
+                hdrs.CONNECTION: "Upgrade",
+                hdrs.SEC_WEBSOCKET_KEY: "dGhlIHNhbXBsZSBub25jZQ==",
+                hdrs.SEC_WEBSOCKET_VERSION: "13",
+            },
+        )
+    )
+
+    # Wait until the handler is running but has not yet called ws.prepare().
+    await handler_started.wait()
+    assert captured_protocol is not None
+
+    # Simulate the client disconnecting abruptly.
+    captured_protocol.force_close()
+
+    # Yield so the handler can resume and hit the ConnectionResetError.
+    await asyncio.sleep(0)
+
+    with contextlib.suppress(aiohttp.ServerDisconnectedError, aiohttp.ClientConnectionResetError):
+        await request_task