[3.5] handle ConnectionResetError, closes #3648 (#3699). (#3734)

(cherry picked from commit 7846174)

Co-authored-by: Gustavo J. A. M. Carneiro <[email protected]>

CHANGES/3648.bugfix

@@ -0,0 +1,2 @@
+Properly handle ConnectionResetError, to silence the "Cannot write to closing
+transport" exception when clients disconnect uncleanly.

aiohttp/web_protocol.py

@@ -444,8 +444,22 @@ async def start(self) -> None:
                             raise RuntimeError("Web-handler should return "
                                                "a response instance, "
                                                "got {!r}".format(resp))
-                await resp.prepare(request)
-                await resp.write_eof()
+                try:
+                    prepare_meth = resp.prepare
+                except AttributeError:
+                    if resp is None:
+                        raise RuntimeError("Missing return "
+                                           "statement on request handler")
+                    else:
+                        raise RuntimeError("Web-handler should return "
+                                           "a response instance, "
+                                           "got {!r}".format(resp))
+                try:
+                    await prepare_meth(request)
+                    await resp.write_eof()
+                except ConnectionResetError:
+                    self.log_debug('Ignored premature client disconnection 2')
+                    break
 
                 # notify server about keep-alive
                 self._keepalive = bool(resp.keep_alive)

tests/test_web_protocol.py

@@ -858,3 +858,30 @@ async def test_two_data_received_without_waking_up_start_task(srv) -> None:
     assert len(srv._messages) == 2
     assert srv._waiter.done()
     await asyncio.sleep(0.01)
+
+
+async def test_client_disconnect(aiohttp_server) -> None:
+
+    async def handler(request):
+        await request.content.read(10)
+        return web.Response()
+
+    logger = mock.Mock()
+    app = web.Application()
+    app._debug = True
+    app.router.add_route('POST', '/', handler)
+    server = await aiohttp_server(app, logger=logger)
+
+    _, writer = await asyncio.open_connection('127.0.0.1', server.port)
+    writer.write("""POST / HTTP/1.1\r
+Connection: keep-alive\r
+Content-Length: 10\r
+Host: localhost:{port}\r
+\r
+""".format(port=server.port).encode("ascii"))
+    await writer.drain()
+    await asyncio.sleep(0.1)
+    writer.write(b"x")
+    writer.close()
+    await asyncio.sleep(0.1)
+    logger.debug.assert_called_with('Ignored premature client disconnection 2')