Fix server disconnect when error after 100-continue (#8876) (#8899)

(cherry picked from commit e058cbd)

Author: Dreamsorcerer

CHANGES/8876.bugfix.rst

@@ -0,0 +1 @@
+Fixed error handling after 100-continue so server sends 500 response instead of disconnecting -- by :user:`Dreamsorcerer`.

aiohttp/web_urldispatcher.py

@@ -341,6 +341,8 @@ async def _default_expect_handler(request: Request) -> None:
     if request.version == HttpVersion11:
         if expect.lower() == "100-continue":
             await request.writer.write(b"HTTP/1.1 100 Continue\r\n\r\n")
+            # Reset output_size as we haven't started the main body yet.
+            request.writer.output_size = 0
         else:
             raise HTTPExpectationFailed(text="Unknown Expect: %s" % expect)
 

tests/test_web_functional.py

@@ -4,7 +4,7 @@
 import pathlib
 import socket
 import zlib
-from typing import Any, Optional
+from typing import Any, NoReturn, Optional
 from unittest import mock
 
 import pytest
@@ -121,6 +121,20 @@ async def handler(request):
         assert resp.status == 500
 
 
+async def test_handler_returns_not_response_after_100expect(
+    aiohttp_server, aiohttp_client
+) -> None:
+    async def handler(request: web.Request) -> NoReturn:
+        raise Exception("foo")
+
+    app = web.Application()
+    app.router.add_get("/", handler)
+    client = await aiohttp_client(app)
+
+    async with client.get("/", expect100=True) as resp:
+        assert resp.status == 500
+
+
 async def test_head_returns_empty_body(aiohttp_client) -> None:
     async def handler(request):
         return web.Response(body=b"test")