Fix lost details on HttpProcessingError (#9052)

CHANGES/9052.bugfix.rst

@@ -0,0 +1 @@
+Fixed exception information getting lost on ``HttpProcessingError`` -- by :user:`Dreamsorcerer`.

aiohttp/client_proto.py

@@ -282,7 +282,15 @@ def data_received(self, data: bytes) -> None:
                         # closed in this case
                         self.transport.close()
                     # should_close is True after the call
-                    self.set_exception(HttpProcessingError(), underlying_exc)
+                    if isinstance(underlying_exc, HttpProcessingError):
+                        exc = HttpProcessingError(
+                            code=underlying_exc.code,
+                            message=underlying_exc.message,
+                            headers=underlying_exc.headers,
+                        )
+                    else:
+                        exc = HttpProcessingError()
+                    self.set_exception(exc, underlying_exc)
                     return
 
                 self._upgraded = upgraded

tests/test_client_functional.py

@@ -4021,10 +4021,6 @@ async def handler(_: web.Request) -> web.Response:
     await session.get("/")
 
 
-@pytest.mark.xfail(
-    reason="#8395 Error message regression for large headers in 3.9.4",
-    raises=AssertionError,
-)
 async def test_header_too_large_error(aiohttp_client: AiohttpClient) -> None:
     """By default when not specifying `max_field_size` requests should fail with a 400 status code."""