Improve performance of starting web requests (#9172)

(cherry picked from commit b93ef57)

CHANGES/9172.misc.rst

@@ -0,0 +1 @@
+9174.misc.rst

aiohttp/web_response.py

@@ -759,10 +759,10 @@ async def write_eof(self, data: bytes = b"") -> None:
             await super().write_eof()
 
     async def _start(self, request: "BaseRequest") -> AbstractStreamWriter:
-        if should_remove_content_length(request.method, self.status):
-            if hdrs.CONTENT_LENGTH in self._headers:
+        if hdrs.CONTENT_LENGTH in self._headers:
+            if should_remove_content_length(request.method, self.status):
                 del self._headers[hdrs.CONTENT_LENGTH]
-        elif not self._chunked and hdrs.CONTENT_LENGTH not in self._headers:
+        elif not self._chunked:
             if isinstance(self._body, Payload):
                 if self._body.size is not None:
                     self._headers[hdrs.CONTENT_LENGTH] = str(self._body.size)

tests/test_web_response.py

@@ -667,6 +667,22 @@ async def write_headers(status_line, headers):
     assert resp.content_length is None
 
 
+async def test_rm_content_length_if_204() -> None:
+    """Ensure content-length is removed for 204 responses."""
+    writer = mock.create_autospec(StreamWriter, spec_set=True, instance=True)
+
+    async def write_headers(status_line, headers):
+        assert hdrs.CONTENT_LENGTH not in headers
+
+    writer.write_headers.side_effect = write_headers
+    req = make_request("GET", "/", writer=writer)
+    payload = BytesPayload(b"answer", headers={"Content-Length": "6"})
+    resp = Response(body=payload, status=204)
+    resp.body = payload
+    await resp.prepare(req)
+    assert resp.content_length is None
+
+
 @pytest.mark.parametrize("status", (100, 101, 204, 304))
 async def test_rm_transfer_encoding_rfc_9112_6_3_http_11(status: int) -> None:
     """Remove transfer encoding for RFC 9112 sec 6.3 with HTTP/1.1."""