Remove Transfer-Encoding for empty body response per RFC 9112 section…

… 6.3

aiohttp/web_response.py

@@ -364,7 +364,31 @@ async def _prepare_headers(self) -> None:
         if self._compression:
             await self._start_compression(request)
 
-        if self._chunked:
+        if self.status in (204, 304) or 100 >= self.status < 200:
+            #
+            # Remove transfer-encoding since there is no body
+            # and this can confuse some clients (e.g. older aiohttp)
+            #
+            # https://datatracker.ietf.org/doc/html/rfc9112#section-6.3
+            #
+            # Any response to a HEAD request and any response with
+            # a 1xx (Informational), 204 (No Content), or 304
+            # (Not Modified) status code is always terminated by
+            # the first empty line after the header fields,
+            # regardless of the header fields present in the message,
+            # and thus cannot contain a message body or trailer section.
+            #
+            if hdrs.TRANSFER_ENCODING in headers:
+                del headers[hdrs.TRANSFER_ENCODING]
+            # HTTP 1.0: https://tools.ietf.org/html/rfc1945#section-10.4
+            # 0 is not a valid value for Content-Length header for HTTP/1.0
+            if hdrs.CONTENT_LENGTH in headers:
+                del headers[hdrs.CONTENT_LENGTH]
+            # HTTP 1.1: https://tools.ietf.org/html/rfc7230#section-3.3.2
+            # 0 is a valid value for Content-Length header for HTTP/1.1
+            if version >= HttpVersion11:
+                headers[hdrs.CONTENT_LENGTH] = "0"
+        elif self._chunked:
             if version != HttpVersion11:
                 raise RuntimeError(
                     "Using chunked encoding is forbidden "

tests/test_web_response.py

@@ -624,6 +624,22 @@ async def write_headers(status_line, headers):
     assert resp.content_length is None
 
 
+async def test_rm_transfer_encoding_rfc_9112_6_3() -> None:
+    """Remove transfer encoding for RFC 9112 sec 6.3."""
+    writer = mock.Mock()
+
+    async def write_headers(status_line, headers):
+        assert hdrs.CONTENT_LENGTH not in headers
+        assert hdrs.TRANSFER_ENCODING not in headers
+
+    writer.write_headers.side_effect = write_headers
+    req = make_request("GET", "/", version=HttpVersion11, writer=writer)
+    resp = Response(body=BytesPayload(b"answer"))
+    resp.enable_compression(ContentCoding.gzip)
+    await resp.prepare(req)
+    assert resp.content_length is None
+
+
 async def test_content_length_on_chunked() -> None:
     req = make_request("GET", "/")
     resp = Response(body=b"answer")