Normalize multiple Transfer-Encoding headers before chunked detection

aiohttp/http_parser.py

@@ -584,8 +584,11 @@ def parse_headers(
             encoding = enc
 
         # chunking
-        te = headers.get(hdrs.TRANSFER_ENCODING)
-        if te is not None:
+        te_values = headers.getall(hdrs.TRANSFER_ENCODING, ())
+        if te_values:
+            # RFC 9110 §5.3: Multiple header fields are equivalent to a single
+            # comma-separated value. Normalize before determining message framing.
+            te = ",".join(v.strip() for v in te_values)
             if self._is_chunked_te(te):
                 chunked = True
 

tests/test_http_parser.py

@@ -399,6 +399,25 @@ def test_duplicate_singleton_header_accepted_in_lax_mode(
     assert len(messages) == 1
 
 
+async def test_response_te_split_headers_chunked(
+    response: HttpResponseParser,
+) -> None:
+    text = (
+        b"HTTP/1.1 200 OK\r\n"
+        b"Transfer-Encoding: gzip\r\n"
+        b"Transfer-Encoding: chunked\r\n"
+        b"\r\n"
+        b"4\r\nWiki\r\n0\r\n\r\n"
+        b"HTTP/1.1 204 No Content\r\n\r\n"
+    )
+    messages, upgrade, tail = response.feed_data(text)
+    assert len(messages) == 2
+    msg1, payload1 = messages[0]
+    assert msg1.chunked
+    assert await payload1.read() == b"Wiki"
+    assert messages[1][0].code == 204
+
+
 def test_duplicate_host_header_rejected(parser: HttpRequestParser) -> None:
     text = (
         b"GET /admin HTTP/1.1\r\n"