gh-157951: Don't read a chunked body for 1xx, 204 and 304 responses in http.client

Lib/http/client.py

@@ -408,6 +408,9 @@ def begin(self, *, _max_headers=None):
             100 <= status < 200 or      # 1xx codes
             self._method == "HEAD"):
             self.length = 0
+            # These responses never have a body, so a Transfer-Encoding
+            # header must not make read() wait for chunks (RFC 9112 6.3).
+            self.chunked = False
 
         # if the connection remains open, and we aren't using chunked, and
         # a content-length was not provided, then assume that the connection

Lib/test/test_httplib.py

@@ -1548,6 +1548,20 @@ def test_chunked_sync(self):
         self.assertEqual(sock.file.read(), extradata.encode("ascii")) #we read to the end
         resp.close()
 
+    def test_bodiless_status_with_chunked_sync(self):
+        """1xx, 204 and 304 responses have no body even with Transfer-Encoding: chunked"""
+        extradata = b"HTTP/1.1 200 OK\r\nContent-Length: 2\r\n\r\nok"
+        for status in (b"204 No Content", b"304 Not Modified"):
+            with self.subTest(status=status):
+                sock = FakeSocket(b"HTTP/1.1 " + status +
+                                  b"\r\nTransfer-Encoding: chunked\r\n\r\n" + extradata)
+                resp = client.HTTPResponse(sock, method="GET")
+                resp.begin()
+                self.assertEqual(resp.read(), b"")
+                # the next response must still be unread
+                self.assertEqual(sock.file.read(), extradata)
+                resp.close()
+
     def test_content_length_sync(self):
         """Check that we don't read past the end of the Content-Length stream"""
         extradata = b"extradata"

Misc/NEWS.d/next/Library/2026-09-22-13-43-27.gh-issue-157951.5xqNEQ.rst

@@ -0,0 +1,3 @@
+Fix :mod:`http.client` blocking in :meth:`~http.client.HTTPResponse.read` when
+a 1xx, 204 or 304 response has a ``Transfer-Encoding: chunked`` header. Such
+responses never have a body.