fix: drop body framing on HEAD/1xx/204/304 in WSGI responses

Mirror the ASGI strip-and-warn behavior (commits 2191832b, 41ec7527,
0d35d2ae) on the WSGI path. Previously gunicorn would forward an
app-supplied Content-Length and body bytes for HEAD requests and
1xx/204/304 responses, violating RFC 9110 / RFC 9112.

- Add _response_omits_body() and _response_forbids_content_length()
  helpers on Response.
- After process_headers, strip Content-Length and clear
  response_length on 1xx/204 (RFC 9110 §6.4.2 forbids it). HEAD and
  304 keep app-supplied Content-Length.
- write() and sendfile() drop body bytes for no-body responses and
  log a single WARNING per request.
- is_chunked() now also covers 1xx via _omits_body.

Fixes #3413

gunicorn/http/wsgi.py

@@ -300,6 +300,8 @@ class Response:
         self.sent = 0
         self.upgrade = False
         self.cfg = cfg
+        self._omits_body = False
+        self._omits_body_warned = False
 
     def force_close(self):
         self.must_close = True
@@ -336,9 +338,34 @@ class Response:
             self.status_code = None
 
         self.process_headers(headers)
+        self._omits_body = self._response_omits_body(
+            self.req.method, self.status_code)
+        if self._omits_body and self._response_forbids_content_length(
+                self.status_code):
+            self.headers = [
+                (k, v) for k, v in self.headers if k.lower() != "content-length"
+            ]
+            self.response_length = None
         self.chunked = self.is_chunked()
         return self.write
 
+    @staticmethod
+    def _response_omits_body(method, status):
+        # RFC 9110: HEAD requests and 1xx/204/304 responses MUST NOT carry
+        # a body, regardless of what the application emits.
+        return (
+            method == "HEAD"
+            or status in (204, 304)
+            or (status is not None and 100 <= status < 200)
+        )
+
+    @staticmethod
+    def _response_forbids_content_length(status):
+        # RFC 9110 §6.4.2: a server MUST NOT send Content-Length on 1xx or
+        # 204. HEAD MAY include the Content-Length the same GET would carry,
+        # and 304 MAY include the Content-Length of the unconditional response.
+        return status == 204 or (status is not None and 100 <= status < 200)
+
     def process_headers(self, headers):
         for name, value in headers:
             if not isinstance(name, str):
@@ -379,12 +406,8 @@ class Response:
             return False
         elif self.req.version <= (1, 0):
             return False
-        elif self.req.method == 'HEAD':
-            # Responses to a HEAD request MUST NOT contain a response body.
-            return False
-        elif self.status_code in (204, 304):
-            # Do not use chunked responses when the response is guaranteed to
-            # not have a response body.
+        elif self._omits_body:
+            # No body permitted (HEAD or 1xx/204/304), so no chunked framing.
             return False
         return True
 
@@ -422,6 +445,15 @@ class Response:
         self.send_headers()
         if not isinstance(arg, bytes):
             raise TypeError('%r is not a byte' % arg)
+        if self._omits_body:
+            if arg and not self._omits_body_warned:
+                log.warning(
+                    "WSGI app sent body bytes on a no-body response "
+                    "(method=%s status=%s); dropping per RFC 9110.",
+                    self.req.method, self.status_code,
+                )
+                self._omits_body_warned = True
+            return
         arglen = len(arg)
         tosend = arglen
         if self.response_length is not None:
@@ -448,6 +480,17 @@ class Response:
         if self.cfg.is_ssl or not self.can_sendfile():
             return False
 
+        if self._omits_body:
+            self.send_headers()
+            if not self._omits_body_warned:
+                log.warning(
+                    "WSGI app sent body bytes on a no-body response "
+                    "(method=%s status=%s); dropping per RFC 9110.",
+                    self.req.method, self.status_code,
+                )
+                self._omits_body_warned = True
+            return True
+
         if not util.has_fileno(respiter.filelike):
             return False
 

tests/test_http.py

@@ -371,3 +371,69 @@ def test_file_wrapper_iterable():
     wrapper2 = FileWrapper(filelike2, blksize=2)
     chunks = list(wrapper2)
     assert chunks == [b"ab", b"c"]
+
+
+def _make_response(method="GET", version=(1, 1)):
+    sock = mock.MagicMock()
+    req = mock.MagicMock()
+    req.method = method
+    req.version = version
+    req.should_close.return_value = False
+    cfg = mock.MagicMock()
+    cfg.is_ssl = False
+    cfg.sendfile = False
+    return Response(req, sock, cfg), sock
+
+
+@pytest.mark.parametrize("status,method,expect_cl", [
+    ("204 No Content", "GET", False),
+    ("100 Continue", "GET", False),
+    ("199 Custom", "GET", False),
+    ("304 Not Modified", "GET", True),
+    ("200 OK", "HEAD", True),
+])
+def test_no_body_response_strips_framing(status, method, expect_cl):
+    """1xx/204 strip Content-Length; HEAD/304 keep app-supplied Content-Length."""
+    resp, _ = _make_response(method=method)
+    body_len = 12
+    resp.start_response(status, [
+        ("Content-Type", "text/plain"),
+        ("Content-Length", str(body_len)),
+    ])
+    header_keys = [k.lower() for k, _ in resp.headers]
+    if expect_cl:
+        assert "content-length" in header_keys
+        assert resp.response_length == body_len
+    else:
+        assert "content-length" not in header_keys
+        assert resp.response_length is None
+    assert resp.chunked is False
+    assert resp._omits_body is True
+
+
+def test_no_body_response_drops_body_and_warns(caplog):
+    resp, sock = _make_response(method="GET")
+    resp.start_response("204 No Content", [
+        ("Content-Type", "text/plain"),
+        ("Content-Length", "5"),
+    ])
+    with caplog.at_level("WARNING", logger="gunicorn.http.wsgi"):
+        resp.write(b"hello")
+        resp.write(b"again")
+    assert resp.sent == 0
+    assert sum(
+        1 for r in caplog.records
+        if "no-body response" in r.getMessage()
+    ) == 1
+
+
+def test_normal_response_unaffected():
+    resp, _ = _make_response(method="GET")
+    resp.start_response("200 OK", [
+        ("Content-Type", "text/plain"),
+        ("Content-Length", "5"),
+    ])
+    assert resp._omits_body is False
+    assert resp.response_length == 5
+    resp.write(b"hello")
+    assert resp.sent == 5