gh-156166: Fix setting and deleting SSLContext._msg_callback (GH-156167)

The setter released the old callback before validating the new value,
so a failed assignment or a deletion removed it.

(cherry picked from commit 67f4d53)

Author: serhiy-storchaka

Lib/test/test_ssl.py

@@ -5266,6 +5266,18 @@ def msg_cb(conn, direction, version, content_type, msg_type, data):
         with self.assertRaises(TypeError):
             client_context._msg_callback = object()
 
+        # the attribute of the underlying C type accepts only a callable
+        # and cannot be deleted
+        descr = _ssl._SSLContext.__dict__['_msg_callback']
+        with self.assertRaises(TypeError):
+            descr.__set__(client_context, object())
+        # a failed assignment does not change the value
+        self.assertIs(client_context._msg_callback, msg_cb)
+        with self.assertRaisesRegex(AttributeError, 'cannot be deleted'):
+            descr.__delete__(client_context)
+        # a failed deletion does not change the value
+        self.assertIs(client_context._msg_callback, msg_cb)
+
     def test_msg_callback_tls12(self):
         client_context, server_context, hostname = testing_context()
         client_context.maximum_version = ssl.TLSVersion.TLSv1_2

Misc/NEWS.d/next/Library/2026-08-21-13-30-00.gh-issue-156166.Xv8pQm.rst

@@ -0,0 +1,3 @@
+:mod:`ssl`: A failed assignment or deletion of the ``_msg_callback``
+attribute of :class:`ssl.SSLContext` no longer removes the current callback.
+Deleting it now raises :exc:`AttributeError` instead of :exc:`TypeError`.

Modules/_ssl/debughelpers.c

@@ -100,20 +100,28 @@ _PySSLContext_set_msg_callback(PyObject *op, PyObject *arg,
                                void *Py_UNUSED(closure))
 {
     PySSLContext *self = PySSLContext_CAST(op);
-    Py_CLEAR(self->msg_cb);
+    if (arg == NULL) {
+        PyErr_Format(PyExc_AttributeError,
+                     "attribute '_msg_callback' of '%.100s' objects "
+                     "cannot be deleted", Py_TYPE(op)->tp_name);
+        return -1;
+    }
+    if (arg != Py_None && !PyCallable_Check(arg)) {
+        PyErr_SetString(PyExc_TypeError,
+                        "not a callable object");
+        return -1;
+    }
+    /* Releasing the old callback can run arbitrary code. */
+    PyObject *old_cb = self->msg_cb;
     if (arg == Py_None) {
+        self->msg_cb = NULL;
         SSL_CTX_set_msg_callback(self->ctx, NULL);
     }
     else {
-        if (!PyCallable_Check(arg)) {
-            SSL_CTX_set_msg_callback(self->ctx, NULL);
-            PyErr_SetString(PyExc_TypeError,
-                            "not a callable object");
-            return -1;
-        }
         self->msg_cb = Py_NewRef(arg);
         SSL_CTX_set_msg_callback(self->ctx, _PySSL_msg_callback);
     }
+    Py_XDECREF(old_cb);
     return 0;
 }