[Python-checkins] gh-117657: Fix data race in `compare_unicode_unicode_threadsafe` (gh-131746)

[colesbury]

https://github.com/python/cpython/commit/5abff6960b4aecb0d5c81c7482cf3faa74e1983d
commit: 5abff6960b4aecb0d5c81c7482cf3faa74e1983d
branch: main
author: Sam Gross <colesb...@gmail.com>
committer: colesbury <colesb...@gmail.com>
date: 2025-03-26T09:01:57-04:00
summary:

gh-117657: Fix data race in `compare_unicode_unicode_threadsafe` (gh-131746)

We can't safely check the type of the found key until we incref it or if we 
know that it's immortal.

files:
M Objects/dictobject.c

diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 614c12bb250ada..f302fc55f4b4d8 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -1412,18 +1412,20 @@ compare_unicode_unicode_threadsafe(PyDictObject *mp, 
PyDictKeysObject *dk,
 {
     PyDictUnicodeEntry *ep = &((PyDictUnicodeEntry *)ep0)[ix];
     PyObject *startkey = _Py_atomic_load_ptr_relaxed(&ep->me_key);
-    assert(startkey == NULL || PyUnicode_CheckExact(startkey));
     if (startkey == key) {
+        assert(PyUnicode_CheckExact(startkey));
         return 1;
     }
     if (startkey != NULL) {
         if (_Py_IsImmortal(startkey)) {
+            assert(PyUnicode_CheckExact(startkey));
             return unicode_get_hash(startkey) == hash && unicode_eq(startkey, 
key);
         }
         else {
             if (!_Py_TryIncrefCompare(&ep->me_key, startkey)) {
                 return DKIX_KEY_CHANGED;
             }
+            assert(PyUnicode_CheckExact(startkey));
             if (unicode_get_hash(startkey) == hash && unicode_eq(startkey, 
key)) {
                 Py_DECREF(startkey);
                 return 1;

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