Make PyDictOrValues thread safe

DinoV · DinoV · commit 75c4227b9b1c · 2024-01-29T16:51:39.000-08:00
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
@@ -5540,6 +5540,9 @@ make_dict_from_instance_attributes(PyInterpreterState *interp,
             track += _PyObject_GC_MAY_BE_TRACKED(val);
         }
     }
+    // free-threading TODO: We'll need to mark the values to be freed via qsbr
+    // so that a resize doesn't free them immediately while another thread
+    // still has a reference to the old values.
     PyObject *res = new_dict(interp, keys, values, used, 0);
     if (track && res) {
         _PyObject_GC_TRACK(res);
@@ -5572,19 +5575,26 @@ _PyObject_MakeInstanceAttributesFromDict(PyObject *obj, PyDictOrValues *dorv)
         return false;
     }
     assert(_PyType_HasFeature(Py_TYPE(obj), Py_TPFLAGS_HEAPTYPE));
+    bool success = true;
+    Py_BEGIN_CRITICAL_SECTION(dict);
     if (dict->ma_keys != CACHED_KEYS(Py_TYPE(obj)) || Py_REFCNT(dict) != 1) {
-        return false;
+        success = false;
+        goto exit;
     }
     assert(dict->ma_values);
     // We have an opportunity to do something *really* cool: dematerialize it!
     _PyDictKeys_DecRef(dict->ma_keys);
     _PyDictOrValues_SetValues(dorv, dict->ma_values);
     OBJECT_STAT_INC(dict_dematerialized);
+    // Lock free readers will need to contend with NULL values here, but
+    // because we locked the dict anyone else will be safe.
     // Don't try this at home, kids:
     dict->ma_keys = NULL;
     dict->ma_values = NULL;
     Py_DECREF(dict);
-    return true;
+exit:
+    Py_END_CRITICAL_SECTION();
+    return success;
 }
 
 int
