bpo-46006: Fix _PyUnicode_EqualToASCIIId() for interned strings

Misc/NEWS.d/next/Core and Builtins/2021-12-15-15-30-04.bpo-46006.T4pbJR.rst

@@ -0,0 +1,4 @@
+Fix string comparisons for subinterpreters: no longer make the assumption
+that two interned strings are not equal if they have different different
+memory addresses. This assumption is no longer true since interned strings have
+been made per interpreter in Python 3.10. Patch by Victor Stinner.

Objects/typeobject.c

@@ -8459,7 +8459,10 @@ update_slot(PyTypeObject *type, PyObject *name)
     for (p = slotdefs; p->name; p++) {
         assert(PyUnicode_CheckExact(p->name_strobj));
         assert(PyUnicode_CheckExact(name));
-        if (p->name_strobj == name) {
+        // bpo-46006: subinterpreters require to compare strings contents, even
+        // if both strings are interned. _PyUnicode_EQ() is required to keep
+        // support for built-in static types in subinterpreters.
+        if (p->name_strobj == name || _PyUnicode_EQ(p->name_strobj, name)) {
             *pp++ = p;
         }
     }

Objects/unicodeobject.c

@@ -11336,11 +11336,18 @@ _PyUnicode_EqualToASCIIId(PyObject *left, _Py_Identifier *right)
         return _PyUnicode_EqualToASCIIString(left, right->string);
     }
 
-    if (left == right_uni)
+    if (left == right_uni) {
         return 1;
-
-    if (PyUnicode_CHECK_INTERNED(left))
-        return 0;
+    }
+    // bpo-46006: The left string cannot be considered as not equal to the
+    // right string if the left string is interned, because the two string
+    // objects can belong to two interpreters.
+    //
+    // While an interpreter is supposed to only access objects that it created
+    // (bpo-40533), in practice in Python 3.11, it remains common that a
+    // subinterpreter access objects of the main interprter. For example,
+    // access attribute names (strings) of static types created by the main
+    // interpreter.
 
     assert(_PyUnicode_HASH(right_uni) != -1);
     Py_hash_t hash = _PyUnicode_HASH(left);