Revert "bpo-40521: Make the empty frozenset per interpreter (pythonGH-21068)"

This reverts commit 261cfed.

Author: rhettinger

Include/internal/pycore_interp.h

@@ -244,8 +244,6 @@ struct _is {
     /* Using a cache is very effective since typically only a single slice is
        created and then deleted again. */
     PySliceObject *slice_cache;
-    // The empty frozenset is a singleton.
-    PyObject *empty_frozenset;
 
     struct _Py_tuple_state tuple;
     struct _Py_list_state list;

Include/internal/pycore_pylifecycle.h

@@ -62,7 +62,6 @@ extern void _PyFrame_Fini(PyThreadState *tstate);
 extern void _PyDict_Fini(PyThreadState *tstate);
 extern void _PyTuple_Fini(PyThreadState *tstate);
 extern void _PyList_Fini(PyThreadState *tstate);
-extern void _PySet_Fini(PyThreadState *tstate);
 extern void _PyBytes_Fini(PyThreadState *tstate);
 extern void _PyFloat_Fini(PyThreadState *tstate);
 extern void _PySlice_Fini(PyThreadState *tstate);

Misc/NEWS.d/next/Core and Builtins/2020-05-20-01-17-34.bpo-40521.wvAehI.rst

@@ -2,8 +2,9 @@ Each interpreter now its has own free lists, singletons and caches:
 
 * Free lists: float, tuple, list, dict, frame, context,
   asynchronous generator.
-* Singletons: empty tuple, empty frozenset, empty bytes string,
+* Singletons: empty tuple, empty bytes string,
   single byte character.
 * Slice cache.
 
 They are no longer shared by all interpreters.
+

Objects/setobject.c

@@ -975,11 +975,12 @@ make_new_set_basetype(PyTypeObject *type, PyObject *iterable)
     return make_new_set(type, iterable);
 }
 
+/* The empty frozenset is a singleton */
+static PyObject *emptyfrozenset = NULL;
+
 static PyObject *
 make_new_frozenset(PyTypeObject *type, PyObject *iterable)
 {
-    PyObject *res;
-
     if (type != &PyFrozenSet_Type) {
         return make_new_set(type, iterable);
     }
@@ -990,7 +991,7 @@ make_new_frozenset(PyTypeObject *type, PyObject *iterable)
             Py_INCREF(iterable);
             return iterable;
         }
-        res = make_new_set((PyTypeObject *)type, iterable);
+        PyObject *res = make_new_set((PyTypeObject *)type, iterable);
         if (res == NULL || PySet_GET_SIZE(res) != 0) {
             return res;
         }
@@ -999,17 +1000,11 @@ make_new_frozenset(PyTypeObject *type, PyObject *iterable)
     }
 
     // The empty frozenset is a singleton
-    PyInterpreterState *interp = _PyInterpreterState_GET();
-    res = interp->empty_frozenset;
-    if (res == NULL) {
-        interp->empty_frozenset = make_new_set((PyTypeObject *)type, NULL);
-        res = interp->empty_frozenset;
-        if (res == NULL) {
-            return NULL;
-        }
+    if (emptyfrozenset == NULL) {
+        emptyfrozenset = make_new_set((PyTypeObject *)type, NULL);
     }
-    Py_INCREF(res);
-    return res;
+    Py_XINCREF(emptyfrozenset);
+    return emptyfrozenset;
 }
 
 static PyObject *
@@ -2304,12 +2299,6 @@ PySet_Add(PyObject *anyset, PyObject *key)
     return set_add_key((PySetObject *)anyset, key);
 }
 
-void
-_PySet_Fini(PyThreadState *tstate)
-{
-    Py_CLEAR(tstate->interp->empty_frozenset);
-}
-
 int
 _PySet_NextEntry(PyObject *set, Py_ssize_t *pos, PyObject **key, Py_hash_t *hash)
 {

Python/pylifecycle.c

@@ -1253,7 +1253,9 @@ finalize_interp_types(PyThreadState *tstate, int is_main_interp)
     _PyAsyncGen_Fini(tstate);
     _PyContext_Fini(tstate);
 
-    _PySet_Fini(tstate);
+    if (is_main_interp) {
+        _PySet_Fini();
+    }
     _PyDict_Fini(tstate);
     _PyList_Fini(tstate);
     _PyTuple_Fini(tstate);