[3.9] bpo-41654: Fix deallocator of MemoryError to account for subclasses (GH-22020) (GH-22045)

When allocating MemoryError classes, there is some logic to use
pre-allocated instances in a freelist only if the type that is being
allocated is not a subclass of MemoryError. Unfortunately in the
destructor this logic is not present so the freelist is altered even
with subclasses of MemoryError..
(cherry picked from commit 9b648a95ccb4c3b14f1e87158f5c9f5dbb2f62c0)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

1 files changed, 12 insertions, 2 deletions

diff --git a/Objects/exceptions.c b/Objects/exceptions.c
index db5e3da..e44ce72 100644
--- a/Objects/exceptions.c
+++ b/Objects/exceptions.c
@@ -2282,8 +2282,12 @@ MemoryError_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 {
     PyBaseExceptionObject *self;
 
-    if (type != (PyTypeObject *) PyExc_MemoryError)
+    /* If this is a subclass of MemoryError, don't use the freelist
+     * and just return a fresh object */
+    if (type != (PyTypeObject *) PyExc_MemoryError) {
         return BaseException_new(type, args, kwds);
+    }
+
     if (memerrors_freelist == NULL)
         return BaseException_new(type, args, kwds);
     /* Fetch object from freelist and revive it */
@@ -2303,8 +2307,14 @@ MemoryError_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 static void
 MemoryError_dealloc(PyBaseExceptionObject *self)
 {
-    _PyObject_GC_UNTRACK(self);
     BaseException_clear(self);
+
+    if (!Py_IS_TYPE(self, (PyTypeObject *) PyExc_MemoryError)) {
+        return Py_TYPE(self)->tp_free((PyObject *)self);
+    }
+
+    _PyObject_GC_UNTRACK(self);
+
     if (memerrors_numfree >= MEMERRORS_SAVE)
         Py_TYPE(self)->tp_free((PyObject *)self);
     else {