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

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.

Lib/test/test_exceptions.py

@@ -1,6 +1,7 @@
 # Python test set -- part 5, built-in exceptions
 
 import copy
+import gc
 import os
 import sys
 import unittest
@@ -1330,6 +1331,36 @@ class ExceptionTests(unittest.TestCase):
             del AssertionError
             self.fail('Expected exception')
 
+    def test_memory_error_subclasses(self):
+        # bpo-41654: MemoryError instances use a freelist of objects that are
+        # linked using the 'dict' attribute when they are inactive/dead.
+        # Subclasses of MemoryError should not participate in the freelist
+        # schema. This test creates a MemoryError object and keeps it alive
+        # (therefore advancing the freelist) and then it creates and destroys a
+        # subclass object. Finally, it checks that creating a new MemoryError
+        # succeeds, proving that the freelist is not corrupted.
+
+        class TestException(MemoryError):
+            pass
+
+        try:
+            raise MemoryError
+        except MemoryError as exc:
+            inst = exc
+
+        try:
+            raise TestException
+        except Exception:
+            pass
+
+        for _ in range(10):
+            try:
+                raise MemoryError
+            except MemoryError as exc:
+                pass
+
+            gc_collect()
+
 
 class ImportErrorTests(unittest.TestCase):
 

Misc/NEWS.d/next/Core and Builtins/2020-08-30-20-38-33.bpo-41654.HtnhAM.rst

@@ -0,0 +1,2 @@
+Fix a crash that occurred when destroying subclasses of
+:class:`MemoryError`. Patch by Pablo Galindo.

Objects/exceptions.c

@@ -2286,8 +2286,11 @@ 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);
+    }
 
     struct _Py_exc_state *state = get_exc_state();
     if (state->memerrors_freelist == NULL) {
@@ -2313,9 +2316,16 @@ MemoryError_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 static void
 MemoryError_dealloc(PyBaseExceptionObject *self)
 {
-    _PyObject_GC_UNTRACK(self);
     BaseException_clear(self);
 
+    /* If this is a subclass of MemoryError, we don't need to
+     * do anything in the free-list*/
+    if (!Py_IS_TYPE(self, (PyTypeObject *) PyExc_MemoryError)) {
+        return Py_TYPE(self)->tp_free((PyObject *)self);
+    }
+
+    _PyObject_GC_UNTRACK(self);
+
     struct _Py_exc_state *state = get_exc_state();
     if (state->memerrors_numfree >= MEMERRORS_SAVE) {
         Py_TYPE(self)->tp_free((PyObject *)self);