8000 bpo-41654: Fix deallocator of MemoryError to account for subclasses (… · python/cpython@9b648a9 · GitHub
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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.
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lines changed

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‎Lib/test/test_exceptions.py

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@@ -1,6 +1,7 @@
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# Python test set -- part 5, built-in exceptions
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import copy
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import gc
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import os
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import sys
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import unittest
@@ -1330,6 +1331,36 @@ def test_assert_shadowing(self):
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del AssertionError
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self.fail('Expected exception')
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def test_memory_error_subclasses(self):
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# bpo-41654: MemoryError instances use a freelist of objects that are
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# linked using the 'dict' attribute when they are inactive/dead.
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# Subclasses of MemoryError should not participate in the freelist
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# schema. This test creates a MemoryError object and keeps it alive
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# (therefore advancing the freelist) and then it creates and destroys a
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# subclass object. Finally, it checks that creating a new MemoryError
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# succeeds, proving that the freelist is not corrupted.
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class TestException(MemoryError):
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pass
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try:
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raise MemoryError
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except MemoryError as exc:
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inst = exc
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try:
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raise TestException
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except Exception:
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pass
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for _ in range(10):
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try:
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raise MemoryError
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except MemoryError as exc:
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pass
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gc_collect()
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class ImportErrorTests(unittest.TestCase):
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‎Misc/NEWS.d/next/Core and Builtins/2020-08-30-20-38-33.bpo-41654.HtnhAM.rst

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Fix a crash that occurred when destroying subclasses of
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:class:`MemoryError`. Patch by Pablo Galindo.

‎Objects/exceptions.c

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

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