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bpo-25782: avoid hang in PyErr_SetObject when current exception has a cycle in its context chain (GH-27626)
Co-authored-by: Dennis Sweeney 36520290+sweeneyde@users.noreply.github.com
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‎Lib/test/test_exceptions.py‎

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@@ -953,6 +953,148 @@ def __del__(self):
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pass
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self.assertEqual(e, (None, None, None))
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def test_raise_does_not_create_context_chain_cycle(self):
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class A(Exception):
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pass
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class B(Exception):
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pass
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class C(Exception):
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pass
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# Create a context chain:
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# C -> B -> A
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# Then raise A in context of C.
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try:
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try:
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raise A
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except A as a_:
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a = a_
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try:
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raise B
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except B as b_:
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b = b_
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try:
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raise C
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except C as c_:
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c = c_
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self.assertIsInstance(a, A)
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self.assertIsInstance(b, B)
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self.assertIsInstance(c, C)
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self.assertIsNone(a.__context__)
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self.assertIs(b.__context__, a)
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self.assertIs(c.__context__, b)
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raise a
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except A as e:
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exc = e
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# Expect A -> C -> B, without cycle
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self.assertIs(exc, a)
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self.assertIs(a.__context__, c)
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self.assertIs(c.__context__, b)
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self.assertIsNone(b.__context__)
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def test_no_hang_on_context_chain_cycle1(self):
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# See issue 25782. Cycle in context chain.
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def cycle():
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try:
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raise ValueError(1)
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except ValueError as ex:
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ex.__context__ = ex
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raise TypeError(2)
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try:
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cycle()
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except Exception as e:
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exc = e
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self.assertIsInstance(exc, TypeError)
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self.assertIsInstance(exc.__context__, ValueError)
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self.assertIs(exc.__context__.__context__, exc.__context__)
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def test_no_hang_on_context_chain_cycle2(self):
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# See issue 25782. Cycle at head of context chain.
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class A(Exception):
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pass
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class B(Exception):
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pass
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class C(Exception):
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pass
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# Context cycle:
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# +-----------+
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# V |
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# C --> B --> A
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with self.assertRaises(C) as cm:
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try:
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raise A()
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except A as _a:
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a = _a
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try:
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raise B()
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except B as _b:
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b = _b
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try:
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raise C()
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except C as _c:
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c = _c
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a.__context__ = c
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raise c
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self.assertIs(cm.exception, c)
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# Verify the expected context chain cycle
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self.assertIs(c.__context__, b)
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self.assertIs(b.__context__, a)
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self.assertIs(a.__context__, c)
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def test_no_hang_on_context_chain_cycle3(self):
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# See issue 25782. Longer context chain with cycle.
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class A(Exception):
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pass
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class B(Exception):
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pass
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class C(Exception):
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pass
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class D(Exception):
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pass
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class E(Exception):
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pass
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# Context cycle:
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# +-----------+
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# V |
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# E --> D --> C --> B --> A
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with self.assertRaises(E) as cm:
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try:
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raise A()
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except A as _a:
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a = _a
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try:
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raise B()
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except B as _b:
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b = _b
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try:
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raise C()
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except C as _c:
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c = _c
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a.__context__ = c
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try:
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raise D()
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except D as _d:
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d = _d
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e = E()
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raise e
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self.assertIs(cm.exception, e)
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# Verify the expected context chain cycle
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self.assertIs(e.__context__, d)
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self.assertIs(d.__context__, c)
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self.assertIs(c.__context__, b)
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self.assertIs(b.__context__, a)
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self.assertIs(a.__context__, c)
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def test_unicode_change_attributes(self):
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# See issue 7309. This was a crasher.
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‎Misc/NEWS.d/next/Core and Builtins/2021-08-07-21-39-19.bpo-25782.B22lMx.rst‎

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1+
Fix bug where ``PyErr_SetObject`` hangs when the current exception has a cycle in its context chain.

‎Python/errors.c‎

Lines changed: 15 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -148,19 +148,33 @@ _PyErr_SetObject(PyThreadState *tstate, PyObject *exception, PyObject *value)
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value = fixed_value;
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}
150150

151-
/* Avoid reference cycles through the context chain.
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/* Avoid creating new reference cycles through the
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context chain, while taking care not to hang on
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pre-existing ones.
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This is O(chain length) but context chains are
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usually very short. Sensitive readers may try
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to inline the call to PyException_GetContext. */
155157
if (exc_value != value) {
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PyObject *o = exc_value, *context;
159+
PyObject *slow_o = o; /* Floyd's cycle detection algo */
160+
int slow_update_toggle = 0;
157161
while ((context = PyException_GetContext(o))) {
158162
Py_DECREF(context);
159163
if (context == value) {
160164
PyException_SetContext(o, NULL);
161165
break;
162166
}
163167
o = context;
168+
if (o == slow_o) {
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/* pre-existing cycle - all exceptions on the
170+
path were visited and checked. */
171+
break;
172+
}
173+
if (slow_update_toggle) {
174+
slow_o = PyException_GetContext(slow_o);
175+
Py_DECREF(slow_o);
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}
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slow_update_toggle = !slow_update_toggle;
164178
}
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PyException_SetContext(value, exc_value);
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}

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