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bpo-25782: avoid hang in PyErr_SetObject when current exception has a cycle in its context chain (GH-27626)

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Co-authored-by: Dennis Sweeney 36520290+sweeneyde@users.noreply.github.com
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Irit Katriel 2021-08-10 10:37:25 +01:00 • committed by GitHub
parent 6b37d0d530
commit d5c217475c
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3 changed files with 158 additions and 1 deletions
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16
Python/errors.c
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@ -148,12 +148,16 @@ _PyErr_SetObject(PyThreadState *tstate, PyObject *exception, PyObject *value)
value = fixed_value;
}
/* Avoid reference cycles through the context chain.
/* Avoid creating new reference cycles through the
context chain, while taking care not to hang on
pre-existing ones.
This is O(chain length) but context chains are
usually very short. Sensitive readers may try
to inline the call to PyException_GetContext. */
if (exc_value != value) {
PyObject *o = exc_value, *context;
PyObject *slow_o = o; /* Floyd's cycle detection algo */
int slow_update_toggle = 0;
while ((context = PyException_GetContext(o))) {
Py_DECREF(context);
if (context == value) {
@ -161,6 +165,16 @@ _PyErr_SetObject(PyThreadState *tstate, PyObject *exception, PyObject *value)
break;
}
o = context;
if (o == slow_o) {
/* pre-existing cycle - all exceptions on the
path were visited and checked. */
break;
}
if (slow_update_toggle) {
slow_o = PyException_GetContext(slow_o);
Py_DECREF(slow_o);
}
slow_update_toggle = !slow_update_toggle;
}
PyException_SetContext(value, exc_value);
}