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bpo-25782: Prevent cycles in the __context__ chain. #20543

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bpo-25782: Prevent cycles in the __context__ chain. #20543

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4 changes: 4 additions & 0 deletions Doc/c-api/exceptions.rst
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Expand Up @@ -601,6 +601,10 @@ Exception Objects
it. There is no type check to make sure that *ctx* is an exception instance.
This steals a reference to *ctx*.

.. versionchanged:: 3.5.2
If *ctx* is *ex*, the function has no any effect. If *ex* is in the
context chain started from *ctx*, it is moved to the start of the chain.
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@pablogsal pablogsal Apr 23, 2021

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Suggested change
If *ctx* is *ex*, the function has no any effect. If *ex* is in the
context chain started from *ctx*, it is moved to the start of the chain.
If *ctx* is *ex*, the function has no effect. If *ex* is in the
context chain started from *ctx*, it is moved to the start of the chain.



.. c:function:: PyObject* PyException_GetCause(PyObject *ex)

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6 changes: 6 additions & 0 deletions Doc/library/exceptions.rst
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Expand Up @@ -66,6 +66,12 @@ In either case, the exception itself is always shown after any chained
exceptions so that the final line of the traceback always shows the last
exception that was raised.

.. versionchanged:: 3.5.2

Setting :attr:`__context__` to self has no any effect. If set
``__context__`` to the chain contained original exception, it is moved to
the start of the context chain.
Comment on lines +71 to +73
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@pablogsal pablogsal Apr 23, 2021

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Suggested change
Setting :attr:`__context__` to self has no any effect. If set
``__context__`` to the chain contained original exception, it is moved to
the start of the context chain.
Setting :attr:`__context__` to self has no effect. If set
``__context__`` to the chain contained an original exception, it is moved to
the start of the context chain.



Base classes
------------
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20 changes: 20 additions & 0 deletions Lib/test/test_exceptions.py
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Expand Up @@ -716,6 +716,26 @@ def __exit__ (self, exc_type, exc_value, exc_tb):
obj = wr()
self.assertIsNone(obj)

def test_context_loop(self):
exc1 = Exception(1)
exc1.__context__ = exc1
self.assertIsNone(exc1.__context__)

exc2 = Exception(2)
exc2.__context__ = exc1
self.assertIs(exc2.__context__, exc1)
exc1.__context__ = exc2
self.assertIs(exc1.__context__, exc2)
self.assertIsNone(exc2.__context__)

exc3 = Exception(3)
exc3.__context__ = exc1
self.assertIs(exc3.__context__, exc1)
exc1.__context__ = exc3
self.assertIs(exc1.__context__, exc3)
self.assertIs(exc3.__context__, exc2)
self.assertIsNone(exc2.__context__)

def test_exception_target_in_nested_scope(self):
# issue 4617: This used to raise a SyntaxError
# "can not delete variable 'e' referenced in nested scope"
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26 changes: 25 additions & 1 deletion Objects/exceptions.c
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Expand Up @@ -356,9 +356,33 @@ PyException_GetContext(PyObject *self)
void
PyException_SetContext(PyObject *self, PyObject *context)
{
Py_XSETREF(_PyBaseExceptionObject_cast(self)->context, context);
assert(PyExceptionInstance_Check(self));
assert(context == NULL || PyExceptionInstance_Check(context));

PyObject *exc = context;
PyObject *prev_exc = NULL;

while (exc != NULL) {
if (exc == self) {
if (prev_exc != NULL) {
/* Move self to the start of the chain. */
((PyBaseExceptionObject *)prev_exc)->context =
((PyBaseExceptionObject *)self)->context;
((PyBaseExceptionObject *)self)->context = context;
}
assert(Py_REFCNT(self) > 1);
Py_DECREF(self);
return;
}
assert(PyExceptionInstance_Check(exc));
prev_exc = exc;
exc = ((PyBaseExceptionObject *)exc)->context;
}

Py_XSETREF(((PyBaseExceptionObject *)self)->context, context);
}


#undef PyExceptionClass_Name

const char *
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