diff options
| -rw-r--r-- | Lib/test/test_exceptions.py | 142 | ||||
| -rw-r--r-- | Misc/NEWS.d/next/Core and Builtins/2021-08-07-21-39-19.bpo-25782.B22lMx.rst | 1 | ||||
| -rw-r--r-- | Python/errors.c | 16 |
3 files changed, 158 insertions, 1 deletions
diff --git a/Lib/test/test_exceptions.py b/Lib/test/test_exceptions.py index 0f7b7f8..cd7050b 100644 --- a/Lib/test/test_exceptions.py +++ b/Lib/test/test_exceptions.py @@ -923,6 +923,148 @@ class ExceptionTests(unittest.TestCase): pass self.assertEqual(e, (None, None, None)) + def test_raise_does_not_create_context_chain_cycle(self): + class A(Exception): + pass + class B(Exception): + pass + class C(Exception): + pass + + # Create a context chain: + # C -> B -> A + # Then raise A in context of C. + try: + try: + raise A + except A as a_: + a = a_ + try: + raise B + except B as b_: + b = b_ + try: + raise C + except C as c_: + c = c_ + self.assertIsInstance(a, A) + self.assertIsInstance(b, B) + self.assertIsInstance(c, C) + self.assertIsNone(a.__context__) + self.assertIs(b.__context__, a) + self.assertIs(c.__context__, b) + raise a + except A as e: + exc = e + + # Expect A -> C -> B, without cycle + self.assertIs(exc, a) + self.assertIs(a.__context__, c) + self.assertIs(c.__context__, b) + self.assertIsNone(b.__context__) + + def test_no_hang_on_context_chain_cycle1(self): + # See issue 25782. Cycle in context chain. + + def cycle(): + try: + raise ValueError(1) + except ValueError as ex: + ex.__context__ = ex + raise TypeError(2) + + try: + cycle() + except Exception as e: + exc = e + + self.assertIsInstance(exc, TypeError) + self.assertIsInstance(exc.__context__, ValueError) + self.assertIs(exc.__context__.__context__, exc.__context__) + + def test_no_hang_on_context_chain_cycle2(self): + # See issue 25782. Cycle at head of context chain. + + class A(Exception): + pass + class B(Exception): + pass + class C(Exception): + pass + + # Context cycle: + # +-----------+ + # V | + # C --> B --> A + with self.assertRaises(C) as cm: + try: + raise A() + except A as _a: + a = _a + try: + raise B() + except B as _b: + b = _b + try: + raise C() + except C as _c: + c = _c + a.__context__ = c + raise c + + self.assertIs(cm.exception, c) + # Verify the expected context chain cycle + self.assertIs(c.__context__, b) + self.assertIs(b.__context__, a) + self.assertIs(a.__context__, c) + + def test_no_hang_on_context_chain_cycle3(self): + # See issue 25782. Longer context chain with cycle. + + class A(Exception): + pass + class B(Exception): + pass + class C(Exception): + pass + class D(Exception): + pass + class E(Exception): + pass + + # Context cycle: + # +-----------+ + # V | + # E --> D --> C --> B --> A + with self.assertRaises(E) as cm: + try: + raise A() + except A as _a: + a = _a + try: + raise B() + except B as _b: + b = _b + try: + raise C() + except C as _c: + c = _c + a.__context__ = c + try: + raise D() + except D as _d: + d = _d + e = E() + raise e + + self.assertIs(cm.exception, e) + # Verify the expected context chain cycle + self.assertIs(e.__context__, d) + self.assertIs(d.__context__, c) + self.assertIs(c.__context__, b) + self.assertIs(b.__context__, a) + self.assertIs(a.__context__, c) + def test_unicode_change_attributes(self): # See issue 7309. This was a crasher. diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-08-07-21-39-19.bpo-25782.B22lMx.rst b/Misc/NEWS.d/next/Core and Builtins/2021-08-07-21-39-19.bpo-25782.B22lMx.rst new file mode 100644 index 0000000..1c52059 --- /dev/null +++ b/Misc/NEWS.d/next/Core and Builtins/2021-08-07-21-39-19.bpo-25782.B22lMx.rst @@ -0,0 +1 @@ +Fix bug where ``PyErr_SetObject`` hangs when the current exception has a cycle in its context chain.
\ No newline at end of file diff --git a/Python/errors.c b/Python/errors.c index 2c020cd..8a2ba8f 100644 --- a/Python/errors.c +++ b/Python/errors.c @@ -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); } |