Expand Up @@ -98,6 +98,16 @@ _PyErr_CreateException(PyObject *exception, PyObject *value) } }
static inline PyObject * GET_CONTEXT(PyObject *exc) { if (exc == NULL) { return NULL; } assert(PyExceptionInstance_Check(exc)); return ((PyBaseExceptionObject *)exc)->context; }
void _PyErr_SetObject(PyThreadState *tstate, PyObject *exception, PyObject *value) { Expand Down Expand Up @@ -136,25 +146,49 @@ _PyErr_SetObject(PyThreadState *tstate, PyObject *exception, PyObject *value) value = fixed_value; }
/* Avoid reference cycles through the context chain. 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; while ((context = PyException_GetContext(o))) { Py_DECREF(context); if (context == value) { PyException_SetContext(o, NULL); break; } o = context; } PyException_SetContext(value, exc_value); if (value == exc_value) { /* This exception is already on top of the stack. */ Py_DECREF(exc_value); } else { Py_DECREF(exc_value); /* Otherwise, push the new value on top. */ PyException_SetContext(value, exc_value);
/* Now we need to destroy any reference cycles that might exist in the chain of contexts. Use Floyd's cycle-finding algorithm to determine if the chain has a cycle or if it reaches NULL. */ PyObject *tortoise = GET_CONTEXT(value); PyObject *hare = GET_CONTEXT(GET_CONTEXT(value)); while (hare != NULL && hare != tortoise) { tortoise = GET_CONTEXT(tortoise); hare = GET_CONTEXT(GET_CONTEXT(hare)); } if (hare != NULL) { /* Making it here means there is a cycle. Now find the _first_ self-intersection. */ tortoise = value; while (tortoise != hare) { tortoise = GET_CONTEXT(tortoise); hare = GET_CONTEXT(hare); } /* Now hare is the first intersection. We want to disconnect hare from its second predecessor. For example: A --> B --> C --> D --> E --> C --> ... becomes A --> B --> C --> D --> E --> NULL, since C is the first intersection. */ PyObject *prev = hare, *next = GET_CONTEXT(hare); while (next != hare) { prev = next; next = GET_CONTEXT(next); } PyException_SetContext(prev, NULL); } }
} if (value != NULL && PyExceptionInstance_Check(value)) tb = PyException_GetTraceback(value); Expand Down