diff options
Diffstat (limited to 'Include/cpython/object.h')
| -rw-r--r-- | Include/cpython/object.h | 86 |
1 files changed, 86 insertions, 0 deletions
diff --git a/Include/cpython/object.h b/Include/cpython/object.h index 70f189c856..3c4bf5bb59 100644 --- a/Include/cpython/object.h +++ b/Include/cpython/object.h @@ -445,6 +445,92 @@ PyAPI_FUNC(int) _PyObject_CheckConsistency( PyObject *op, int check_content); + +/* Trashcan mechanism, thanks to Christian Tismer. + +When deallocating a container object, it's possible to trigger an unbounded +chain of deallocations, as each Py_DECREF in turn drops the refcount on "the +next" object in the chain to 0. This can easily lead to stack overflows, +especially in threads (which typically have less stack space to work with). + +A container object can avoid this by bracketing the body of its tp_dealloc +function with a pair of macros: + +static void +mytype_dealloc(mytype *p) +{ + ... declarations go here ... + + PyObject_GC_UnTrack(p); // must untrack first + Py_TRASHCAN_BEGIN(p, mytype_dealloc) + ... The body of the deallocator goes here, including all calls ... + ... to Py_DECREF on contained objects. ... + Py_TRASHCAN_END // there should be no code after this +} + +CAUTION: Never return from the middle of the body! If the body needs to +"get out early", put a label immediately before the Py_TRASHCAN_END +call, and goto it. Else the call-depth counter (see below) will stay +above 0 forever, and the trashcan will never get emptied. + +How it works: The BEGIN macro increments a call-depth counter. So long +as this counter is small, the body of the deallocator is run directly without +further ado. But if the counter gets large, it instead adds p to a list of +objects to be deallocated later, skips the body of the deallocator, and +resumes execution after the END macro. The tp_dealloc routine then returns +without deallocating anything (and so unbounded call-stack depth is avoided). + +When the call stack finishes unwinding again, code generated by the END macro +notices this, and calls another routine to deallocate all the objects that +may have been added to the list of deferred deallocations. In effect, a +chain of N deallocations is broken into (N-1)/(PyTrash_UNWIND_LEVEL-1) pieces, +with the call stack never exceeding a depth of PyTrash_UNWIND_LEVEL. + +Since the tp_dealloc of a subclass typically calls the tp_dealloc of the base +class, we need to ensure that the trashcan is only triggered on the tp_dealloc +of the actual class being deallocated. Otherwise we might end up with a +partially-deallocated object. To check this, the tp_dealloc function must be +passed as second argument to Py_TRASHCAN_BEGIN(). +*/ + +/* The new thread-safe private API, invoked by the macros below. */ +PyAPI_FUNC(void) _PyTrash_thread_deposit_object(PyObject*); +PyAPI_FUNC(void) _PyTrash_thread_destroy_chain(void); + +#define PyTrash_UNWIND_LEVEL 50 + +#define Py_TRASHCAN_BEGIN_CONDITION(op, cond) \ + do { \ + PyThreadState *_tstate = NULL; \ + /* If "cond" is false, then _tstate remains NULL and the deallocator \ + * is run normally without involving the trashcan */ \ + if (cond) { \ + _tstate = PyThreadState_GET(); \ + if (_tstate->trash_delete_nesting >= PyTrash_UNWIND_LEVEL) { \ + /* Store the object (to be deallocated later) and jump past \ + * Py_TRASHCAN_END, skipping the body of the deallocator */ \ + _PyTrash_thread_deposit_object(_PyObject_CAST(op)); \ + break; \ + } \ + ++_tstate->trash_delete_nesting; \ + } + /* The body of the deallocator is here. */ +#define Py_TRASHCAN_END \ + if (_tstate) { \ + --_tstate->trash_delete_nesting; \ + if (_tstate->trash_delete_later && _tstate->trash_delete_nesting <= 0) \ + _PyTrash_thread_destroy_chain(); \ + } \ + } while (0); + +#define Py_TRASHCAN_BEGIN(op, dealloc) Py_TRASHCAN_BEGIN_CONDITION(op, \ + Py_TYPE(op)->tp_dealloc == (destructor)(dealloc)) + +/* For backwards compatibility, these macros enable the trashcan + * unconditionally */ +#define Py_TRASHCAN_SAFE_BEGIN(op) Py_TRASHCAN_BEGIN_CONDITION(op, 1) +#define Py_TRASHCAN_SAFE_END(op) Py_TRASHCAN_END + #ifdef __cplusplus } #endif |