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Clear weakrefs in garbage found by the GC (#16495) 

Fix a bug due to the interaction of weakrefs and the cyclic garbage
collector. We must clear any weakrefs in garbage in order to prevent
their callbacks from executing and causing a crash.
This commit is contained in:
Neil Schemenauer 2019-09-30 10:06:45 -07:00 committed by Łukasz Langa
parent c9a413ede4
commit bcda460baf
2 changed files with 20 additions and 0 deletions
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3
Misc/NEWS.d/next/Core and Builtins/2019-09-30-09-33-21.bpo-38006.UYlJum.rst Normal file
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@ -0,0 +1,3 @@
Fix a bug due to the interaction of weakrefs and the cyclic garbage
collector. We must clear any weakrefs in garbage in order to prevent their
callbacks from executing and causing a crash.

17
Modules/gcmodule.c
View File

@ -678,6 +678,21 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
op = FROM_GC(gc);
next = GC_NEXT(gc);
if (PyWeakref_Check(op)) {
/* A weakref inside the unreachable set must be cleared. If we
* allow its callback to execute inside delete_garbage(), it
* could expose objects that have tp_clear already called on
* them. Or, it could resurrect unreachable objects. One way
* this can happen is if some container objects do not implement
* tp_traverse. Then, wr_object can be outside the unreachable
* set but can be deallocated as a result of breaking the
* reference cycle. If we don't clear the weakref, the callback
* will run and potentially cause a crash. See bpo-38006 for
* one example.
*/
_PyWeakref_ClearRef((PyWeakReference *)op);
}
if (! PyType_SUPPORTS_WEAKREFS(Py_TYPE(op)))
continue;
@ -733,6 +748,8 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
* is moved to wrcb_to_call in this case.
*/
if (gc_is_collecting(AS_GC(wr))) {
/* it should already have been cleared above */
assert(wr->wr_object == Py_None);
continue;
}