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| -rw-r--r-- | Modules/gcmodule.c | 19 |
1 files changed, 19 insertions, 0 deletions
diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c index 6442fe5..e6aabe4 100644 --- a/Modules/gcmodule.c +++ b/Modules/gcmodule.c @@ -214,6 +214,25 @@ update_refs(PyGC_Head *containers) for (; gc != containers; gc = gc->gc.gc_next) { assert(gc->gc.gc_refs == GC_REACHABLE); gc->gc.gc_refs = FROM_GC(gc)->ob_refcnt; + /* Python's cyclic gc should never see an incoming refcount + * of 0: if something decref'ed to 0, it should have been + * deallocated immediately at that time. + * Possible cause (if the assert triggers): a tp_dealloc + * routine left a gc-aware object tracked during its teardown + * phase, and did something-- or allowed something to happen -- + * that called back into Python. gc can trigger then, and may + * see the still-tracked dying object. Before this assert + * was added, such mistakes went on to allow gc to try to + * delete the object again. In a debug build, that caused + * a mysterious segfault, when _Py_ForgetReference tried + * to remove the object from the doubly-linked list of all + * objects a second time. In a release build, an actual + * double deallocation occurred, which leads to corruption + * of the allocator's internal bookkeeping pointers. That's + * so serious that maybe this should be a release-build + * check instead of an assert? + */ + assert(gc->gc.gc_refs != 0); } } |