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[Łukasz Langa]

To: python-checkins at python.org
Subject: Clear weakrefs in garbage found by the GC (GH-16495) (#16499)
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https://github.com/python/cpython/commit/92ca515ee1efbdc51678e12d105ad642c9b9=
cc13
commit: 92ca515ee1efbdc51678e12d105ad642c9b9cc13
branch: 3.8
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.co=
m>
committer: =C5=81ukasz Langa <lukasz at langa.pl>
date: 2019-09-30T19:27:46+02:00
summary:

Clear weakrefs in garbage found by the GC (GH-16495) (#16499)

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.
(cherry picked from commit bcda460baf25062ab68622b3f043f52b9db4d21d)

Co-authored-by: Neil Schemenauer <nas-github at arctrix.com>

files:
A Misc/NEWS.d/next/Core and Builtins/2019-09-30-09-33-21.bpo-38006.UYlJum.rst
M Modules/gcmodule.c

diff --git a/Misc/NEWS.d/next/Core and Builtins/2019-09-30-09-33-21.bpo-38006=
.UYlJum.rst b/Misc/NEWS.d/next/Core and Builtins/2019-09-30-09-33-21.bpo-3800=
6.UYlJum.rst
new file mode 100644
index 000000000000..c2dafae0afab
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2019-09-30-09-33-21.bpo-38006.UYlJum=
.rst=09
@@ -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.
diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c
index b2ee56623cd4..8bdbafef167e 100644
--- a/Modules/gcmodule.c
+++ b/Modules/gcmodule.c
@@ -678,6 +678,21 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
         op =3D FROM_GC(gc);
         next =3D GC_NEXT(gc);
=20
+        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;
=20
@@ -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 =3D=3D Py_None);
                 continue;
             }
=20