SF 1055820: weakref callback vs gc vs threads
In cyclic gc, clear weakrefs to unreachable objects before allowing any
Python code (weakref callbacks or __del__ methods) to run.
This is a critical bugfix, affecting all versions of Python since weakrefs
were introduced. I'll backport to 2.3.
diff --git a/Misc/NEWS b/Misc/NEWS
index 4aba0fc..001152a 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -32,6 +32,17 @@
Core and builtins
-----------------
+- Bug #1055820 Cyclic garbage collection was not protecting against that
+ calling a live weakref to a piece of cyclic trash could resurrect an
+ insane mutation of the trash if any Python code ran during gc (via
+ running a dead object's __del__ method, running another callback on a
+ weakref to a dead object, or via any Python code run in any other thread
+ that managed to obtain the GIL while a __del__ or callback was running
+ in the thread doing gc). The most likely symptom was "impossible"
+ ``AttributeEror`` exceptions, appearing seemingly at random, on weakly
+ referenced objects. The cure was to clear all weakrefs to unreachable
+ objects before allowing any callbacks to run.
+
- Bug #1054139 _PyString_Resize() now invalidates its cached hash value.
Extension Modules