Retract the claim that this is always safe if PyMem_{Del, DEL, Free, FREE}
are called without the GIL. It's incredibly unlikely to fail, but I can't
make this bulletproof without either adding a lock for exclusion, or
giving up on growing the arena base-address vector (it would be safe if
this were a static array).
diff --git a/Objects/obmalloc.c b/Objects/obmalloc.c
index 5ac5c35..500afba 100644
--- a/Objects/obmalloc.c
+++ b/Objects/obmalloc.c
@@ -403,6 +403,10 @@
* only make a supposed-to-succeed case fail by mistake).
* Read the above 50 times before changing anything in this
* block.
+ * XXX Fudge. This is still vulnerable: there's nothing
+ * XXX to stop the bad-guy thread from picking up the
+ * XXX current value of arenas, but not indexing off of it
+ * XXX until after the PyMem_FREE(oldarenas) below completes.
*/
uptr *oldarenas;
int newmax = maxarenas + (maxarenas >> 1);