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I think your patch is about as elegant as it can be.  The
approach looks sound but I believe you forgot to clear the
wr_callback pointer.  I'm attaching an incremental patch
(relative to patch3.txt).

Regarding gc_weakref.txt, I think we should add a note at
the top explaining the critical rule.  I.e. after we can
computed the unreachable set (aka trash), it's okay to
invoke code that can access non-trash but we cannot allow
code to run that can reference trash.

There is only one minor issue that I can think of.  We
decide which callbacks to invoke inside
move_troublemakers().  It's possible that unreachable
weakrefs become reachable when move_finalizer_reachable() is
called (i.e. weakrefs that would be trash except that they
got revived because they were referenced by an object with a
__del__ method).  However, since those weakrefs would have
been trash I think it's okay not to invoke their callbacks.