----------
> Guido:
> > Well, it *also* abuses the ob_refcnt field.
Neil:
> My patch fixes that too (by abusing the gc_prev pointer to chain trash
> together).
I think I haven't seen that patch yet.
> > How about this wild idea (which Tim & I had simultaneously
> > yesterday): change the trashcan code to simply leave the
> > object in the GC generational list, and let the GC code
> > special-case objects with zero refcnt so that they are
> > eventually properly disposed?
>
> That could probably work. What happens when the GC is disabled?
The trashcan code is also disabled. Better not create cycles *or*
deeply nested containers. They are similar anyway. :-)
> There is insidious bug here. Andrew helped me walk through it and I
> think we figured it out. First here's the code to trigger it:
>
> import gc
>
> class Ouch:
> def __del__(self):
> for x in range(20):
> list([])
>
> def f():
> gc.set_threshold(5)
> while 1:
> t = () # <-- here
> for i in range(300):
> t = [t, Ouch()]
> f()
>
> The line marked with "here" is where things go wrong. t used to refer
> to a long chain of [t, Ouch()]. The SETLOCAL macro in ceval calls
> Py_XDECREF(GETLOCAL(i)). That starts the deallocation of the list
> structure. Ouch.__del__ gets called can creates some more objects,
> triggering a collection. The f frame's traverse gets called and tries
> to follow the pointer for the t local. It points to memory that was
> freed by _PyTrash_destroy_chain.
Yes, Tim & I figured this out just before lunch, too. :-(
> Hmm, now that I think about it the GC is not needed to trigger the bug:
>
> import gc
> gc.disable()
> import sys
>
> class Ouch:
> def __del__(self):
> print f_frame.f_locals['t']
>
> def f():
> global f_frame
> f_frame = sys._getframe()
> while 1:
> t = ()
> for i in range(300):
> t = [t, Ouch()]
>
> f()
>
> I haven't figured out the correct solution yet. I'm just giving a
> status update. :-)
This is my patch:
Index: ceval.c
===================================================================
RCS file: /cvsroot/python/python/dist/src/Python/ceval.c,v
retrieving revision 2.308
diff -c -r2.308 ceval.c
*** ceval.c 24 Mar 2002 19:25:00 -0000 2.308
--- ceval.c 28 Mar 2002 18:21:09 -0000
***************
*** 554,561 ****
/* Local variable macros */
#define GETLOCAL(i) (fastlocals[i])
! #define SETLOCAL(i, value) do { Py_XDECREF(GETLOCAL(i)); \
! GETLOCAL(i) = value; } while (0)
/* Start of code */
--- 554,562 ----
/* Local variable macros */
#define GETLOCAL(i) (fastlocals[i])
! #define SETLOCAL(i, value) do { PyObject *tmp = GETLOCAL(i); \
! GETLOCAL(i) = value; \
! Py_XDECREF(tmp); } while (0)
/* Start of code */
--Guido van Rossum (home page: http://www.python.org/~guido/)
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