#10963: More functorial constructions
-------------------------------------+-------------------------------------
Reporter: nthiery | Owner: stumpc5
Type: enhancement | Status: needs_work
Priority: major | Milestone: sage-5.13
Component: categories | Resolution:
Keywords: | Merged in:
Authors: Nicolas M. Thiéry | Reviewers: Simon King, Frédéric
Report Upstream: N/A | Chapoton
Branch: | Work issues:
Dependencies: #11224, #8327, | Commit:
#10193, #12895, #14516, #14722, | Stopgaps:
#13589, #14471, #15069, #15094, |
#11688, #13394 |
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Comment (by nbruin):
Replying to [comment:159 jdemeyer]:
> This happens sometimes:
> {{{
> Exception RuntimeError: 'maximum recursion depth exceeded while
calling a Python object' in <sage.structure.coerce_dict.TripleDictEraser
object at 0x17814b0> ignored
> Exception RuntimeError: 'maximum recursion depth exceeded while
calling a Python object' in <cyfunction
WeakValueDictionary.__init__.<locals>.callback at 0x2421410> ignored
> }}}
Looks similar to #15069, so the most likely scenario is that there is a
very complicated data structure that gets garbage collected and that the
decref of something initiates a chain of subsequent decrefs that is more
than 1000 deep.
It seems there are unresolved issues in python with this stuff. See
[http://bugs.python.org/issue483469] for an even worse (segmentation fault
inducing!) problem with `__del__`. It looks like the python "maximum
recursion depth" is avoided there via a similar trick to #15069, leading
do a C-stack overflow as a result (hence the harder crash). Indeed:
{{{
sage: class A: pass
sage: a=A(); prev=a;
sage: from sage.structure.coerce_dict import MonoDict
sage: M = MonoDict(11)
sage: for i in range(10^5): newA = A(); M[prev] = newA; prev = newA
sage: del a
Segmentation fault
}}}
(the value `10^5` may need adjustment, depending on your C-stack), showing
that with the fix on #15069 we only postpone the problem with some order
of magnitudes, and get a worse problem instead.
I suspect we're hitting here the same problem (note that for a
`WeakValueDictionary` we have to chain in the other direction):
{{{
sage: a=A(); prev=a;
sage: M=WeakValueDictionary()
sage: for i in range(10^3+10): newA = A(); M[newA] = prev; prev = newA
sage: del a
Exception RuntimeError: 'maximum recursion depth exceeded while calling a
Python object' in <cyfunction
WeakValueDictionary.__init__.<locals>.callback at 0x6a527d0> ignored
}}}
This problem goes away if we instead define
{{{
sage: class A(object): pass
}}}
Probably, old-style objects do not participate in the "trashcan" but new-
style objects do (see #13901 and
[http://trac.cython.org/cython_trac/ticket/797 cython ticket#797]; we need
this on cython classes too), which flattens call-stacks during
deallocation.
The problem also doesn't occur with `weakref.WeakValueDictionary`,
probably also because there are sufficiently many general python
structures involved to let the trashcan kick in.
Oddly enough, replacing `object` above by `SageObject` or `Parent` seems
to also work, so the scenario we're running into is probably not exactly
what described here.
--
Ticket URL: <http://trac.sagemath.org/ticket/10963#comment:162>
Sage <http://www.sagemath.org>
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