#14159: Don't install callbacks on values of TripleDict, MonoDict
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Reporter: nbruin | Owner: tbd
Type: PLEASE CHANGE | Status: new
Priority: major | Milestone: sage-5.8
Component: PLEASE CHANGE | Resolution:
Keywords: | Work issues:
Report Upstream: N/A | Reviewers:
Authors: | Merged in:
Dependencies: | Stopgaps:
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Comment (by nbruin):
Ouch, I looked at that code and you're correct: That doesn't overwrite the
value at all (it's really a cache). Incidentally, at that code:
{{{
try:
H = _cache[key]()
except KeyError:
H = None
if H is not None:
# Are domain or codomain breaking the unique parent condition?
if H.domain() is X and H.codomain() is Y:
return H
}}}
Since `X,Y` are part of key, which is looked up by identity, the "domain"
and "codomain" test should really be superfluous. So perhaps we want
{{{
if H is not None:
assert H.domain() is X and H.codomain() is Y
return H
}}}
since something is going very wrong if this is not the case! (H would not
be allowed to sit there in the dict).
Furthermore, `H` has strong references to `X,Y,category` in the form of
`H.domain()`, `H.codomain()` and `H.homset_category()`, so if H is alive
then so must the keys. Which means that the callback on H must happen (or
be purged) before those on `X,Y` etc., or at least in the same GC.
so their `id`s can't get reused until their callbacks have happened or
have been discarded.
I think that only leaves the possibility that `_cache` got silently
corrupted due to a GC at an inopportune time before this particular error.
We cannot rule that out, but it's extremely unlikely: It requires a bucket
to be rather full and buckets are typically not full at all (ideally only
one entry).
On the other hand, since this is a global _cache, it does get to hold
quite a few keys, so it's quite likely that it underwent a `resize`
operation, where a corruption was much more likely to happen.
It's rather difficult to do an accurate postmortem with so little data.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/14159#comment:10>
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