#13394: Write a WeakValueDictionary with safer key removal
-------------------------------+--------------------------------------------
Reporter: nbruin | Owner: rlm
Type: enhancement | Status: new
Priority: major | Milestone: sage-5.4
Component: memleak | Resolution:
Keywords: | Work issues:
Report Upstream: N/A | Reviewers:
Authors: | Merged in:
Dependencies: | Stopgaps:
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Comment (by nbruin):
The standard Python `WeakValueDictionary` is built on top of a normal
dictionary, built on top of a normal dictionary. Let's say we have
{{{
W = WeakValueDictionary()
}}}
There is an underlying `D = dict`. If we do
{{{
W[k] = v
}}}
then it really executes
{{{
D[k] = KeyedRef(v, k, remove_callback)
}}}
This is a weak reference to v, so v can still get collected. If it does,
the callback can ensure that
{{{
del D[k]
}}}
gets executed. This locates the entry by computing the hash of `h`,
finding the right bucket and finding an equal key by comparison. When we
create the entry, we have better lookup information, though: We already
have `hash(k)` and `id(v)`. These two don't necessarily pin down the entry
in the dictionary, but in our applications they do and for a
`WeakValueDictionary` identical `v` would be the same (dead) `KeyedRef`
anyway, so it wouldn't hurt to remove them all. We need one more method on
our dictionary:
{{{
D.remove_entry_by_hash_and_value_id(h,i):
"""
remove all entries { k : v } from the dictionary that have
hash(k) == h and id(v) == i
"""
}}}
Then the callback can be
{{{
function remove(ref):
D.remove_entry_by_hash_and_value_id(hash(ref.key),id(ref))
}}}
and the `KeyedRef` that gets stored would be
{{{
KeyedRef(v,hash(k),remove)
}}}
(normal precautions to put the dict reference in a closure or a class
should apply here in defining `remove`)
Or one could just immediately integrate the `KeyedRefs` in the design. See
also #13387 for discussions about dictionary designs.
In either case, the main effect is that removal does not cause any hashing
or comparison methods to be called on `k` anymore.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/13394#comment:2>
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