Weak values keep whole dictionary alive?

[eschnett]

While browsing dict.jl, I've come across this function:

function add_weak_value(t::Dict, k, v)
    t[k] = WeakRef(v)
    finalizer(v, x->weak_value_delete!(t, k, x))
    return t
end

This looks as if the lambda passed to the finalizer keeps the whole dictionary (t) alive. This sounds dangerous if dictionaries with weak keys are transient (which may not be the "usual" way they are used).

[vtjnash]

the lambda will be destroyed after the finalizer is run, allowing the dictionary to be freed. in what situation do you foresee this being dangerous?

[eschnett]

I am thinking of a case where the dictionary can be garbage collected, but
one of the weak values is still alive (i.e. reachable independent of the
dictionary). In this case, it will hold the whole dictionary hostage until
the value itself becomes unreachable.

-erik

On Sunday, November 16, 2014, Jameson Nash notifications@github.com wrote:

the lambda will be destroyed after the finalizer is run, allowing the
dictionary to be freed. in what situation do you foresee this being
dangerous?

Reply to this email directly or view it on GitHub.<
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Erik Schnetter schnetter@gmail.com
http://www.perimeterinstitute.ca/personal/eschnetter/

[JeffBezanson]

Maybe we should go with the plan described by the TODO in add_weak_key, and remove weak keys/values lazily instead of with finalizers. That might be faster too, since it avoids the overhead of adding and running finalizers.

We could also consider adding this behavior to Dict instead of using a separate type.

Also, add_weak_value doesn't seem to be used anywhere.

[oxinabox]

I think this can be closed as the problematic method add_weak_value no longer exists in master.