Guaranteeing either idempotence or an error message seems to require that the stable pointer (but not what it points to) remain allocated until there are no more references to it. I don't know enough about these things to say how hard or expensive that might be. Another possibility might be to avoid reallocating a pointer too soon after it's freed; that would give a decent chance at an error and might be less expensive.

On Feb 4, 2018 8:38 PM, "Gershom B" <gershomb@gmail.com> wrote:

I was just reading Roman Cheplyaka’s very interesting blog-post here: https://ro-che.info/articles/2018-02-03-stableptr-undefined-behavior.

As he points out, the docs for `freeStablePtr` say

"if the stable pointer is passed to deRefStablePtr or freeStablePtr, the behaviour is undefined.”

And indeed we can observe weird behavior as a result of sucn an error.

A deRef of a stable pointer is arguably the sort of sharp-edge we know how to code to avoid. But a double free is a bit trickier. Would it be worth adding a bit more overhead to make such an operation idempotent?

Additionally, would it be worthwhile to add `withStablePtr` to the `Foreign.StablePtr` module? I imagine there are cases that it won’t cover, but it would at least encourage good discipline in the cases that it does handle. The evident utility of such a function is witnessed by its existence in a few different codebases, not least the Win32 library (https://hackage.haskell.org/package/Win32-2.6.2.0/docs/System-Win32-Types.html#v:withStablePtr)

Cheers,
Gershom

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