Preventing double-free error with `stablePtr`

[Simon Peyton Jones]

Catching double-frees does sound like a good idea to me.

Also, is the wisdom in Roman’s post captured in the StablePtr docs?  If not, can someone do that?

Simon

From: Libraries [mailto:libraries-bounces at haskell.org] On Behalf Of Gershom B
Sent: 05 February 2018 01:38
To: Haskell Libraries <libraries at haskell.org>
Subject: Preventing double-free error with `stablePtr`

I was just reading Roman Cheplyaka’s very interesting blog-post here: https://ro-che.info/articles/2018-02-03-stableptr-undefined-behavior<https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fro-che.info%2Farticles%2F2018-02-03-stableptr-undefined-behavior&data=02%7C01%7Csimonpj%40microsoft.com%7C10dc6ba9e74d4ec2bbbe08d56c39361a%7Cee3303d7fb734b0c8589bcd847f1c277%7C1%7C0%7C636533915338797696&sdata=Y3cT1XpzbPoD%2BTsf2qSkFhxPMkDeyRqV073fe2IRvaY%3D&reserved=0>.

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<https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fhackage.haskell.org%2Fpackage%2FWin32-2.6.2.0%2Fdocs%2FSystem-Win32-Types.html%23v%3AwithStablePtr&data=02%7C01%7Csimonpj%40microsoft.com%7C10dc6ba9e74d4ec2bbbe08d56c39361a%7Cee3303d7fb734b0c8589bcd847f1c277%7C1%7C0%7C636533915338797696&sdata=L45oTzaUhG8G7U%2Fp%2FIV85smqkxbub5UQqvGA%2B9cGuJI%3D&reserved=0>)

Cheers,
Gershom
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