[Haskell-cafe] ResourceT "unmasked" during allocation?

[Michael Snoyman]

Yes, there's a good reason: it's a bug :). I wrote that code a while ago,
and had a mistaken understanding of how bracket works at the time. Thanks
for noticing this, I'll release a new version shortly.

On Mon, Nov 7, 2016 at 2:30 AM, Viktor Dukhovni <ietf-dane at dukhovni.org>
wrote:

> I am looking at using ResourceT to manage the acquisiition and
> deallocation of network sockets.  In comparing resourceT's `allocate`
> with `bracket` I was somewhat perplexed to find that `allocate` in
> Control.Monad.Trans.Resource seems to not mask exceptions during
> allocation (see "No mask" comment I inserted):
>
>         allocateRIO :: IO a -> (a -> IO ()) -> ResourceT IO (ReleaseKey, a)
>         allocateRIO acquire rel = ResourceT $ \istate -> liftIO $ E.mask $
> \restore -> do
>             a <- restore acquire
>             --   ^^^ No mask ^^^
>             key <- register' istate $ rel a
>             return (key, a)
>
>         allocate :: MonadResource m
>                  => IO a -- ^ allocate
>                  -> (a -> IO ()) -- ^ free resource
>                  -> m (ReleaseKey, a)
>         allocate a = liftResourceT . allocateRIO a
>
> By contrast in Exception.Base we see that `bracket` masks
> the initialization action:
>
>         bracket before after thing =
>           mask $ \restore -> do
>             a <- before
>             --   ^^^ masked! ^^^
>             r <- restore (thing a) `onException` after a
>             _ <- after a
>             return r
>
> This seems to suggest that ResourceT is more exposes to resource
> leaks via asynchronous exceptions.  Say if a file descriptor is a
> allocated, but an exception interrupts "allocate" before it can
> register the resource.
>
> Perhaps I am missing something, or there are good reasons for
> the difference in approach.  Can anyone shed some light on the
> reason why ResourceT differs (seemingly) from bracket in this
> regard.
>
> --
>         Viktor.
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