[Haskell-cafe] Finalizer registered with System.Mem.Weak.addFinalizer is called surprisingly early

[Antoine Latter]

On Wed, Mar 9, 2011 at 3:54 PM, balodja <balodja at zlug.asia> wrote:
> Hello,
>
> I use addFinalizer from System.Mem.Weak for performing some actions on
> data just before that is reclaimed by GC. Registered in this way
> finalizer is called surprisingly early for me, much earlier the data is
> tend to be collected.
>
> I've attached source code of simple two-threaded program. Source code
> also can be found at [1]. It is pretty simple and should be readable
> enough.
>

Does it work if you change:

>  forM_ [1..1000000] $ \x -> do
>    send socket (show x)
>    threadDelay (10*1000)

to:

>  forM_ [1..1000000] $ \x -> do
>    send socket (show x)
>    touch socket
>    threadDelay (10*1000)

Where "touch" is found in Control.Monad.Primitive of the package
"primitive": http://hackage.haskell.org/packages/archive/primitive/0.3.1/doc/html/Control-Monad-Primitive.html#v:touch

I'm guessing what happens is that optimizations make the 'Socket'
constructor go away, so the finalizer runs as soon as it can. The
'touch' function might force it to stick around. You'll probably want
to look at the low-level compiler output to make sure:

http://hackage.haskell.org/package/ghc-core

Does anyone else have any good tutorials to link to?

Antoine

> Program is intentioned to pass messages through transactional variable
> (TVar) for printing by worker thread. Messages consist of formatted to
> String Int values from 1 to 1000000. So the output should look like
> this:
>
>> 1
>> 2
>> ...
>> 1000000
>
> But running the program gives something like that:
>
>> 1
>> 2
>> ...
>> 283
>> finalizing
>> Test: thread blocked indefinitely in an STM transaction
>
> As far as I see, finalizer is called too early. If someone could point
> me out how to do the right thing, that would be helpful.
>
> Operaton system: Debian Squeeze
> Compiler: GHC 6.12.1
> Compiling options: none, just "ghc --make Test.hs"
>
> Thanks,
> balodja
>
> [1] http://hpaste.org/44648
>
>
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