Avoiding BlockedIndefinitelyOnSTM exceptions

[Neil Davies]

Gabriel

Is the underlying issue one of “scope” - STM variables have global scope, would a batter approach to be to create scope of such things and then some overall recovery mechanism could handle such an exception within that scope?

Neil

On 14 Jul 2014, at 03:30, Gabriel Gonzalez <gabriel439 at gmail.com> wrote:

> I have what may sound like an unusual request: I would like to automatically avoid `BlockedIndefinitelyOnSTM` exceptions with a primitive that looks something like this:
> 
>    safe :: STM a -> STM (Maybe a)
> 
> This hypothetical `safe` primitive would attempt a transaction, and if `ghc` detects that this transaction would fail because of an `BlockedIndefinitelyOnSTM` exception it will return `Nothing` instead of throwing an uncatchable exception.
> 
> I originally simulated a limited form of this behavior using `pipes-concurrency`.  I instrumented the garbage collector (using weak references) to detect when an STM variable was garbage collected and to safely cancel any transactions that depended on those variables.  You can see the implementation here:
> 
> https://github.com/Gabriel439/Haskell-Pipes-Concurrency-Library/blob/23e7e2dab472b7e4cde7bea31227a917ce5d5375/src/Pipes/Concurrent.hs#L170
> 
> The original purpose behind this was to easily read and write to a channel without having to count references to the channel.  I reasoned that the garbage collector *already knew* how many open references there were to channel, so I thought "why not use the garbage collector to gracefully cancel transactions that would block just before they would trigger the exception?"
> 
> This worked really well up until ghc-7.8 changed something and the above trick no longer works.  To be honest, I'm surprised that it ever worked at all, which is why I'm not requesting restoring the original behavior.  Instead, I think providing something like the above `safe` primitive would be nicer, if possible.
> 
> Would it be possible to implement something like `safe`?
> 
> Alternatively, is it possible to make the `BlockedIndefinitelyOnSTM` exception catchable?
> 
> P.S.  I'm also interested in learning more about what may have caused the change in behavior in the transition from ghc-7.6 to ghc-7.8.  What changes were made to the interaction between STM and weak references that may have triggered this?
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