[Haskell-cafe] enumerators: exception that can't be catched

[Yuras Shumovich]

Hi,

Thank you for the reply.

Unlikely it is the case (if I understand it correctly). The exception is
thrown by "enumSocket", I added traces to prove that. And it is
propagated to
"runWithSocket" ( http://hackage.haskell.org/packages/archive/websockets/0.7.4.0/doc/html/src/Network-WebSockets-Socket.html#runWithSocket ), so that "Data.Enumerator.run" returns "Left". So actually it is not an IO exception, but it is thrown via "throwError".

Looks like I don't have other options except to reimplement websockets
protocol myself :(

Thanks,
Yuras

On Tue, 2013-08-27 at 15:40 -0400, Ben Doyle wrote:
> This is partially guesswork, but the code to catchWSError looks
> dubious:
> 
> 
>     catchWsError :: WebSockets p a
>              -> (SomeException -> WebSockets p a)
>              -> WebSockets p a
>   catchWsError act c = WebSockets $ do
>       env <- ask
>       let it  = peelWebSockets env $ act
>           cit = peelWebSockets env . c
>       lift $ it `E.catchError` cit
>     where
>       peelWebSockets env = flip runReaderT env . unWebSockets
> 
> Look at `cit`. It runs the recovery function, then hands the underlying Iteratee the existing environment. That's fine if `act` is at fault, but there are Iteratee- and IO-ish things in WebSocketsEnv---if one of `envSink` or `envSendBuilder` is causing the exception, it'll just get re-thrown after `E.catchError`. (I think. That's the guesswork part.)
> So check how `envSendBuilder` is built up, and see if there's a way it could throw an exception on client disconnect.
> 
> 
> On Tue, Aug 27, 2013 at 10:28 AM, Yuras Shumovich
> <shumovichy at gmail.com> wrote:
>         Hello,
>         
>         I'm debugging an issue in "websockets" package,
>         https://github.com/jaspervdj/websockets/issues/42
>         
>         I'm not familiar with "enumerator" package (websockets are
>         based on it),
>         so I'm looking for help. The exception is throws inside
>         "enumSocket"
>         enumerator using
>         "throwError" ( http://hackage.haskell.org/packages/archive/network-enumerator/0.1.5/doc/html/src/Network-Socket-Enumerator.html#enumSocket ), but I can't catch it with "catchError". It is propagated to "run" function:
>            <interactive>: recv: resource vanished (Connection reset by
>         peer)
>         
>         The question is: how is it possible? could it be a bug in
>         "enumerator"
>         package?
>         
>         Thanks,
>         Yuras
>         
>         
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