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

Ben Doyle benjamin.peter.doyle at gmail.com
Tue Aug 27 21:40:19 CEST 2013

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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> Haskell-Cafe at haskell.org
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