[Haskell-cafe] Conduit Error Output: Control.Monad.Trans.Resource.stateCleanup

[Michael Snoyman]

On Tue, Feb 21, 2012 at 7:40 AM, Michael Snoyman <michael at snoyman.com> wrote:
> On Tue, Feb 21, 2012 at 5:46 AM, Lyndon Maydwell <maydwell at gmail.com> wrote:
>> Hi Michael, Café.
>>
>> I'm writing some code using the conduit library and am encountering
>> the following error output (while the program appears to function
>> correctly) when using Data.Conduit.Lazy.
>>
>> The error given is:
>>
>>> profile_simple_test_data: Control.Monad.Trans.Resource.stateCleanup: There is a bug in the implementation. The mutable state is being accessed after cleanup. Please contact the maintainers.
>>
>> A reduced code snippet that generates this error is (also attached):
>>
>>> import Control.Monad
>>> import System.Environment
>>> import Control.Monad.IO.Class (liftIO)
>>> import System.IO
>>> import Data.Conduit.Lazy
>>> import Data.List (sort)
>>>
>>> import Data.Conduit
>>>
>>> import Prelude hiding (map)
>>>
>>> main = getArgs >>= process
>>>
>>> process args = mapM_ sorted args
>>>
>>> sorted x = runResourceT (lazyConsume $ sourceFeed x) >>= (mapM_ print . id)
>>>
>>> sourceFeed :: ResourceIO m => FilePath -> Source m String
>>> sourceFeed file = sourceIO
>>>     (openFile file ReadMode)
>>>     hClose
>>>     (\h -> liftIO $ do
>>>         eof <- hIsEOF h
>>>         if eof
>>>             then return IOClosed
>>>             else fmap IOOpen $ hGetLine h)
>>
>> when run over any text file.
>>
>> I may be doing something inconsistent with the correct use of sourceIO
>> or lazyConsume, however, I tried to follow the example at
>> http://www.yesodweb.com/home/snoyberg/blogs/conduit/conduit/source/source.ditamap?nav=nav-2
>> as closely as possible.
>>
>> Is this a bug, or simply an incorrect use of Conduit?
>
> I haven't fully debugged this yet. There's certainly a bug in the
> implementation of ResourceT, but the sample program is also wrong. You
> can't pass the result from a call to lazyConsume outside the scope of
> its ResourceT; the correct way to write sorted would be:
>
>    sorted x = runResourceT $ lazyConsume (sourceFeed x) >>= mapM_
> (liftIO . print)
>
> My guess is that this is a fallout from the transition away from
> mutable variables: lazyConsume no longer has any way of knowing that
> its ResourceT has already been terminated. Perhaps a simple solution
> would be to expose a primitive that checks if the ResourceT block has
> already been finalized.
>
> Michael

I've added a test case for this bug, and fixed it. The commit is:

https://github.com/snoyberg/conduit/commit/87e890fe7ee58686d20cabba15dd37f18ba66620

The basic idea is to add an extra constructor to represent when the
ResourceT has already been closed, and expose a function
resourceActive to check the state. Can you check if this solves your
problem?

Michael