The MonadControlIO instance of RegionT allows unsafe forkIO

[basvandijk]

In regions-0.9 I removed support for forking threads because it allowed you to use a closed handle in a forked thread. Unfortunately I just realized that it's still possible to fork threads in a region. The reason is that I've derived a MonadControlIO instance for RegionT which enables you to use forkIO as demonstrated by the following program:

{-# LANGUAGE UnicodeSyntax, NoImplicitPrelude, KindSignatures #-}

module Main where

-- from base:
import Data.Function      ( ($) )
import Control.Concurrent ( ThreadId, forkIO, threadDelay )
import Control.Monad      ( (>>=), liftM, void )
import System.IO          ( IO )

-- from transformers:
import Control.Monad.IO.Class ( liftIO )

-- from regions:
import Control.Monad.Trans.Region ( RegionT, runRegionT )

-- from safer-file-handles:
import System.IO.SaferFileHandles ( openFile
                                 , IOMode(ReadMode)
                                 , hGetContents
                                 , putStrLn
                                 )

-- from pathtype:
import System.Path.Posix ( asAbsFile )

-- from monad-control:
import Control.Exception.Control ( mask_ )
import Control.Monad.IO.Control  ( MonadControlIO, liftControlIO )

main ∷ IO ()
main = do runRegionT region
         threadDelay 1500000

region ∷ MonadControlIO pr ⇒ RegionT s pr ()
region = do
 putStrLn "Running region"

 h ← openFile (asAbsFile "/etc/passwd") ReadMode

 _ ← liftForkIO $ do
          putStrLn "Forked region"
          liftIO $ threadDelay 1000000
          hGetContents h >>= putStrLn

 liftIO $ threadDelay 500000
 putStrLn "Exiting region"

liftForkIO ∷ MonadControlIO m ⇒ m α → m ThreadId
liftForkIO m = liftControlIO $ \runInIO →
                forkIO $ void $ runInIO m

Executing main yields the following error:

> main
Running region
Forked region
Exiting region
<interactive>: /etc/passwd: hGetContents: illegal operation (handle is closed)

I think the only solution is to drop the derived MonadControlIO and MonadTransControl instances. Unfortunately the packages that use regions require this instance because they need to use mask_ when opening resources. Here an example from safer-file-handles:

openFile ∷ (MonadControlIO pr, AbsRelClass ar)
        ⇒ FilePath ar
        → IOMode ioMode
        → RegionT s pr
            (RegionalFileHandle ioMode (RegionT s pr))
openFile = openNormal E.openFile

openNormal open = \filePath ioMode → mask_ $ do
 h ← liftIO $ open (getPathString filePath) ioMode
 ch ← onExit $ sanitizeIOError $ hClose h
 return $ RegionalFileHandle h ch

I guess I have to solve this by providing a custom mask_ function or using MonadCatchIO-transformers as I did before.

I'm going to think about the best solution. In the mean time just don't use something like liftForkIO.

[basvandijk]

Fixed !