MVar deadlock when using Literate Haskell and multiple capabilities

[snoyberg]

Take two files, one an input file for hlint foo.lhs:

> main = return ()

And another program using the hlint library (hlint.hs):

import Language.Haskell.HLint
import Control.Exception

main = do
    x <- try $ hlint ["foo.lhs"] >> return ()
    putStrLn "after call to hlint"
    print (x :: Either SomeException ())
    putStrLn "after call to print"

Now try the following commands:

$ ghc --make -threaded -rtsopts hlint.hs
$ ./hlint
after call to hlint
Left In file foo.lhs at line 1: comment line before program line.

after call to print
$ ./hlint +RTS -N2
hlint: In file foo.lhs at line 1: comment line before program line.

after call to hlint
Left thread blocked indefinitely in an MVar operation
after call to print

Notice how the exception changes to thread blocked. This looks like some problematic parallelism.

[ndmitchell]

Thanks for the fantastic test case, certainly worthy of a beer when I eventually meet you. I'm taking a look now.

[ndmitchell]

Fixed - my parallelism function didn't deal with exceptions (it was written many years ago). I've released hlint-1.9.3 with the fix.

[snoyberg]

Awesome, thanks!

By the way, are you going to be at ICFP by any chance?

[ndmitchell]

Yep, I will be.

[snoyberg]

Cool, I'll see you there!