Subtle bug in Data.ByteString and/or GHC (perhaps)

[Donald Bruce Stewart]

Very intersting. Possibly something unsafe happening in there.

Duncan, care to take a look?

-- Don

nad:
> Hi,
> 
> The following program prints different outputs depending on whether
> optimisations are turned on or not (using GHC 6.6.1 and binary 0.3):
> 
>   module Bug (main) where
> 
>   import Data.Binary
>   import Data.Binary.Put
>   import Data.Binary.Get
>   import Data.Binary.Builder
>   import qualified Data.ByteString.Lazy as BL
>   import qualified Data.ByteString.Base as BB
> 
>   append' :: BL.ByteString -> BL.ByteString -> BL.ByteString
>   append' = BL.append
>   -- append' (BB.LPS xs) (BB.LPS ys) = BB.LPS (xs ++ ys)
> 
>   encode' :: Char -> BL.ByteString
>   encode' x = encode (st `seq` 'a') `append'` toLazyByteString builder
>     where (st, builder) = unPut (put x)
> 
>   decode' :: BL.ByteString -> Char
>   decode' s = decode s'
>     where (_, s', _) = runGetState (get :: Get Char) s 0
> 
>   main :: IO ()
>   main = mapM_ (print . test) "abc"
>     where test x = decode' (encode' x) == x
> 
>   $ ghc --make -O Bug.hs -main-is Bug.main -o bug
>   [...]
>   $ ./bug
>   True
>   False
>   False
>   $ rm Bug.o; ghc --make Bug.hs -main-is Bug.main -o bug
>   [...]
>   $ ./bug
>   True
>   True
>   True
> 
> If the commented-out version of append' (which is a bit lazier than
> the other one) is used instead, then this problem disappears. The
> commented-out version is the one used in the darcs version of
> Data.ByteString, so the problem above has, in a sense, already been
> fixed.
> 
> However, it is disconcerting that the result of a program can depend
> on optimisation flags, and changing the strictness of a function
> should only make a (pure) program more or less defined, not change a
> result from True to False. Hence I wonder if anyone knows the real
> cause of this bug, and what the risk of encountering similar bugs in
> the future is. It took lots of time to find and fix the problem above
> (which of course came up in a larger piece of code), so I hope that I
> won't encounter similar problems again.
> 
> My guess is that the problem has something to do with unsound rewrite
> rules in Data.ByteString, by the way. Or maybe the problem lies in
> GHC's optimiser.
> 
> -- 
> /NAD
> 
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