Demonstrate a bug with fast-logger when concurrently writing chunks larger than the buffer size

large-fast-logger.hs

-- Compiled with:
--
-- stack --resolver lts-6.17 ghc --package async --package fast-logger -- -threaded -rtsopts -with-rtsopts -N large-fast-logger.hs

{-# LANGUAGE OverloadedStrings #-}
import Control.Concurrent.Async (Concurrently (..))
import Control.Monad            (unless, zipWithM_)
import Data.Foldable            (traverse_)
import Data.Monoid              ((<>))
import Data.String              (fromString)
import System.Log.FastLogger    (LoggerSet, newFileLoggerSet, pushLogStr,
                                 rmLoggerSet)

main :: IO ()
main = do
    loggerSet <- newFileLoggerSet 16 "log.txt" -- really small on purpose
    runConcurrently $ traverse_ (Concurrently . run loggerSet) ['A'..'Z']
    rmLoggerSet loggerSet
    output <- readFile "log.txt"
    zipWithM_ checkLine [1..] (lines output)

run :: LoggerSet -> Char -> IO ()
run loggerSet c =
    pushLogStr loggerSet str
  where
    str = fromString (replicate 99 c) <> "\n"

checkLine :: Int -> String -> IO ()
checkLine lineNumber chars = do
    unless (length chars == 99)
        $ putStrLn $ "Does not have 99 characters: " ++ show lineNumber
    case chars of
        [] -> return () -- already mentioned above
        x:xs
            | any (/= x) xs ->
                putStrLn $ "Mismatched characters: " ++ show lineNumber
            | otherwise -> return ()