Do not overwrite syntax.css

liquidhaskell-boot/src/Language/Haskell/Liquid/UX/Annotate.hs

@@ -20,6 +20,7 @@ module Language.Haskell.Liquid.UX.Annotate
   , annErrors
   ) where
 
+import           Control.Monad (unless)
 import           Data.Hashable
 import qualified Data.Text.IO as Text
 import           Data.String
@@ -132,10 +133,16 @@ copyFileCreateParentDirIfMissing src tgt = do
   Dir.createDirectoryIfMissing False $ tempDirectory tgt
   Dir.copyFile src tgt
 
+-- | Creates the parent directory and writes the file if the
+-- file does not exist already.
+-- This prevents failure if multiple threads are trying to write to
+-- the same file.
 writeFileCreateParentDirIfMissing :: T.Text -> FilePath -> IO ()
 writeFileCreateParentDirIfMissing s tgt = do
   Dir.createDirectoryIfMissing False $ tempDirectory tgt
-  Text.writeFile tgt s
+  exists <- Dir.doesFileExist tgt
+  unless exists $
+    Text.writeFile tgt s
 
 writeFilesOrStrings :: FilePath -> [Either FilePath String] -> IO ()
 writeFilesOrStrings tgtFile = mapM_ $ either (`copyFileCreateParentDirIfMissing` tgtFile) (tgtFile `appendFile`)