add BackgroundWriter to ensure that files are fully written

builder/src/BackgroundWriter.hs

@@ -0,0 +1,42 @@
+{-# LANGUAGE BangPatterns #-}
+module BackgroundWriter
+  ( Scope
+  , withScope
+  , writeBinary
+  )
+  where
+
+
+import Control.Concurrent (forkIO)
+import Control.Concurrent.MVar (MVar, newEmptyMVar, newMVar, putMVar, takeMVar)
+import qualified Data.Binary as Binary
+import Data.Foldable (traverse_)
+
+import qualified File
+
+
+
+-- BACKGROUND WRITER
+
+
+newtype Scope =
+  Scope (MVar [MVar ()])
+
+
+withScope :: (Scope -> IO a) -> IO a
+withScope callback =
+  do  workList <- newMVar []
+      result <- callback (Scope workList)
+      mvars <- takeMVar workList
+      traverse_ takeMVar mvars
+      return result
+
+
+writeBinary :: (Binary.Binary a) => Scope -> FilePath -> a -> IO ()
+writeBinary (Scope workList) path value =
+  do  mvar <- newEmptyMVar
+      _ <- forkIO (File.writeBinary path value >> putMVar mvar ())
+      oldWork <- takeMVar workList
+      let !newWork = mvar:oldWork
+      putMVar workList newWork
+

elm.cabal

@@ -78,6 +78,7 @@ Executable elm
 
         -- from builder/
         Build
+        BackgroundWriter
         Deps.Bump
         Deps.Diff
         Deps.Registry