generalize withNullHandle to MonadIO

Assistant.hs

@@ -100,18 +100,20 @@ startDaemon assistant foreground startdelay cannotrun listenhost startbrowser =
 		let flag = "GIT_ANNEX_OUTPUT_REDIR"
 		createAnnexDirectory (parentDir logfile)
 		ifM (liftIO $ isNothing <$> getEnv flag)
-			( liftIO $ withFile devNull WriteMode $ \nullh -> do
+			( liftIO $ withNullHandle $ \nullh -> do
 				loghandle <- openLog logfile
 				e <- getEnvironment
 				cmd <- programPath
 				ps <- getArgs
-				(_, _, _, pid) <- createProcess (proc cmd ps)
+				let p = (proc cmd ps)
 					{ env = Just (addEntry flag "1" e)
 					, std_in = UseHandle nullh
 					, std_out = UseHandle loghandle
 					, std_err = UseHandle loghandle
 					}
-				exitWith =<< waitForProcess pid
+				exitcode <- withCreateProcess p $ \_ _ _ pid ->
+					waitForProcess pid
+				exitWith exitcode
 			, start (Utility.Daemon.foreground (Just pidfile)) $
 				case startbrowser of
 					Nothing -> Nothing

Assistant/WebApp/DashBoard.hs

@@ -137,8 +137,8 @@ openFileBrowser = do
 	ifM (liftIO $ inPath cmd)
 		( do
 			let run = void $ liftIO $ forkIO $ do
-				(Nothing, Nothing, Nothing, pid) <- createProcess p
-				void $ waitForProcess pid
+				withCreateProcess p $ \_ _ _ pid -> void $
+					waitForProcess pid
 			run
 #ifdef mingw32_HOST_OS
 			{- On windows, if the file browser is not

Utility/Process.hs

@@ -50,6 +50,7 @@ import Utility.Exception
 import System.Exit
 import System.IO
 import System.Log.Logger
+import Control.Monad.IO.Class
 import Control.Concurrent
 import qualified Control.Exception as E
 import Control.Monad
@@ -207,8 +208,10 @@ withOEHandles creator p a = creator p' $ a . oeHandles
 		, std_err = CreatePipe
 		}
 
-withNullHandle :: (Handle -> IO a) -> IO a
-withNullHandle = withFile devNull WriteMode
+withNullHandle :: (MonadIO m, MonadMask m) => (Handle -> m a) -> m a
+withNullHandle = bracket
+	(liftIO $ openFile devNull WriteMode)
+	(liftIO . hClose)
 
 -- | Forces the CreateProcessRunner to run quietly;
 -- both stdout and stderr are discarded.
@@ -230,7 +233,7 @@ feedWithQuietOutput
 	-> CreateProcess
 	-> (Handle -> IO a)
 	-> IO a
-feedWithQuietOutput creator p a = withFile devNull WriteMode $ \nullh -> do
+feedWithQuietOutput creator p a = withNullHandle $ \nullh -> do
 	let p' = p
 		{ std_in = CreatePipe
 		, std_out = UseHandle nullh