Refactor MVar SignalType to TMVar SignalType

Replace MVar with TMVar from the STM package. This is common for ghc
now. Since STM is used everywhere else in the src it also adds no
additional dependencies.

The main reason for this switch is, that readMVar, swapMVar, etc. are
only atomically if there is no other producer for this MVar i.e.
putMVar. For example readMVar is a combination of putMVar and takeMVar.
Due to scheduling and readMVar's non-atomicity it is possible that
values written to the MVar appear in the wrong order.

Using TMVar fixes this problem, since it allows really atomical
read/swap operations.

1 files changed, 11 insertions, 10 deletions

diff --git a/src/Xmobar.hs b/src/Xmobar.hs
index 2dbba11..79234d2 100644
--- a/src/Xmobar.hs
+++ b/src/Xmobar.hs
@@ -75,7 +75,7 @@ runX :: XConf -> X () -> IO ()
 runX xc f = runReaderT f xc
 
 -- | Starts the main event loop and threads
-startLoop :: XConf -> MVar SignalType -> [[(Maybe ThreadId, TVar String)]] -> IO ()
+startLoop :: XConf -> TMVar SignalType -> [[(Maybe ThreadId, TVar String)]] -> IO ()
 startLoop xcfg@(XConf _ _ w _ _) sig vs = do
     tv <- atomically $ newTVar []
     _ <- forkIO (checker tv [] vs sig `catch`
@@ -107,16 +107,16 @@ startLoop xcfg@(XConf _ _ w _ _) sig vs = do
 #endif
           ev <- getEvent e
           case ev of
-            ConfigureEvent {} -> putMVar signal Reposition
-            ExposeEvent {} -> putMVar signal Wakeup
-            RRScreenChangeNotifyEvent {} -> putMVar signal Reposition
+            ConfigureEvent {} -> atomically $ putTMVar signal Reposition
+            ExposeEvent {} -> atomically $ putTMVar signal Wakeup
+            RRScreenChangeNotifyEvent {} -> atomically $ putTMVar signal Reposition
             _ -> return ()
 
 -- | Send signal to eventLoop every time a var is updated
 checker :: TVar [String]
            -> [String]
            -> [[(Maybe ThreadId, TVar String)]]
-           -> MVar SignalType
+           -> TMVar SignalType
            -> IO ()
 checker tvar ov vs signal = do
       nval <- atomically $ do
@@ -124,16 +124,16 @@ checker tvar ov vs signal = do
               guard (nv /= ov)
               writeTVar tvar nv
               return nv
-      putMVar signal Wakeup
+      atomically $ putTMVar signal Wakeup
       checker tvar nval vs signal
     where
       concatV = fmap concat . mapM (readTVar . snd)
 
 
 -- | Continuously wait for a signal from a thread or a interrupt handler
-eventLoop :: TVar [String] -> XConf -> MVar SignalType -> IO ()
+eventLoop :: TVar [String] -> XConf -> TMVar SignalType -> IO ()
 eventLoop tv xc@(XConf d _ w fs cfg) signal = do
-      typ <- takeMVar signal
+      typ <- atomically $ takeTMVar signal
       case typ of
          Wakeup -> do
             runX xc (updateWin tv)
@@ -186,7 +186,7 @@ eventLoop tv xc@(XConf d _ w fs cfg) signal = do
 
 -- | Runs a command as an independent thread and returns its thread id
 -- and the TVar the command will be writing to.
-startCommand :: MVar SignalType
+startCommand :: TMVar SignalType
              -> (Runnable,String,String)
              -> IO (Maybe ThreadId, TVar String)
 startCommand sig (com,s,ss)
@@ -196,7 +196,8 @@ startCommand sig (com,s,ss)
     | otherwise       = do var <- atomically $ newTVar is
                            let cb str = atomically $ writeTVar var (s ++ str ++ ss)
                            h <- forkIO $ start com cb
-                           _ <- forkIO $ trigger com ( maybe (return ()) (putMVar sig) )
+                           _ <- forkIO $ trigger com
+                                       $ maybe (return ()) (atomically . putTMVar sig)
                            return (Just h,var)
     where is = s ++ "Updating..." ++ ss