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, 7 insertions, 7 deletions

diff --git a/src/Signal.hs b/src/Signal.hs
index a003859..44fe4f9 100644
--- a/src/Signal.hs
+++ b/src/Signal.hs
@@ -19,7 +19,7 @@
 module Signal where
 
 import Data.Typeable (Typeable)
-import Control.Concurrent
+import Control.Concurrent.STM
 import Control.Exception hiding (handle)
 import System.Posix.Signals
 
@@ -52,19 +52,19 @@ parseSignalType :: String -> Maybe SignalType
 parseSignalType = fmap fst . safeHead . reads
 
 -- | Signal handling
-setupSignalHandler :: IO (MVar SignalType)
+setupSignalHandler :: IO (TMVar SignalType)
 setupSignalHandler = do
-   tid   <- newEmptyMVar
+   tid   <- newEmptyTMVarIO
    installHandler sigUSR2 (Catch $ updatePosHandler tid) Nothing
    installHandler sigUSR1 (Catch $ changeScreenHandler tid) Nothing
    return tid
 
-updatePosHandler :: MVar SignalType -> IO ()
+updatePosHandler :: TMVar SignalType -> IO ()
 updatePosHandler sig = do
-   putMVar sig Reposition
+   atomically $ putTMVar sig Reposition
    return ()
 
-changeScreenHandler :: MVar SignalType -> IO ()
+changeScreenHandler :: TMVar SignalType -> IO ()
 changeScreenHandler sig = do
-   putMVar sig ChangeScreen
+   atomically $ putTMVar sig ChangeScreen
    return ()