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{- making the Annex monad available across threads
-
- Copyright 2012 Joey Hess <joey@kitenet.net>
-}
{-# LANGUAGE BangPatterns #-}
module Assistant.ThreadedMonad where
import Common.Annex
import qualified Annex
import Control.Concurrent
import Control.Exception (throw)
{- The Annex state is stored in a MVar, so that threaded actions can access
- it. -}
type ThreadState = MVar Annex.AnnexState
{- Stores the Annex state in a MVar.
-
- Once the action is finished, retrieves the state from the MVar.
-}
withThreadState :: (ThreadState -> Annex a) -> Annex a
withThreadState a = do
state <- Annex.getState id
mvar <- liftIO $ newMVar state
r <- a mvar
newstate <- liftIO $ takeMVar mvar
Annex.changeState (const newstate)
return r
{- Runs an Annex action, using the state from the MVar.
-
- This serializes calls by threads. -}
runThreadState :: ThreadState -> Annex a -> IO a
runThreadState mvar a = do
startstate <- takeMVar mvar
-- catch IO errors and rethrow after restoring the MVar
!(r, newstate) <- catchIO (Annex.run startstate a) $ \e -> do
putMVar mvar startstate
throw e
putMVar mvar newstate
return r
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