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diff --git a/Assistant/ThreadedMonad.hs b/Assistant/ThreadedMonad.hs
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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
+
+{- 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
+	!(r, newstate) <- Annex.run startstate a
+	putMVar mvar newstate
+	return r