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{- making the Annex monad available across threads
-
- Copyright 2012 Joey Hess <joey@kitenet.net>
-
- Licensed under the GNU GPL version 3 or higher.
-}
module Assistant.ThreadedMonad where
import Common.Annex
import qualified Annex
import Control.Concurrent
import Data.Tuple
{- 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; only one thread can run in Annex at a
- time. -}
runThreadState :: ThreadState -> Annex a -> IO a
runThreadState mvar a = modifyMVar mvar $ \state -> swap <$> Annex.run state a
{- Runs an Annex action, using a copy of the state from the MVar.
-
- The state modified by the action is thrown away, so it's up to the
- action to perform any necessary shutdown tasks in order for state to not
- be lost. And it's up to the caller to resynchronise with any changes
- the action makes to eg, the git-annex branch.
-}
unsafeRunThreadState :: ThreadState -> Annex a -> IO a
unsafeRunThreadState mvar a = do
state <- readMVar mvar
Annex.eval state a
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