e34046de38
It's a mvar lookup either way, but I think this way will be slightly more efficient. And it reduces the number of places where it's checked to 1.
69 lines
1.7 KiB
Haskell
69 lines
1.7 KiB
Haskell
{- Handle for the Keys database.
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-
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- Copyright 2015 Joey Hess <id@joeyh.name>
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-:
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- Licensed under the GNU GPL version 3 or higher.
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-}
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module Database.Keys.Handle (
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DbHandle,
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newDbHandle,
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unavailableDbHandle,
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DbState(..),
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withDbState,
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flushDbQueue,
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closeDbHandle,
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) where
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import qualified Database.Queue as H
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import Utility.Exception
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import Control.Concurrent
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import Control.Monad.IO.Class (liftIO, MonadIO)
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import Control.Applicative
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import Prelude
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-- The MVar is always left full except when actions are run
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-- that access the database.
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newtype DbHandle = DbHandle (MVar DbState)
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-- The database can be closed or open, but it also may have been
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-- tried to open (for read) and didn't exist yet or is not readable.
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data DbState = DbClosed | DbOpen H.DbQueue | DbUnavailable
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newDbHandle :: IO DbHandle
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newDbHandle = DbHandle <$> newMVar DbClosed
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unavailableDbHandle :: IO DbHandle
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unavailableDbHandle = DbHandle <$> newMVar DbUnavailable
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-- Runs an action on the state of the handle, which can change its state.
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-- The MVar is empty while the action runs, which blocks other users
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-- of the handle from running.
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withDbState
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:: (MonadIO m, MonadCatch m)
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=> DbHandle
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-> (DbState -> m (v, DbState))
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-> m v
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withDbState (DbHandle mvar) a = do
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st <- liftIO $ takeMVar mvar
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go st `onException` (liftIO $ putMVar mvar st)
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where
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go st = do
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(v, st') <- a st
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liftIO $ putMVar mvar st'
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return v
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flushDbQueue :: DbHandle -> IO ()
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flushDbQueue (DbHandle mvar) = go =<< readMVar mvar
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where
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go (DbOpen qh) = H.flushDbQueue qh
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go _ = return ()
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closeDbHandle :: DbHandle -> IO ()
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closeDbHandle h = withDbState h go
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where
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go (DbOpen qh) = do
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H.closeDbQueue qh
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return ((), DbClosed)
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go st = return ((), st)
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