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diff --git a/Assistant/Types/TransferSlots.hs b/Assistant/Types/TransferSlots.hs
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+{- git-annex assistant transfer slots
+ -
+ - Copyright 2012 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+{-# LANGUAGE DeriveDataTypeable #-}
+
+module Assistant.Types.TransferSlots where
+
+import Assistant.Types.DaemonStatus
+import Logs.Transfer
+
+import qualified Control.Exception as E
+import qualified Control.Concurrent.MSemN as MSemN
+import Data.Typeable
+
+type TransferSlots = MSemN.MSemN Int
+
+{- A special exception that can be thrown to pause or resume a transfer, while
+ - keeping its slot in use. -}
+data TransferException = PauseTransfer | ResumeTransfer
+	deriving (Show, Eq, Typeable)
+
+instance E.Exception TransferException
+
+type TransferSlotRunner = DaemonStatusHandle -> TransferSlots -> TransferGenerator -> IO ()
+type TransferGenerator = IO (Maybe (Transfer, TransferInfo, IO ()))
+
+{- Number of concurrent transfers allowed to be run from the assistant.
+ -
+ - Transfers launched by other means, including by remote assistants,
+ - do not currently take up slots.
+ -}
+numSlots :: Int
+numSlots = 1
+
+newTransferSlots :: IO TransferSlots
+newTransferSlots = MSemN.new numSlots