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diff --git a/Assistant/TransferSlots.hs b/Assistant/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.
+ -}
+
+module Assistant.TransferSlots where
+
+import Control.Exception
+import Control.Concurrent
+
+type TransferSlots = QSemN
+
+{- 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 = newQSemN numSlots
+
+{- Waits until a transfer slot becomes available, and runs a transfer
+ - action in the slot.
+ -}
+inTransferSlot :: TransferSlots -> IO a -> IO a
+inTransferSlot s = bracket_ (waitQSemN s 1) (signalQSemN s 1)