add TransferScanner thread

Efficiently finding transfers that need to be done to get two repos back
in sync seems like an interesting problem.

Assistant/ScanRemotes.hs

@@ -0,0 +1,41 @@
+{- git-annex assistant remotes needing scanning
+ -
+ - Copyright 2012 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Assistant.ScanRemotes where
+
+import Common.Annex
+import Data.Function
+
+import Control.Concurrent.STM
+import Data.Time.Clock
+import qualified Data.Map as M
+
+type ScanRemoteMap = TMVar (M.Map Remote UTCTime)
+
+{- The TMVar starts empty, and is left empty when there are no remotes
+ - to scan. -}
+newScanRemoteMap :: IO ScanRemoteMap
+newScanRemoteMap = atomically newEmptyTMVar
+
+{- Blocks until there is a remote that needs to be scanned.
+ - Processes remotes added most recently first. -}
+getScanRemote :: ScanRemoteMap -> IO Remote
+getScanRemote v = atomically $ do
+	m <- takeTMVar v
+	let newest = Prelude.head $ reverse $
+		map fst $ sortBy (compare `on` snd) $ M.toList m
+	putTMVar v $ M.delete newest m
+	return newest
+
+{- Adds new remotes that need scanning to the map. -}
+addScanRemotes :: ScanRemoteMap -> [Remote] -> IO ()
+addScanRemotes _ [] = return ()
+addScanRemotes v rs = do
+	now <- getCurrentTime
+	atomically $ do
+		m <- fromMaybe M.empty <$> tryTakeTMVar v
+		putTMVar v $ foldr (`M.insert` now) m rs