add TransferScanner thread

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

Assistant/Threads/TransferScanner.hs

@@ -0,0 +1,34 @@
+{- git-annex assistant thread to scan remotes to find needed transfers
+ -
+ - Copyright 2012 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Assistant.Threads.TransferScanner where
+
+import Assistant.Common
+import Assistant.ScanRemotes
+import Assistant.TransferQueue
+import Assistant.ThreadedMonad
+import Logs.Transfer
+import Types.Remote
+import Utility.ThreadScheduler
+
+thisThread :: ThreadName
+thisThread = "TransferScanner"
+
+{- This thread scans remotes, to find transfers that need to be made to
+ - keep their data in sync. The transfers are queued with lot priority. -}
+transferScannerThread :: ThreadState -> ScanRemoteMap -> TransferQueue -> IO ()
+transferScannerThread st scanremotes transferqueue = do
+	runEvery (Seconds 2) $ do
+		r <- getScanRemote scanremotes
+		needtransfer <- scan st r
+		forM_ needtransfer $ \(f, t) ->
+			queueLaterTransfer transferqueue f t
+
+scan :: ThreadState -> Remote -> IO [(AssociatedFile, Transfer)]
+scan st r = do
+	debug thisThread ["scanning", show r]
+	return [] -- TODO