avoid spawning new transfer thread until a slot becomes available

Assistant/Threads/TransferScanner.hs

@@ -19,7 +19,7 @@ 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. -}
+ - keep their data in sync. The transfers are queued with low priority. -}
 transferScannerThread :: ThreadState -> ScanRemoteMap -> TransferQueue -> IO ()
 transferScannerThread st scanremotes transferqueue = do
 	runEvery (Seconds 2) $ do

Assistant/TransferSlots.hs

@@ -29,9 +29,10 @@ newTransferSlots = newQSemN numSlots
 {- Waits until a transfer slot becomes available, and runs a transfer
  - action in the slot, in its own thread. -}
 inTransferSlot :: TransferSlots -> ThreadState -> Annex a -> IO ThreadId
-inTransferSlot s st a = forkIO $ bracket_ start done run
+inTransferSlot s st a = do
+	waitQSemN s 1
+	forkIO $ bracket_ noop done run	
 	where
-		start = waitQSemN s 1
 		done = transferComplete s
 		run = unsafeRunThreadState st a