move DaemonStatus manipulation out of the Annex monad to IO

I've convinced myself that nothing in DaemonStatus can deadlock,
as it always keepts the TMVar full. That was the only reason it was in the
Annex monad.

Assistant/DaemonStatus.hs

@@ -15,7 +15,7 @@ import Utility.NotificationBroadcaster
 import Logs.Transfer
 import qualified Command.Sync
 
-import Control.Concurrent
+import Control.Concurrent.STM
 import System.Posix.Types
 import Data.Time.Clock.POSIX
 import Data.Time
@@ -41,7 +41,8 @@ data DaemonStatus = DaemonStatus
 
 type TransferMap = M.Map Transfer TransferInfo
 
-type DaemonStatusHandle = MVar DaemonStatus
+{- This TMVar is never left empty, so accessing it will never block. -}
+type DaemonStatusHandle = TMVar DaemonStatus
 
 newDaemonStatus :: IO DaemonStatus
 newDaemonStatus = do
@@ -56,21 +57,19 @@ newDaemonStatus = do
 		, notificationBroadcaster = nb
 		}
 
-getDaemonStatus :: DaemonStatusHandle -> Annex DaemonStatus
-getDaemonStatus = liftIO . readMVar
+getDaemonStatus :: DaemonStatusHandle -> IO DaemonStatus
+getDaemonStatus = atomically . readTMVar
 
-modifyDaemonStatus_ :: DaemonStatusHandle -> (DaemonStatus -> DaemonStatus) -> Annex ()
-modifyDaemonStatus_ handle a = do
-	nb <- liftIO $ modifyMVar handle $ \s -> return
-		(a s, notificationBroadcaster s)
-	liftIO $ sendNotification nb
+modifyDaemonStatus_ :: DaemonStatusHandle -> (DaemonStatus -> DaemonStatus) -> IO ()
+modifyDaemonStatus_ handle a = modifyDaemonStatus handle $ \s -> (a s, ())
 
-modifyDaemonStatus :: DaemonStatusHandle -> (DaemonStatus -> (DaemonStatus, b)) -> Annex b
+modifyDaemonStatus :: DaemonStatusHandle -> (DaemonStatus -> (DaemonStatus, b)) -> IO b
 modifyDaemonStatus handle a = do
-	(b, nb) <- liftIO $ modifyMVar handle $ \s -> do
-		let (s', b) = a s
-		return $ (s', (b, notificationBroadcaster s))
-	liftIO $ sendNotification nb
+	(b, nb) <- atomically $ do
+		(s, b) <- a <$> takeTMVar handle
+		putTMVar handle s
+		return $ (b, notificationBroadcaster s)
+	sendNotification nb
 	return b
 
 {- Updates the cached ordered list of remotes from the list in Annex
@@ -78,10 +77,10 @@ modifyDaemonStatus handle a = do
 updateKnownRemotes :: DaemonStatusHandle -> Annex ()
 updateKnownRemotes dstatus = do
 	remotes <- Command.Sync.syncRemotes []
-	modifyDaemonStatus_ dstatus $
+	liftIO $ modifyDaemonStatus_ dstatus $
 		\s -> s { knownRemotes = remotes }
 
-{- Load any previous daemon status file, and store it in the MVar for this
+{- Load any previous daemon status file, and store it in a MVar for this
  - process to use as its DaemonStatus. Also gets current transfer status. -}
 startDaemonStatus :: Annex DaemonStatusHandle
 startDaemonStatus = do
@@ -90,7 +89,7 @@ startDaemonStatus = do
 		catchDefaultIO (readDaemonStatusFile file) =<< newDaemonStatus
 	transfers <- M.fromList <$> getTransfers
 	remotes <- Command.Sync.syncRemotes []
-	liftIO $ newMVar status
+	liftIO $ atomically $ newTMVar status
 		{ scanComplete = False
 		, sanityCheckRunning = False
 		, currentTransfers = transfers
@@ -102,18 +101,17 @@ startDaemonStatus = do
  -}
 daemonStatusThread :: ThreadState -> DaemonStatusHandle -> IO ()
 daemonStatusThread st handle = do
-	bhandle <- runThreadState st $ 
-		liftIO . newNotificationHandle
-			=<< notificationBroadcaster <$> getDaemonStatus handle
+	bhandle <- newNotificationHandle
+		=<< notificationBroadcaster <$> getDaemonStatus handle
 	checkpoint
 	runEvery (Seconds tenMinutes) $ do
-		liftIO $ waitNotification bhandle
+		waitNotification bhandle
 		checkpoint
 	where
-		checkpoint = runThreadState st $ do
-			file <- fromRepo gitAnnexDaemonStatusFile
+		checkpoint = do
 			status <- getDaemonStatus handle
-			liftIO $ writeDaemonStatusFile file status
+			file <- runThreadState st $ fromRepo gitAnnexDaemonStatusFile
+			writeDaemonStatusFile file status
 
 {- Don't just dump out the structure, because it will change over time,
  - and parts of it are not relevant. -}
@@ -167,12 +165,12 @@ tenMinutes :: Int
 tenMinutes = 10 * 60
 
 {- Mutates the transfer map. -}
-adjustTransfers :: DaemonStatusHandle -> (TransferMap -> TransferMap) -> Annex ()
+adjustTransfers :: DaemonStatusHandle -> (TransferMap -> TransferMap) -> IO ()
 adjustTransfers dstatus a = modifyDaemonStatus_ dstatus $
 	\s -> s { currentTransfers = a (currentTransfers s) }
 
 {- Removes a transfer from the map, and returns its info. -}
-removeTransfer :: DaemonStatusHandle -> Transfer -> Annex (Maybe TransferInfo)
+removeTransfer :: DaemonStatusHandle -> Transfer -> IO (Maybe TransferInfo)
 removeTransfer dstatus t = modifyDaemonStatus dstatus go
 	where
 		go s =