send notifications when the TransferQueue is changed

The fun part was making it move things from TransferQueue to currentTransfers
entirely atomically. Which will avoid inconsistent display if the WebApp
renders the current status at just the wrong time. STM to the rescue!

Assistant/DaemonStatus.hs

@@ -36,6 +36,7 @@ data DaemonStatus = DaemonStatus
 	-- Ordered list of remotes to talk to.
 	, knownRemotes :: [Remote]
 	-- Clients can use this to wait on changes to the DaemonStatus
+	-- and other related things like the TransferQueue.
 	, notificationBroadcaster :: NotificationBroadcaster
 	}
 
@@ -72,6 +73,12 @@ modifyDaemonStatus handle a = do
 	sendNotification nb
 	return b
 
+{- Can be used to send a notification that the daemon status, or other
+ - associated thing, like the TransferQueue, has changed. -}
+notifyDaemonStatusChange :: DaemonStatusHandle -> IO ()
+notifyDaemonStatusChange handle = sendNotification
+	=<< notificationBroadcaster <$> atomically (readTMVar handle)
+
 {- Updates the cached ordered list of remotes from the list in Annex
  - state. -}
 updateKnownRemotes :: DaemonStatusHandle -> Annex ()
@@ -164,7 +171,16 @@ afterLastDaemonRun timestamp status = maybe False (< t) (lastRunning status)
 tenMinutes :: Int
 tenMinutes = 10 * 60
 
-{- Mutates the transfer map. -}
+{- Mutates the transfer map. Runs in STM so that the transfer map can
+ - be modified in the same transaction that modifies the transfer queue.
+ - Note that this does not send a notification of the change; that's left
+ - to the caller. -}
+adjustTransfersSTM :: DaemonStatusHandle -> (TransferMap -> TransferMap) -> STM ()
+adjustTransfersSTM dstatus a = do
+	s <- takeTMVar dstatus
+	putTMVar dstatus $ s { currentTransfers = a (currentTransfers s) }
+
+{- Variant that does send notifications. -}
 adjustTransfers :: DaemonStatusHandle -> (TransferMap -> TransferMap) -> IO ()
 adjustTransfers dstatus a = modifyDaemonStatus_ dstatus $
 	\s -> s { currentTransfers = a (currentTransfers s) }