scan multiple remotes in one pass

The expensive transfer scan now scans a whole set of remotes in one pass.
So at startup, or when network comes up, it will run only once.

Note that this can result in transfers from/to higher cost remotes being
queued before other transfers of other content from/to lower cost remotes.
Before, low cost remotes were scanned first and all their transfers came
first. When multiple transfers are queued for a key, the lower cost ones
are still queued first. However, this could result in transfers from slow
remotes running for a long time while transfers of other data from faster
remotes waits.

I expect to make the transfer queue smarter about ordering
and/or make it allow multiple transfers at a time, which should eliminate
this annoyance. (Also, it was already possible to get into that situation,
for example if the network was up, lots of transfers from slow remotes
might be queued, and then a disk is mounted and its faster transfers have
to wait.)

Also note that this means I don't need to improve the code in
Assistant.Sync that currently checks if any of the reconnected remotes
have diverged, and if so, queues scans of all of them. That had been very
innefficient, but now doesn't matter.

Assistant/Threads/TransferScanner.hs

@@ -36,13 +36,13 @@ transferScannerThread st dstatus scanremotes transferqueue = do
 	where
 		go scanned = do
 			threadDelaySeconds (Seconds 2)
-			(r, info) <- getScanRemote scanremotes
-			if fullScan info || not (S.member r scanned)
+			(rs, infos) <- unzip <$> getScanRemote scanremotes
+			if any fullScan infos || any (`S.notMember` scanned) rs
 				then do
-					expensiveScan st dstatus transferqueue r
-					go (S.insert r scanned)
+					expensiveScan st dstatus transferqueue rs
+					go (S.union scanned (S.fromList rs))
 				else do
-					failedTransferScan st dstatus transferqueue r
+					mapM_ (failedTransferScan st dstatus transferqueue) rs
 					go scanned
 		{- All available remotes are scanned in full on startup,
 		 - for multiple reasons, including:
@@ -92,39 +92,39 @@ failedTransferScan st dstatus transferqueue r = do
 		dequeue t = void $ runThreadState st $ inRepo $
 			liftIO . tryIO . removeFile . failedTransferFile t
 
-{- This is a expensive scan through the full git work tree.
+{- This is a expensive scan through the full git work tree, finding
+ - files to download from or upload to any of the remotes.
  - 
  - The scan is blocked when the transfer queue gets too large. -}
-expensiveScan :: ThreadState -> DaemonStatusHandle -> TransferQueue -> Remote -> IO ()
-expensiveScan st dstatus transferqueue r = do
-	liftIO $ debug thisThread ["starting scan of", show r]
-	void $ alertWhile dstatus (scanAlert r) $ do
+expensiveScan :: ThreadState -> DaemonStatusHandle -> TransferQueue -> [Remote] -> IO ()
+expensiveScan st dstatus transferqueue rs = do
+	liftIO $ debug thisThread ["starting scan of", show rs]
+	void $ alertWhile dstatus (scanAlert rs) $ do
 		g <- runThreadState st $ fromRepo id
 		files <- LsFiles.inRepo [] g
 		go files
 		return True
-	liftIO $ debug thisThread ["finished scan of", show r]
+	liftIO $ debug thisThread ["finished scan of", show rs]
 	where
 		go [] = noop
 		go (f:fs) = do
-			v <- runThreadState st $ whenAnnexed check f
-			case v of
-				Nothing -> noop
-				Just t -> do
-					debug thisThread ["queuing", show t]
-					enqueue f t
+			mapM_ (enqueue f) =<< catMaybes <$> runThreadState st
+				(ifAnnexed f findtransfers $ return [])
 			go fs
-			where
-				check _ (key, _) = ifM (inAnnex key)
-					( helper key Upload False =<< remoteHas r key
-					, helper key Download True =<< remoteHas r key
-					)
-				helper key direction x y
-					| x == y = return $ Just $
-						Transfer direction (Remote.uuid r) key
-					| otherwise = return Nothing
-				
-		enqueue f t = queueTransferWhenSmall transferqueue dstatus (Just f) t r
+		enqueue f (r, t) = do
+			debug thisThread ["queuing", show t]
+			queueTransferWhenSmall transferqueue dstatus (Just f) t r
+		findtransfers (key, _) = do
+			locs <- loggedLocations key
+			let use a = return $ map (a key locs) rs
+			ifM (inAnnex key)
+				( use $ check Upload False
+				, use $ check Download True
+				)
+		check direction want key locs r
+			| (Remote.uuid r `elem` locs) == want = Just $
+				(r, Transfer direction (Remote.uuid r) key)
+			| otherwise = Nothing
 
 remoteHas :: Remote -> Key -> Annex Bool
 remoteHas r key = elem