remove the TChan component from the data structure

The code to maintain that TChan in parallel with the list was buggy,
the two were not always the same. And all that TChan was needed for was
blocking on the next transfer, which can be accomplished just as well by
checking the size and retrying, thanks to STM.

Also, this is faster, and uses less memory. Total win.

Assistant/TransferQueue.hs

@@ -29,12 +29,8 @@ import qualified Types.Remote as Remote
 import Control.Concurrent.STM
 import qualified Data.Map as M
 
-{- The transfer queue consists of a channel listing the transfers to make;
- - the size of the queue is also tracked, and a list is maintained
- - in parallel to allow for reading. -}
 data TransferQueue = TransferQueue
-	{ queue :: TChan (Transfer, TransferInfo)
-	, queuesize :: TVar Int
+	{ queuesize :: TVar Int
 	, queuelist :: TVar [(Transfer, TransferInfo)]
 	}
 
@@ -43,8 +39,7 @@ data Schedule = Next | Later
 
 newTransferQueue :: IO TransferQueue
 newTransferQueue = atomically $ TransferQueue
-	<$> newTChan
-	<*> newTVar 0
+	<$> newTVar 0
 	<*> newTVar []
 
 {- Reads the queue's content without blocking or changing it. -}
@@ -91,13 +86,12 @@ queueTransfers schedule q dstatus k f direction = do
 
 enqueue :: Schedule -> TransferQueue -> DaemonStatusHandle -> Transfer -> TransferInfo -> IO ()
 enqueue schedule q dstatus t info
-	| schedule == Next = go unGetTChan (new:)
-	| otherwise = go writeTChan (\l -> l++[new])
+	| schedule == Next = go (new:)
+	| otherwise = go (\l -> l++[new])
 	where
 		new = (t, info)
-		go modqueue modlist = do
+		go modlist = do
 			atomically $ do
-				void $ modqueue (queue q) new
 				void $ modifyTVar' (queuesize q) succ
 				void $ modifyTVar' (queuelist q) modlist
 			void $ notifyTransfer dstatus
@@ -121,7 +115,7 @@ queueTransferAt wantsz schedule q dstatus f t remote = do
 queueTransferWhenSmall :: TransferQueue -> DaemonStatusHandle -> AssociatedFile -> Transfer -> Remote -> IO ()
 queueTransferWhenSmall = queueTransferAt 10 Later
 
-{- Blocks until a pending transfer is available from the queue,
+{- Blocks until a pending transfer is available in the queue,
  - and removes it.
  -
  - Checks that it's acceptable, before adding it to the
@@ -131,15 +125,19 @@ queueTransferWhenSmall = queueTransferAt 10 Later
  - where an observer sees an inconsistent status. -}
 getNextTransfer :: TransferQueue -> DaemonStatusHandle -> (TransferInfo -> Bool) -> IO (Maybe (Transfer, TransferInfo))
 getNextTransfer q dstatus acceptable = atomically $ do
-	void $ modifyTVar' (queuesize q) pred
-	void $ modifyTVar' (queuelist q) (drop 1)
-	r@(t, info) <- readTChan (queue q)
-	if acceptable info
-		then do
-			adjustTransfersSTM dstatus $
-				M.insertWith' const t info
-			return $ Just r
-		else return Nothing
+	sz <- readTVar (queuesize q)
+	if sz < 1
+		then retry -- blocks until queuesize changes
+		else do
+			(r@(t,info):rest) <- readTVar (queuelist q)
+			writeTVar (queuelist q) rest
+			void $ modifyTVar' (queuesize q) pred
+			if acceptable info
+				then do
+					adjustTransfersSTM dstatus $
+						M.insertWith' const t info
+					return $ Just r
+				else return Nothing
 
 {- Moves transfers matching a condition from the queue, to the
  - currentTransfers map. -}
@@ -165,8 +163,4 @@ dequeueTransfersSTM q c = do
 		<$> readTVar (queuelist q)
 	void $ writeTVar (queuesize q) (length ts)
 	void $ writeTVar (queuelist q) ts
-	drain
-	forM_ ts $ unGetTChan (queue q)
 	return removed
-	where
-		drain = maybe noop (const drain) =<< tryReadTChan (queue q)