add two long-running XMPP push threads, no more inversion of control

I hope this will be easier to reason about, and less buggy. It was
certianly easier to write!

An immediate benefit is that with a traversable queue of push requests to
select from, the threads can be a lot fairer about choosing which client to
service next.

Assistant/NetMessager.hs

@@ -14,7 +14,6 @@ import Assistant.Types.NetMessager
 
 import Control.Concurrent.STM
 import Control.Concurrent.MSampleVar
-import Control.Exception as E
 import qualified Data.Set as S
 import qualified Data.Map as M
 import qualified Data.DList as D
@@ -69,73 +68,39 @@ checkImportantNetMessages (storedclient, sentclient) = go <<~ netMessager
 		sent <- M.lookup sentclient <$> (readTMVar $ sentImportantNetMessages nm)
 		return (fromMaybe S.empty stored, fromMaybe S.empty sent)
 
-{- Runs an action that runs either the send or receive side of a push.
- - Only one such action per side can run at a time. Other pushes, for
- - the same, or other clients, need to wait their turn.
- -
- - Only one push is allowed to wait per client.
- - There is no point in building up more.
- -
- - While the push is running, netMessagesPush will get messages put into it
- - relating to this push, while any messages relating to other pushes
- - on the same side go to netMessagesDeferred. Once the push finishes,
- - those deferred messages will be fed to handledeferred for processing.
- -}
-runPush :: PushSide -> ClientID -> Assistant Bool -> Assistant Bool
-runPush side clientid a = do
-	debugmsg "preparing to run"
-	nm <- getAssistant netMessager
-	let setup = do
-		(canrun, release) <- atomically $ checkcanrun nm
-		if canrun
-			then atomically $ waittorun nm release
-			else return (False, noop)
-	let cleanup (_, release) = atomically release
-	go <- asIO1 $ \(run, _) ->
-		if run
-			then do
-				debugmsg "started running"
-				r <- a
-				debugmsg "finished running"
-				{- Empty the inbox, because stuff may have
-				 - been left in it if the push failed. -}
-				emptyInbox clientid side
-				return r
-			else do
-				debugmsg "skipping running"
-				return False
-	r <- liftIO $ E.bracket setup cleanup go
-	return r
+{- Queues a push initiation message in the queue for the appropriate
+ - side of the push but only if there is not already an initiation message
+ - from the same client in the queue. -}
+queuePushInitiation :: NetMessage -> Assistant ()
+queuePushInitiation msg@(Pushing clientid stage) = do
+	tv <- getPushInitiationQueue side
+  	liftIO $ atomically $ do
+		r <- tryTakeTMVar tv
+		case r of
+			Nothing -> putTMVar tv [msg]
+			Just l -> do
+				let !l' = msg : filter differentclient l
+				putTMVar tv l'
   where
-	debugmsg s = netMessagerDebug clientid [s, show side]
-	-- check that this is one of the two threads allowed
-	-- to run at the same time, pushing to the same client
-	-- on the same side
-  	checkcanrun nm = do
-		let v = getSide side $ netMessagerPushThreadCount nm
-		m <- readTVar v
-		case M.lookup clientid m of
-			Just count
-				| count > 2 -> return (False, noop)
-			_ -> do
-				writeTVar v $
-					M.insertWith' (+) clientid 1 m
-				let release = modifyTVar' v $
-					M.insertWith' (-) clientid 1
-				return (True, release)
-	-- block until this is the only thread performing
-	-- a push on this side, to any client
-	waittorun nm release = do
-		let v = getSide side $ netMessagerPushRunning nm
-		ifM (isNothing <$> tryReadTMVar v)
-			( do
-				putTMVar v clientid
-				let release' = do
-					void $ takeTMVar v
-					release
-				return (True, release')
-			, retry
-			)
+  	side = pushDestinationSide stage
+	differentclient (Pushing cid _) = cid /= clientid
+	differentclient _ = True
+queuePushInitiation _ = noop
+
+{- Waits for a push inititation message to be received, and runs
+ - function to select a message from the queue. -}
+waitPushInitiation :: PushSide -> ([NetMessage] -> (NetMessage, [NetMessage])) -> Assistant NetMessage
+waitPushInitiation side selector = do
+	tv <- getPushInitiationQueue side
+	liftIO $ atomically $ do
+		q <- takeTMVar tv
+		if null q
+			then retry
+			else do
+				let (msg, !q') = selector q
+				unless (null q') $
+					putTMVar tv q'
+				return msg
 
 {- Stores messages for a push into the appropriate inbox.
  -
@@ -198,7 +163,11 @@ emptyInbox clientid side = do
 	
 getInboxes :: PushSide -> Assistant Inboxes
 getInboxes side =
-	getSide side . netMessagesInboxes <$> getAssistant netMessager
+	getSide side . netMessagerInboxes <$> getAssistant netMessager
+
+getPushInitiationQueue :: PushSide -> Assistant (TMVar [NetMessage])
+getPushInitiationQueue side =
+	getSide side . netMessagerPushInitiations <$> getAssistant netMessager
 
 netMessagerDebug :: ClientID -> [String] -> Assistant ()
 netMessagerDebug clientid l = debug $