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/Threads/XMPPClient.hs

@@ -20,7 +20,6 @@ import qualified Remote
 import Utility.ThreadScheduler
 import Assistant.WebApp (UrlRenderer)
 import Assistant.WebApp.Types hiding (liftAssistant)
-import Assistant.WebApp.Configurators.XMPP (checkCloudRepos)
 import Assistant.Alert
 import Assistant.Pairing
 import Assistant.XMPP.Git
@@ -108,10 +107,8 @@ xmppClient urlrenderer d creds =
 		maybe noop (inAssistant . pairMsgReceived urlrenderer stage u selfjid) (parseJID c)
 	handle _ (GotNetMessage m@(Pushing _ pushstage))
 		| isPushNotice pushstage = inAssistant $ handlePushNotice m
-		| isPushInitiation pushstage = inAssistant $ do
-			let checker = checkCloudRepos urlrenderer
-			void $ forkIO <~> handlePushInitiation checker m
-		| otherwise = void $ inAssistant $ storeInbox m
+		| isPushInitiation pushstage = inAssistant $ queuePushInitiation m
+		| otherwise = inAssistant $ storeInbox m
 	handle _ (Ignorable _) = noop
 	handle _ (Unknown _) = noop
 	handle _ (ProtocolError _) = noop

Assistant/Threads/XMPPPusher.hs

@@ -0,0 +1,81 @@
+{- git-annex XMPP pusher threads
+ -
+ - This is a pair of threads. One handles git send-pack,
+ - and the other git receive-pack. Each thread can be running at most
+ - one such operation at a time.
+ -
+ - Why not use a single thread? Consider two clients A and B.
+ - If both decide to run a receive-pack at the same time to the other,
+ - they would deadlock with only one thread. For larger numbers of
+ - clients, the two threads are also sufficient.
+ -
+ - Copyright 2013 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Assistant.Threads.XMPPPusher where
+
+import Assistant.Common
+import Assistant.NetMessager
+import Assistant.Types.NetMessager
+import Assistant.WebApp (UrlRenderer)
+import Assistant.WebApp.Configurators.XMPP (checkCloudRepos)
+import Assistant.XMPP.Git
+
+import Control.Exception as E
+
+xmppSendPackThread :: UrlRenderer -> NamedThread
+xmppSendPackThread = pusherThread "XMPPSendPack" SendPack
+
+xmppReceivePackThread :: UrlRenderer -> NamedThread
+xmppReceivePackThread = pusherThread "XMPPReceivePack" ReceivePack
+
+pusherThread :: String -> PushSide -> UrlRenderer -> NamedThread
+pusherThread threadname side urlrenderer = namedThread threadname $ go Nothing
+  where
+  	go lastpushedto = do
+		msg <- waitPushInitiation side $ selectNextPush lastpushedto
+		debug ["started running push", logNetMessage msg]
+
+		runpush <- asIO $ runPush checker msg
+		r <- liftIO (E.try runpush :: IO (Either SomeException (Maybe ClientID)))
+		let successful = case r of
+			Right (Just _) -> True
+			_ -> False
+
+		{- Empty the inbox, because stuff may have
+		 - been left in it if the push failed. -}
+		let justpushedto = getclient msg
+		maybe noop (`emptyInbox` side) justpushedto
+
+		debug ["finished running push", logNetMessage msg, show successful]
+		go $ if successful then justpushedto else lastpushedto
+	
+	checker = checkCloudRepos urlrenderer
+
+	getclient (Pushing cid _) = Just cid
+	getclient _ = Nothing
+
+{- Select the next push to run from the queue.
+ - The queue cannot be empty!
+ -
+ - We prefer to select the most recently added push, because its requestor
+ - is more likely to still be connected.
+ -
+ - When passed the ID of a client we just pushed to, we prefer to not
+ - immediately push again to that same client. This avoids one client 
+ - drowing out others. So pushes from the client we just pushed to are 
+ - relocated to the beginning of the list, to be processed later.
+ -}
+selectNextPush :: Maybe ClientID -> [NetMessage] -> (NetMessage, [NetMessage])
+selectNextPush _ (m:[]) = (m, []) -- common case
+selectNextPush _ [] = error "selectNextPush: empty list"
+selectNextPush lastpushedto l = go [] l
+  where
+	go (r:ejected) [] = (r, ejected)
+	go rejected (m:ms) = case m of
+		(Pushing clientid _)
+			| Just clientid /= lastpushedto -> (m, rejected ++ ms)
+		_ -> go (m:rejected) ms
+  	go [] [] = undefined