assistant: Get back in sync with XMPP remotes after network reconnection, and on startup.

Make manualPull send push requests over XMPP.

When reconnecting with remotes, those that are XMPP remotes cannot
immediately be pulled from and scanned, so instead maintain a set of
(probably) desynced remotes, and put XMPP remotes on it. (This set could be
used in other ways later, if we can detect we're out of sync with other
types of remotes.)

The merger handles detecting when a XMPP push is received from a desynced
remote, and triggers a scan then, if they have in fact diverged.

This has one known bug: A single XMPP remote can have multiple clients
behind it. When this happens, only the UUID of one client is recorded
as the UUID of the XMPP remote. Pushes from the other XMPP clients will not
trigger a scan. If the client whose UUID is expected responds to the push
request, it'll work, but when that client is offline, we're SOL.

Assistant/Threads/Merger.hs

@@ -10,12 +10,18 @@ module Assistant.Threads.Merger where
 import Assistant.Common
 import Assistant.TransferQueue
 import Assistant.BranchChange
+import Assistant.DaemonStatus
+import Assistant.ScanRemotes
 import Utility.DirWatcher
 import Utility.Types.DirWatcher
 import qualified Annex.Branch
 import qualified Git
 import qualified Git.Branch
 import qualified Command.Sync
+import Annex.TaggedPush
+import Remote (remoteFromUUID)
+
+import qualified Data.Set as S
 
 {- This thread watches for changes to .git/refs/, and handles incoming
  - pushes. -}
@@ -64,13 +70,16 @@ onAdd file
 	| ".lock" `isSuffixOf` file = noop
 	| isAnnexBranch file = do
 		branchChanged
-		whenM (liftAnnex Annex.Branch.forceUpdate) $
-			queueDeferredDownloads "retrying deferred download" Later
+		diverged <- liftAnnex Annex.Branch.forceUpdate
+		when diverged $
+			unlessM handleDesynced $
+				queueDeferredDownloads "retrying deferred download" Later
 	| "/synced/" `isInfixOf` file = do
 		mergecurrent =<< liftAnnex (inRepo Git.Branch.current)
 	| otherwise = noop
   where
 	changedbranch = fileToBranch file
+
 	mergecurrent (Just current)
 		| equivBranches changedbranch current = do
 			debug
@@ -80,6 +89,22 @@ onAdd file
 			void $ liftAnnex  $ Command.Sync.mergeFrom changedbranch
 	mergecurrent _ = noop
 
+	handleDesynced = case branchTaggedBy changedbranch of
+		Nothing -> return False
+		Just u -> do
+			s <- desynced <$> getDaemonStatus				
+			if S.member u s
+				then do
+					modifyDaemonStatus_ $ \st -> st
+						{ desynced = S.delete u s }
+					mr <- liftAnnex $ remoteFromUUID u
+					case mr of
+						Just r -> do
+							addScanRemotes True [r]
+							return True
+						Nothing -> return False
+				else return False
+
 equivBranches :: Git.Ref -> Git.Ref -> Bool
 equivBranches x y = base x == base y
   where