global webapp redirects, to finish upgrades

When an automatic upgrade completes, or when the user clicks on the upgrade
button in one webapp, but also has it open in another browser window/tab,
we have a problem: The current web server is going to stop running in
minutes, but there is no way to send a redirect to the web browser to the
new url.

To solve this, used long polling, so the webapp is always listening for
urls it should redirect to. This allows globally redirecting every open
webapp. Works great! Tested with 2 web browsers with 2 tabs each.
May be useful for other purposes later too, dunno.

The overhead is 2 http requests per page load in the webapp. Due to yesod's
speed, this does not seem to noticibly delay it. Only 1 of the requests
could possibly block the page load, the other is async.

Assistant/WebApp/Notifications.hs

@@ -28,6 +28,7 @@ import qualified Data.Text as T
 #ifndef WITH_OLD_YESOD
 import qualified Data.Aeson.Types as Aeson
 #endif
+import Control.Concurrent
 
 {- Add to any widget to make it auto-update using long polling.
  -
@@ -82,6 +83,9 @@ getNotifierRepoListR reposelector = notifierUrl route getRepoListBroadcaster
   where
 	route nid = RepoListR nid reposelector
 
+getNotifierGlobalRedirR :: Handler RepPlain
+getNotifierGlobalRedirR = notifierUrl GlobalRedirR getGlobalRedirBroadcaster
+
 getTransferBroadcaster :: Assistant NotificationBroadcaster
 getTransferBroadcaster = transferNotifier <$> getDaemonStatus
 
@@ -93,3 +97,20 @@ getBuddyListBroadcaster =  getBuddyBroadcaster <$> getAssistant buddyList
 
 getRepoListBroadcaster :: Assistant NotificationBroadcaster
 getRepoListBroadcaster =  syncRemotesNotifier <$> getDaemonStatus
+
+getGlobalRedirBroadcaster :: Assistant NotificationBroadcaster
+getGlobalRedirBroadcaster =  globalRedirNotifier <$> getDaemonStatus
+
+getGlobalRedirR :: NotificationId -> Handler Text
+getGlobalRedirR nid = do
+	tid <- liftIO myThreadId
+	liftIO $ do
+		hPutStrLn stderr $ show ("getGlobalRedirR waiting", tid)
+		hFlush stderr
+	waitNotifier getGlobalRedirBroadcaster nid
+	v <- globalRedirUrl <$> liftAssistant getDaemonStatus
+	liftIO $ do
+		hPutStrLn stderr $ show ("getGlobalRedirR got a val", v, tid)
+		hFlush stderr
+	maybe (getGlobalRedirR nid) (return . T.pack)
+		=<< globalRedirUrl <$> liftAssistant getDaemonStatus