webapp: Fix a race that sometimes caused alerts or other notifications to be missed if they occurred while a page was loading.

When a page is loaded, the javascript requests an notification url, and
does long polling on the url to be informed of changes. But if a change
occured before the notification url was requested, it would not be notified
of that change, and so the page display would not update.

I fixed this by *always* updating the page display after it gets
the notification url. This is extra work, but the overhead is not noticable
in the other overhead of loading a page.

(A nicer way would be to somehow record the version of a page initially
loaded, and then compare it with the current version when getting the
notification url, and only force an update if it's changed. But getting
the "version" of the different parts of the page that use long polling
is difficult.)

Assistant/Threads/DaemonStatus.hs

@@ -17,7 +17,7 @@ import Utility.NotificationBroadcaster
  -}
 daemonStatusThread :: NamedThread
 daemonStatusThread = namedThread "DaemonStatus" $ do
-	notifier <- liftIO . newNotificationHandle
+	notifier <- liftIO . newNotificationHandle False
 		=<< changeNotifier <$> getDaemonStatus
 	checkpoint
 	runEvery (Seconds tenMinutes) <~> do

Assistant/Threads/TransferPoller.hs

@@ -21,7 +21,7 @@ import qualified Data.Map as M
 transferPollerThread :: NamedThread
 transferPollerThread = namedThread "TransferPoller" $ do
 	g <- liftAnnex gitRepo
-	tn <- liftIO . newNotificationHandle =<<
+	tn <- liftIO . newNotificationHandle True =<<
 		transferNotifier <$> getDaemonStatus
 	forever $ do
 		liftIO $ threadDelay 500000 -- 0.5 seconds

Assistant/WebApp.hs

@@ -30,7 +30,7 @@ waitNotifier getbroadcaster nid = liftAssistant $ do
 newNotifier :: forall sub. (Assistant NotificationBroadcaster) -> GHandler sub WebApp NotificationId
 newNotifier getbroadcaster = liftAssistant $ do
 	b <- getbroadcaster
-	liftIO $ notificationHandleToId <$> newNotificationHandle b
+	liftIO $ notificationHandleToId <$> newNotificationHandle True b
 
 {- Adds the auth parameter as a hidden field on a form. Must be put into
  - every form. -}

Utility/NotificationBroadcaster.hs

@@ -40,14 +40,23 @@ data NotificationHandle = NotificationHandle NotificationBroadcaster Notificatio
 newNotificationBroadcaster :: IO NotificationBroadcaster
 newNotificationBroadcaster = atomically $ newTMVar []
 
-{- Allocates a notification handle for a client to use. -}
-newNotificationHandle :: NotificationBroadcaster -> IO NotificationHandle
-newNotificationHandle b = NotificationHandle
+{- Allocates a notification handle for a client to use.
+ -
+ - An immediate notification can be forced the first time waitNotification
+ - is called on the handle. This is useful in cases where a notification
+ - may be sent while the new handle is being constructed. Normally,
+ - such a notification would be missed. Forcing causes extra work,
+ - but ensures such notifications get seen.
+ -}
+newNotificationHandle :: Bool -> NotificationBroadcaster -> IO NotificationHandle
+newNotificationHandle force b = NotificationHandle
 	<$> pure b
 	<*> addclient
   where
 	addclient = do
-		s <- newEmptySV
+		s <- if force
+			then newSV ()
+			else newEmptySV
 		atomically $ do
 			l <- takeTMVar b
 			putTMVar b $ l ++ [s]

debian/changelog

@@ -2,6 +2,8 @@ git-annex (4.20130324) UNRELEASED; urgency=low
 
   * Group subcommands into sections in usage. Closes: #703797
   * Per-command usage messages.
+  * webapp: Fix a race that sometimes caused alerts or other notifications
+    to be missed if they occurred while a page was loading.
 
  -- Joey Hess <joeyh@debian.org>  Mon, 25 Mar 2013 10:21:46 -0400