watch for changes to transfer info files, to update progress bars on upload

This is handled differently for inotify, which can track modifications of
existing files, and kqueue, which cannot (TTBOMK). On the inotify side,
the TransferWatcher just waits for the file to be updated and reads the new
bytesComplete. On the kqueue side, the TransferPoller has to re-read the
file every update (currently 0.5 seconds, might need to increase that).

I did think about working around kqueue's limitations by somehow creating
a new file each time the size changed. But cleaning up all the files that
would result seemed difficult. And really, this is not a lot worse than
the TransferWatcher's behavior for downloads, which stats a file every 0.5
seconds. As long as the OS has decent file caching behavior..

Assistant/Threads/TransferWatcher.hs

@@ -30,6 +30,7 @@ transferWatcherThread st dstatus transferqueue = thread $ do
 	let hooks = mkWatchHooks
 		{ addHook = hook onAdd
 		, delHook = hook onDel
+		, modifyHook = hook onModify
 		, errHook = hook onErr
 		}
 	void $ watchDir dir (const False) hooks id
@@ -71,6 +72,25 @@ onAdd st dstatus _ file _ = case parseTransferFile file of
 				{ transferRemote = r }
 		sameuuid t r = Remote.uuid r == transferUUID t
 
+{- Called when a transfer information file is updated.
+ -
+ - The only thing that should change in the transfer info is the
+ - bytesComplete, so that's the only thing updated in the DaemonStatus. -}
+onModify :: Handler
+onModify _ dstatus _ file _ = do
+	case parseTransferFile file of
+		Nothing -> noop
+		Just t -> go t =<< readTransferInfoFile Nothing file
+	where
+		go _ Nothing = noop
+		go t (Just newinfo) = alterTransferInfo dstatus t $ \info ->
+			info { bytesComplete = bytesComplete newinfo }
+
+{- This thread can only watch transfer sizes when the DirWatcher supports
+ - tracking modificatons to files. -}
+watchesTransferSize :: Bool
+watchesTransferSize = modifyTracked
+
 {- Called when a transfer information file is removed. -}
 onDel :: Handler
 onDel st dstatus transferqueue file _ = case parseTransferFile file of