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/TransferPoller.hs

@@ -12,6 +12,7 @@ import Assistant.ThreadedMonad
 import Assistant.DaemonStatus
 import Logs.Transfer
 import Utility.NotificationBroadcaster
+import qualified Assistant.Threads.TransferWatcher as TransferWatcher
 
 import Control.Concurrent
 import qualified Data.Map as M
@@ -42,9 +43,20 @@ transferPollerThread st dstatus = thread $ do
 				sz <- catchMaybeIO $
 					fromIntegral . fileSize
 						<$> getFileStatus f
-				when (bytesComplete info /= sz && isJust sz) $
-					alterTransferInfo dstatus t $
-						\i -> i { bytesComplete = sz }
-			{- Can't poll uploads, instead the upload code
-			 - updates the files. -}
+				newsize t info sz
+			{- Uploads don't need to be polled for when the
+			 - TransferWatcher thread can track file
+			 - modifications. -}
+			| TransferWatcher.watchesTransferSize = noop
+			{- Otherwise, this code polls the upload progress
+			 - by reading the transfer info file. -}
+			| otherwise = do
+				let f = transferFile t g
+				mi <- catchDefaultIO Nothing $
+					readTransferInfoFile Nothing f
+				maybe noop (newsize t info . bytesComplete) mi
+		newsize t info sz
+			| bytesComplete info /= sz && isJust sz = 
+				alterTransferInfo dstatus t $
+					\i -> i { bytesComplete = sz }
 			| otherwise = noop