only start ref change watcher thread once per P2P connection

This is more efficient. Note that the peer will get CHANGED messages for
all refs changed since the connection opened, even if those changes
happened before it sent NOTIFYCHANGE.

P2P/Annex.hs

@@ -25,10 +25,8 @@ import Utility.Metered
 
 import Control.Monad.Free
 
--- When we're serving a peer, we know their uuid, and can use it to update
--- transfer logs.
 data RunMode
-	= Serving UUID
+	= Serving UUID ChangedRefsHandle
 	| Client
 
 -- Full interpreter for Proto, that can receive and send objects.
@@ -115,18 +113,17 @@ runLocal runmode runner a = case a of
 				protoaction False
 				next
 			Right _ -> runner next
-	WaitRefChange next -> do
-		v <- tryNonAsync $ bracket
-			watchChangedRefs
-			(liftIO . stopWatchingChangedRefs)
-			(liftIO . waitChangedRefs)
-		case v of
-			Left e -> return (Left (show e))
-			Right changedrefs -> runner (next changedrefs)
+	WaitRefChange next -> case runmode of
+		Serving _ h -> do
+			v <- tryNonAsync $ liftIO $ waitChangedRefs h
+			case v of
+				Left e -> return (Left (show e))
+				Right changedrefs -> runner (next changedrefs)
+		_ -> return $ Left "change notification not implemented for client"
   where
 	transfer mk k af ta = case runmode of
 		-- Update transfer logs when serving.
-		Serving theiruuid -> 
+		Serving theiruuid _ -> 
 			mk theiruuid k af noRetry ta noNotification
 		-- Transfer logs are updated higher in the stack when
 		-- a client.