.

go/zodb/open.go

@@ -196,27 +196,56 @@ const (
 // watcher dispatches events from driver to subscribers and serves
 // {Add,Del}Watch requests.
 func (s *storage) watcher() {
-	for {
-		select {
-		case req := <-s.watchReq:
-			switch req.op {
-			case addWatch:
-				s.watchTab[req.watchq] = struct{}{}
+	// staging place for AddWatch requests.
+	//
+	// during event delivery to registered watchqs, add/del requests are
+	// also served - not to get stuck and support clients who do DelWatch
+	// and no longer receive from their watchq. However we cannot register
+	// added watchq immediately, because it is undefined whether or not
+	// we'll see it while iterating watchTab. So we queue what was added
+	// and flush it on the beginning of each cycle.
+	var addq map[chan<- Event]struct{}
+	addqFlush := func() {
+		for watchq := range addq {
+			s.watchTab[watchq] = struct{}{}
+		}
+		addq = make(map[chan<- Event]struct{})
+	}
+	handleReq := func(req watchRequest) {
+		switch req.op {
+		case addWatch:
+			addq[req.watchq] = struct{}{}
 
-			case delWatch:
-				delete(s.watchTab, req.watchq)
+		case delWatch:
+			delete(s.watchTab, req.watchq)
+			delete(addq, req.watchq)
 
-			default:
-				panic("bad watch request op")
-			}
+		default:
+			panic("bad watch request op")
+		}
 
-			req.ack <- s.drvHead
+		req.ack <- s.drvHead
+	}
+
+	// close all subscribers's watchq on close
+	// XXX AddWatch/DelWatch after watcher exits?
+	defer func() {
+		addqFlush()
+		for watchq := range s.watchTab {
+			close(watchq)
+		}
+	}()
+
+	for {
+		addqFlush() // register staged AddWatch(s)
+
+		select {
+		case req := <-s.watchReq:
+			handleReq(req)
 
 		case event, ok := <-s.drvWatchq:
 			if !ok {
 				// storage closed
-				// XXX close all subscribers' watchq?
-				// XXX AddWatch/DelWatch after watcher exits?
 				return
 			}
 
@@ -235,8 +264,13 @@ func (s *storage) watcher() {
 
 			// deliver event to all watchers
 			for watchq := range s.watchTab {
-				// XXX + select and handle DelWatch
-				watchq <- event
+				select {
+				case req := <-s.watchReq:
+					handleReq(req)
+
+				case watchq <- event:
+					// ok
+				}
 			}
 		}
 	}