Fix channel message delay

This commit fixes an issue where a new channel message isn't
sent right away. Rather it would wait in the queue until
another next message is queued.

This issue wasn't common and likely would be difficult to
discover on a busy production machine. But occasionally popped
up in a failed test.

See #763

Author: tidwall

internal/server/pubsub.go

@@ -327,24 +327,22 @@ func (s *Server) liveSubscription(
 	go func() {
 		log.Debugf("pubsub open")
 		defer log.Debugf("pubsub closed")
+		target.cond.L.Lock()
+		defer target.cond.L.Unlock()
 		for {
-			var msgs []submsg
-			target.cond.L.Lock()
-			if len(target.msgs) > 0 {
-				msgs = target.msgs
+			for len(target.msgs) > 0 {
+				msgs := target.msgs
 				target.msgs = nil
+				target.cond.L.Unlock()
+				for _, msg := range msgs {
+					writeMessage(msg)
+				}
+				target.cond.L.Lock()
 			}
-			target.cond.L.Unlock()
-			for _, msg := range msgs {
-				writeMessage(msg)
-			}
-			target.cond.L.Lock()
 			if target.closed {
-				target.cond.L.Unlock()
-				return
+				break
 			}
 			target.cond.Wait()
-			target.cond.L.Unlock()
 		}
 	}()