add an option to configure the Subscription output queue length

pubsub.go

@@ -1150,6 +1150,16 @@ func (p *PubSub) Subscribe(topic string, opts ...SubOpt) (*Subscription, error)
 	return topicHandle.Subscribe(opts...)
 }
 
+// WithBufferSize is a Subscribe option to customize the size of the subscribe output buffer.
+// The default length is 32 but it can be configured to avoid dropping messages if the consumer is not reading fast
+// enough.
+func WithBufferSize(size int) SubOpt {
+	return func(sub *Subscription) error {
+		sub.ch = make(chan *Message, size)
+		return nil
+	}
+}
+
 type topicReq struct {
 	resp chan []string
 }

topic.go

@@ -141,7 +141,6 @@ func (t *Topic) Subscribe(opts ...SubOpt) (*Subscription, error) {
 
 	sub := &Subscription{
 		topic: t.topic,
-		ch:    make(chan *Message, 32),
 		ctx:   t.p.ctx,
 	}
 
@@ -152,6 +151,11 @@ func (t *Topic) Subscribe(opts ...SubOpt) (*Subscription, error) {
 		}
 	}
 
+	if sub.ch == nil {
+		// apply the default size
+		sub.ch = make(chan *Message, 32)
+	}
+
 	out := make(chan *Subscription, 1)
 
 	t.p.disc.Discover(sub.topic)