Add documentation for the RequestPartialMessages topic option

and fix a bug where we would skip sending them full messages if they
requested partial messages and we disabled partial messages.

Author: MarcoPolo

gossipsub.go

@@ -1375,7 +1375,7 @@ func (gs *GossipSubRouter) rpcs(msg *Message) iter.Seq2[peer.ID, *RPC] {
 			if pid == from || pid == peer.ID(msg.GetFrom()) {
 				continue
 			}
-			if peerStates, ok := gs.p.topics[topic]; ok && peerStates[pid].requestsPartial {
+			if gs.peerWantsPartial(pid, topic) {
 				// The peer requested partial messages. We'll skip sending them full messages
 				continue
 			}
@@ -1387,6 +1387,11 @@ func (gs *GossipSubRouter) rpcs(msg *Message) iter.Seq2[peer.ID, *RPC] {
 	}
 }
 
+func (gs *GossipSubRouter) peerWantsPartial(p peer.ID, topic string) bool {
+	peerStates, ok := gs.p.topics[topic]
+	return ok && gs.extensions.myExtensions.PartialMessages && peerStates[p].requestsPartial
+}
+
 func (gs *GossipSubRouter) Join(topic string) {
 	gmap, ok := gs.mesh[topic]
 	if ok {

pubsub.go

@@ -1543,8 +1543,21 @@ type TopicOptions struct {
 
 // RequestPartialMessages requests that peers, if they support it, send us
 // partial messages on this topic.
+//
+// If a peer does not support partial messages, this has no effect, and the peer
+// will continue sending us full messages
+//
+// It is an error to use this option if partial messages are not enabled
 func RequestPartialMessages() TopicOpt {
 	return func(t *Topic) error {
+		gs, ok := t.p.rt.(*GossipSubRouter)
+		if !ok {
+			return errors.New("partial messages only supported by gossipsub")
+		}
+
+		if !gs.extensions.myExtensions.PartialMessages {
+			return errors.New("partial messages are not enabled")
+		}
 		t.requestPartialMessages = true
 		return nil
 	}