Fix gossipsub race condition for heartbeat

[brandonwestcott]

In using gossipsub on a system with a number of ephemeral peers, we noticed messages would occasionally fail to route to their intended targets even though a subscribe had been received.

In a basic example, given there are 2 peers connected, PeerA & PeerB. For TopicA, PeerA is not in the mesh, but has seen and gossiped it before to PeerC, which is now disconnected and no longer subscribed.

When a Subscribe request for TopicA is issued from PeerB, then PeerA adds that peer onto the PubSub.topics map:

go-libp2p-pubsub/pubsub.go

Lines 566 to 573 in 49274b0

	if subopt.GetSubscribe() {
	tmap, ok := p.topics[t]
	if !ok {
	tmap = make(map[peer.ID]struct{})
	p.topics[t] = tmap
	}
	tmap[rpc.from] = struct{}{}

However, its not that map that is used for publishing messages back out from PeerA when its not in the mesh:

go-libp2p-pubsub/gossipsub.go

Lines 233 to 242 in 49274b0

	gmap, ok = gs.fanout[topic]
	if !ok {
	// we don't have any, pick some
	peers := gs.getPeers(topic, GossipSubD, func(peer.ID) bool { return true })
	if len(peers) > 0 {
	gmap = peerListToMap(peers)
	gs.fanout[topic] = gmap
	}
	}

Line #233 there uses GossipSubRouter.fanout as its map, which is updated during the heartbeat process:

go-libp2p-pubsub/gossipsub.go

Lines 441 to 448 in 49274b0

	for topic, peers := range gs.fanout {
	// check whether our peers are still in the topic
	for p := range peers {
	_, ok := gs.p.topics[topic][p]
	if !ok {
	delete(peers, p)
	}
	}

In between the 1 second of a heartbeat, these two maps can be out of date with each other, which is to be expected. However, on

go-libp2p-pubsub/gossipsub.go

Line 234 in 49274b0

if !ok {

the !ok only works as a fallback for the initial iteration, if the map is empty because all other Peers have been unsubscribed (PeerC) then it doesn't fallback to getPeers.

This PR is to change that conditional to check for empty map cases so that the fallback still happens. The other option is to make the key for the topic be set back to nil instead of an empty map inside the heartbeat process.

[vyzo]

thank you!