fix(gossipsub): make sure we have fanout peers when publish

[ackintosh]

Description

This PR is to submit a fix in Lighthouse. sigp/lighthouse#6738

An InsufficientPeers error can occur under a particular condition, even if we have peers subscribed to a topic.

Notes & open questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[elenaf9]

Is it deliberate that we don't remove an entry from self.fanout when the last peer in the its list is removed?

[ackintosh]

@elenaf9 Thanks for your review!

Is it deliberate that we don't remove an entry from self.fanout when the last peer in the its list is removed?

Yes, because if we choose the way of removing an entry, we need to ensure that it's removed without any omissions. From a quick look at the code, it seems that removal would need to be handled in at least two places. In contrast, doing is_empty check during publishing ensures that we can reliably avoid iterating on an empty list.

[jxs]

Thanks Akihito! LGTM