inefficiency and dormant panic and permanent message loss in gossip

[rade]

I just found the following in Router.SendAllGossipDown:

        if gossip := channel.gossiper.Gossip(); gossip != nil {
            channel.SendDown(conn, channel.gossiper.Gossip())

which unnecessarily invokes Gossip() twice. Which is inefficient.

And if Gossip() returned nil it can lead to a panic further along (in the GossipSender's GossipData.Merge invocation), or it can cause the GossipSender goroutine to terminate (since it treats nil as a stop), which would cause all subsequent gossip on that particular channel+connection to be discarded.

Fortunately that can't happen since only SurrogateGossiper.Gossip() returns nil, and does so always, hence would not get past the first line of the above code.