Don't replicate zero length feeds about cabal-core

[hackergrrl]

Possible edge case: I connect to a cabal, read some backlog, then leave. I return the next day, still not having posted anything yet, and now nobody will ever replicate my feed again (because it's size=0).

[hackergrrl]

cc @telamon

[telamon]

@noffle yes, that pretty much sums it up, if you're a lurker then you won't unnecessarily tax the network you'll just act as another seed,
And if a user writes a message his feed will no longer be empty and thus it will no longer be filtered.

[hackergrrl]

Ah, but how will other users know that that person has a non-empty feed if they refuse to sync with them when it's length=0 locally? :)

[telamon]

@noffle The peer shouldn't be filtered, only zero length feeds would be rejected. So a user with a zero-length writer should behave like --seed mode until he appends an entry.

I'm thinking of letting other peers know there's a new feed that is qualified for replication the same way that the active feed forwarding works.

When an peer with established connections suddenly becomes aware of a new feed the _onFeedReplicated handler is invoked that forwards the feed to all other connected peers that dosen't have it yet.

So the same needs to be done on the peer that has a zero-length writer when the first entry is written.
Maybe cabal could invoke the above _onFeedReplicated() or startConversation() method which should reannounce previously filtered feeds.

But anyway, at the moment of writing, in order to manually notify connected peers that your writer is no-longer empty we could do the following:

const writer = multi.feed('local')
writer.on('append', () => {
  if (writer.length === 1) {
    multi.replicationManager.connections.forEach(conn => {
      multi.replicationManager._onFeedReplicated(conn, writer.key)
    })
  }
})

I'm adding to my TODO to expose an manager#advertise(key) method to wrap up the above loop in a nicer package.

Did i manage to answer the question?