PeerSharingServer refactorisation

[Marcin Szamotulski]

computePeerSharing should be defined in Ouroboros.Network.PeerSharing. We don't need to export it, it should be used directly by peerSharingServer. This will simplify the interface we expose to consensus, e.g. no callback in the daApplicationInitiatorResponderMode, but instead requires access to PublicPeerSelectionState which is fine, we do that for other mini-protocols as well. Ideally daApplicationIntiatorResponderMode wouldn't it as an argument, other protocols don't need to pass similar information if I recall correctly.

[Armando Santos]

IIRC it is only how it is now because polymorphism shenanigans and because we need access to the PeerSelectionState. Since peerSharingServer is used in consensus code when setting up the miniprotocols and since we need access to PeerSelectionState, we have to partially apply computePeerSharingServer which is defined in ouroboros-network Diffusion.P2P (module that has access to PeerSelectionState) to daApplicationInitiatorResponderMode.

If we want to move computePeerSharing to Ouroboros.Network.PeerSharing and call it directly in peerSharingServer we still need a way to pass an STM action to read the PeerSelectionState to consensus. So maybe the cleanest way to do this is to define a PeerSharing consensus API dictionary and use that

[Marcin Szamotulski]

We can pass TVar PublicPeerSelectionState to Diffusion through DiffusionArguments (even better just pass atomically . writeTVar publicPeerSelectionStateVar :: PublickPeerSelectionState -> m () - this requires some changes, but it's possible). Then we will have access to the TVar outside of Diffusion and thus allow us to create peerSharingServer outside of the diffusion context, it will be responsibility of consensus to create the TVar and pass it to the right places. This is already true for other mini-protocols, that they create some context outside of diffusion, e.g. the bracketSyncWithFetchClient requires FetchClientRegistry which is part of NodeKernel, maybe that's the right place for the TVar too.

[Marcin Szamotulski]

NodeKernel contains other pieces of diffusion state. We can also make this more explicit by providing State (diffusion stuff is supposed to be imported qualified so it will be Diffusion.State), e.g.

data State peeraddr = State {
  dsFetchClientRegistry      :: FetchClientRegistry (ConnectionId peeraddr) hdr blk m,
  
  dfPeerSharingRegistry      :: PeerSharingRegistry peeraddr m,
  
  dfPublicPeerSelectionState :: TVar m PublicPeerSelectionState
  -- ^ the new thing which we add to `NodeKernel`
}

newState :: m (State peeraddr)
newState = ...

And then consolidate it in NodeKernel in a single field getDiffusionState :: NodeKernel -> Diffusion.State

I would define provide it in Ouroboros.Network.Diffusion.State module and re-export from Ouroboros.Network.Diffusion.

This way in the future we can extend NodeKernel in in a simple way without much changes to ouroboros-consensus.

We could also define our own initState function, similar to initiNodeKernel. It would fork blockFetchLogic, such function will need to take arguments which are constructed by consensus, so I am not sure if this is worth it. If we do so then Diffusion.State can also contain getFetchMode.