make localhost a stateless implementation

[colin-axner]

Now that the light client module has the context for all its interface functions, the localhost implementation can be stateless as it should only read off the existing ibc store and not need to store any information under its client store

We should probably add a migration to prune unnecessary state and addition we should remove the additional disc writes that happen on every block to update the height in the localhost client state (set in the client store)

Can be removed:

• begin blocker logic for updating localhost
• UpdateLocalhostClient function in 02-client keeper

Originally posted by @colin-axner in #5866 (comment)

[gjermundgaraba]

I started looking into this and I found a couple of things that I wanted to discuss around the implementation.

1 Client state verification

After making localhost stateless a direct membership proof of the client state merklePath := commitmenttypes.NewMerklePath(host.FullClientStatePath(exported.LocalhostClientID)) is not possible (since it doesn't exist in state anymore).

To get tests to pass I currently just put a check in to return early with no error if we are trying to verify that.

// The localhost client is stateless, so if we need to verify the localhost client state, we can just return nil to verify
if merklePath.KeyPath[1] == host.FullClientStatePath(ModuleName) {
	return nil
}

Is this the way to do this, or is this actually something we don't need to support and just adjust the tests?

2 Client state queries in 02-client

In 02-client there is also some consequences for quering client states. For getting a single client state, this is easy enough with a clientID check:

if clientID == exported.LocalhostClientID {
	return localhost.NewClientState(types.GetSelfHeight(ctx)), true // Or route the call to the light client module directly
}

But for the paginated call to get ClientStates we don't get the localhost client state anymore from the store prefix.NewStore(ctx.KVStore(k.storeKey), host.KeyClientStorePrefix). My current solution to get tests passing was to just always add it to the list in that query:

clientStates := types.IdentifiedClientStates{types.NewIdentifiedClientState(exported.LocalhostClientID, localhost.NewClientState(types.GetSelfHeight(ctx)))}
// then do the actually query
pageRes, err := query.FilteredPaginate(store, req.Pagination, func(key, value []byte, accumulate bool) (bool, error) {
...

[colin-axner]

Great questions

1. This should not be necessary. Client state membership proofs are currently used during the connection handshake, but the localhost is a special connection which is pre-created. Doing a membership proof on a localhost client, is equivalent to calling localhost.Status() (so I think it's safe to allow it return an error if someone called VerifyMembership on the localhost client state

2. I think the solution you went with makes sense 👍 Localhost always exists on a chain, it can simply be disabled, but should still be returned in client state queries in the same way non-active clients will be