Description

Credits to @hwwhww for bringing this to our attention.

In fork choice, Lighthouse checks checkpoint epochs:

However, the specification declares:

    correct_justified = (
        store.justified_checkpoint.epoch == GENESIS_EPOCH
        or head_state.current_justified_checkpoint == store.justified_checkpoint
    )
    correct_finalized = (
        store.finalized_checkpoint.epoch == GENESIS_EPOCH
        or head_state.finalized_checkpoint == store.finalized_checkpoint
    )

We have to check "checkpoints" are equal. So Lighthouse needs to update to check both epoch and root, not just the epoch.

Steps to resolve

In order to support this, we need to add the root information to the ProtoNode:

... and also the ProtoArray:

This will require a database migration. I think it should be fairly simple to migrate the ProtoNode, the roots of all the justified ancestors should be in the ProtoArray, so we should just be able to find the roots by just iterating backwards until we find the first ancestor where ancestor.slot <= start_slot(node.justified_epoch).

For ProtoArray, I think we should be able to get the root for the justified checkpoint from the ForkChoiceStore, which is persisted alongside it. We'd need to do some more reading to ensure there's a 1:1 relationship between these values, though.