Enable light client data backfill by tracking best `SyncAggregate`

[etan-status]

Beacon nodes can only compute light client data locally if they have the corresponding BeaconState available. This is not the case for blocks before the initially synced checkpoint state. The p2p-interface defines endpoints to sync light client data, but it only supports forward sync.

To enable beacon nodes to backfill light client data, we must ensure that a malicious peer cannot convince us of fraudulent data. While it is possible to verify light client data against the locally backfilled blocks, blocks are not necessarily available anymore on libp2p as they are subject to MIN_EPOCHS_FOR_BLOCK_REQUESTS. Light client data stays relevant for more than 5 months, and without validating it against local block data it is impossible to distinguish canonical light client data from fraudulent light client data that eventually culminates in a shared history; the old periods in that case could still be manipulated. Furthermore, agreeing on canonical data improves caching performance and is relevant, e.g., for the portal network.

To support efficient proof that a LightClientUpdate is canonical, it is proposed to minimally extend the BeaconState to track the best SyncAggregate of the current and previous sync committee period, according to an implementation-independent ranking function. The proposed ranking function is compatible with what consensus nodes implementing https://github.com/ethereum/consensus-specs/pull/3553 are already making available across libp2p and REST transports. It is based on and compatible with the is_better_update function in specs/altair/light-client/sync-protocol.md.

There are three minor differences to is_better_update:

1. is_better_update runs in the LC, so runs without fork choice. It needs extra conditions to prefer older data over newer data. The BeaconState ranking function can use simpler logic.
2. The LC is always initialized from a post-Altair finalized checkpoint. This assumption does not hold in theoretical edge cases, requiring an extra guard for ALTAIR_FORK_EPOCH in the BeaconState function.
3. is_better_update has to deal with BNs serving incomplete data while they are still backfilling. This is not the case with BeaconState.

Once the data is available in the BeaconState, a light client data backfill protocol could be defined that serves, for past periods:

1. A LightClientUpdate from requested period + 1 that proves that the entirety of period is finalized.
2. BeaconState.historical_summaries[period].block_summary_root at (1)'s attested_header.beacon.state_root + Merkle proof.
3. For each epoch's slot 0 block within requested period, the corresponding LightClientHeader + Merkle multi-proof for the block's inclusion into (2)'s block_summary_root.
4. For each of the entries from (3) with beacon.slot within period, the current_sync_committee_branch + Merkle proof for constructing LightClientBootstrap.
5. If (4) is not empty, the requested period's current_sync_committee.
6. The best LightClientUpdate from period, if one exists, + Merkle proof that its sync_aggregate + signature_slot is selected as the canonical best one in (1)'s attested_header.beacon.state_root.

Only the proof in (6) depends on BeaconState tracking the best light client data. This modification would enshrine the logic of a subset of is_better_update, but does not require adding any LightClientXyz data structures to the BeaconState.

[etan-status]

Context: https://hackmd.io/@etan-status/electra-lc

This is the next step needed to decentralize BeaconState syncing!

A deeply finalized checkpoint root could be integrated into the network's config.yaml files, which is then used to fast-forward to the latest finalized state root using light client sync, via libp2p (decentralized). For that, we need historical light client data to be syncable, though. But that requires a working backfill protocol.

Also related: https://github.com/ethpandaops/checkpointz/issues/143

[dapplion]

This should be placed as an EIP in specs/_features now that's modifying consensus data structures. Have only skimmed through but approach looks good

[leolara]

I am closing this issue because it seems stale. Please, do not hesitate to reopen it if this is a mistake