Replaces `process_slot_for_light_client_store` which force updates the
`LightClientStore` automatically based on `finalized_header` age with
`try_light_client_store_force_update` which may be manually called based
on use case dependent heuristics if light client sync appears stuck.
Not all use cases share the same risk profile.
Adds `create_light_client_bootstrap` and `create_light_client_update`
functions as a reference implementation for serving light client data.
This also enables a new test harness to verify that light client data
gets applied to a `LightClientStore` as expected.
`LightClientUpdate` structures currently use different merkle proof root
depending on the presence of `finalized_header`. By always rooting it in
the same state (the `attested_header.state_root`), logic gets simpler.
Caveats:
- In periods of extended non-finality, `update.finalized_header` may now
be outdated by several sync committee periods. The old implementation
rejected such updates as the `next_sync_committee` in them was stale,
but the new implementation can properly handle this case.
- The `next_sync_committee` can no longer be considered finalized based
on `is_finality_update`. Instead, waiting until `finalized_header` is
in the `attested_header`'s sync committee period is now necessary.
- Because `update.finalized_header > store.finalized_header` no longer
holds (for updates with finality), an `is_better_update` helper is
added to improve `best_valid_update` tracking (in the past, finalized
updates with supermajority participation would always directly apply)
This PR builds on prior work from:
- @hwwhww at https://github.com/ethereum/consensus-specs/pull/2829
The existing README has a reference to an alias type `Bytes[N]` that has been removed from the repo so it is not clear what it exactly refers to.
This PR updates the type to the equivalent `List[T, N]` using more recent SSZ typing syntax.
Building merkle proofs is required functionality for implementing light
client sync. Although the spec currently only defines a function to
verify merkle proofs (`is_valid_merkle_branch`) there are still a few
PySpec unit tests that produce merkle proofs. This patch adds a new
generator to extract test vectors from those static unit tests, so that
light client implementations can validate their merkle proof logic.
