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be outdated by several sync committee periods.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
Some tests are currently restricted to a single phase using @with_phases
even though they could likely run unchanged in later phases. This patch
changes the default for such tests to also run in later phases. If the
beacon chain changes enough in later phases to break these tests, this
highlights that the tests need to be adjusted or extended accordingly.
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.
