This PR changes the blsync checkpoint init logic so that even if the
initialization fails with a certain server and an error log message is
printed, the server goes back to its initial state and is allowed to
retry initialization after the failure delay period. The previous logic
had an `ssDone` server state that did put the server in a permanently
unusable state once the checkpoint init failed for an apparently
permanent reason. This was not the correct behavior because different
servers behave differently in case of overload and sometimes the
response to a permanently missing item is not clearly distinguishable
from an overload response. A safer logic is to never assume anything to
be permanent and always give a chance to retry.
The failure delay formula is also fixed; now it is properly capped at
`maxFailureDelay`. The previous formula did allow the delay to grow
unlimited if a retry was attempted immediately after each delay period.
This PR adds an extra mechanism to sync.HeadSync that tries to retrieve the latest finality update from every server each time it sends an optimistic update in a new epoch (unless we already have a validated finality update attested in the same epoch).
Note that this is not necessary and does not happen if the new finality update is delivered before the optimistic update. The spec only mandates light_client_finality_update events when a new epoch is finalized. If the chain does not finalize for a while then we might need an explicit request that returns a finality proof that proves the same finality epoch from the latest attested epoch.
Here we add a beacon chain light client for use by geth.
Geth can now be configured to run against a beacon chain API endpoint,
without pointing a CL to it. To set this up, use the `--beacon.api` flag. Information
provided by the beacon chain is verified, i.e. geth does not blindly trust the beacon
API endpoint in this mode. The root of trust are the beacon chain 'sync committees'.
The configured beacon API endpoint must provide light client data. At this time, only
Lodestar and Nimbus provide the necessary APIs.
There is also a standalone tool, cmd/blsync, which uses the beacon chain light client
to drive any EL implementation via its engine API.
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Co-authored-by: Felix Lange <fjl@twurst.com>
