## Issue Addressed
Closes https://github.com/sigp/lighthouse/issues/2327
## Proposed Changes
This is an extension of some ideas I implemented while working on `tree-states`:
- Cache the indexed attestations from blocks in the `ConsensusContext`. Previously we were re-computing them 3-4 times over.
- Clean up `import_block` by splitting each part into `import_block_XXX`.
- Move some stuff off hot paths, specifically:
- Relocate non-essential tasks that were running between receiving the payload verification status and priming the early attester cache. These tasks are moved after the cache priming:
- Attestation observation
- Validator monitor updates
- Slasher updates
- Updating the shuffling cache
- Fork choice attestation observation now happens at the end of block verification in parallel with payload verification (this seems to save 5-10ms).
- Payload verification now happens _before_ advancing the pre-state and writing it to disk! States were previously being written eagerly and adding ~20-30ms in front of verifying the execution payload. State catchup also sometimes takes ~500ms if we get a cache miss and need to rebuild the tree hash cache.
The remaining task that's taking substantial time (~20ms) is importing the block to fork choice. I _think_ this is because of pull-tips, and we should be able to optimise it out with a clever total active balance cache in the state (which would be computed in parallel with payload verification). I've decided to leave that for future work though. For now it can be observed via the new `beacon_block_processing_post_exec_pre_attestable_seconds` metric.
Co-authored-by: Michael Sproul <micsproul@gmail.com>
## Issue Addressed
Part of https://github.com/sigp/lighthouse/issues/3651.
## Proposed Changes
Add a flag for enabling the light client server, which should be checked before gossip/RPC traffic is processed (e.g. https://github.com/sigp/lighthouse/pull/3693, https://github.com/sigp/lighthouse/pull/3711). The flag is available at runtime from `beacon_chain.config.enable_light_client_server`.
Additionally, a new method `BeaconChain::with_mutable_state_for_block` is added which I envisage being used for computing light client updates. Unfortunately its performance will be quite poor on average because it will only run quickly with access to the tree hash cache. Each slot the tree hash cache is only available for a brief window of time between the head block being processed and the state advance at 9s in the slot. When the state advance happens the cache is moved and mutated to get ready for the next slot, which makes it no longer useful for merkle proofs related to the head block. Rather than spend more time trying to optimise this I think we should continue prototyping with this code, and I'll make sure `tree-states` is ready to ship before we enable the light client server in prod (cf. https://github.com/sigp/lighthouse/pull/3206).
## Additional Info
I also fixed a bug in the implementation of `BeaconState::compute_merkle_proof` whereby the tree hash cache was moved with `.take()` but never put back with `.restore()`.
