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Add --light-client-server flag and state cache utils (#3714)

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## 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()`.
This commit is contained in:
Michael Sproul
2022-11-11 11:03:18 +00:00
parent c591fcd201
commit 3be41006a6
9 changed files with 99 additions and 8 deletions
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21
beacon_node/beacon_chain/src/snapshot_cache.rs
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@@ -298,6 +298,27 @@ impl<T: EthSpec> SnapshotCache<T> {
})
}
/// Borrow the state corresponding to `block_root` if it exists in the cache *unadvanced*.
///
/// Care must be taken not to mutate the state in an invalid way. This function should only
/// be used to mutate the *caches* of the state, for example the tree hash cache when
/// calculating a light client merkle proof.
pub fn borrow_unadvanced_state_mut(
&mut self,
block_root: Hash256,
) -> Option<&mut BeaconState<T>> {
self.snapshots
.iter_mut()
.find(|snapshot| {
// If the pre-state exists then state advance has already taken the state for
// `block_root` and mutated its tree hash cache. Rather than re-building it while
// holding the snapshot cache lock (>1 second), prefer to return `None` from this
// function and force the caller to load it from disk.
snapshot.beacon_block_root == block_root && snapshot.pre_state.is_none()
})
.map(|snapshot| &mut snapshot.beacon_state)
}
/// If there is a snapshot with `block_root`, clone it and return the clone.
pub fn get_cloned(
&self,