State cache tweaks (#7095)

Backport of:

- https://github.com/sigp/lighthouse/pull/7067

For:

- https://github.com/sigp/lighthouse/issues/7039


  - Prevent writing to state cache when migrating the database
- Add `state-cache-headroom` flag to control pruning
- Prune old epoch boundary states ahead of mid-epoch states
- Never prune head block's state
- Avoid caching ancestor states unless they are on an epoch boundary
- Log when states enter/exit the cache

Co-authored-by: Eitan Seri-Levi <eserilev@ucsc.edu>

beacon_node/http_api/src/attestation_performance.rs

@@ -126,8 +126,11 @@ pub fn get_attestation_performance<T: BeaconChainTypes>(
 
     // Load state for block replay.
     let state_root = prior_block.state_root();
+
+    // This branch is reached from the HTTP API. We assume the user wants
+    // to cache states so that future calls are faster.
     let state = chain
-        .get_state(&state_root, Some(prior_slot))
+        .get_state(&state_root, Some(prior_slot), true)
         .and_then(|maybe_state| maybe_state.ok_or(BeaconChainError::MissingBeaconState(state_root)))
         .map_err(unhandled_error)?;
 

beacon_node/http_api/src/block_packing_efficiency.rs

@@ -285,8 +285,10 @@ pub fn get_block_packing_efficiency<T: BeaconChainTypes>(
     // Load state for block replay.
     let starting_state_root = first_block.state_root();
 
+    // This branch is reached from the HTTP API. We assume the user wants
+    // to cache states so that future calls are faster.
     let starting_state = chain
-        .get_state(&starting_state_root, Some(prior_slot))
+        .get_state(&starting_state_root, Some(prior_slot), true)
         .and_then(|maybe_state| {
             maybe_state.ok_or(BeaconChainError::MissingBeaconState(starting_state_root))
         })

beacon_node/http_api/src/block_rewards.rs

@@ -43,8 +43,10 @@ pub fn get_block_rewards<T: BeaconChainTypes>(
         .map_err(unhandled_error)?
         .ok_or_else(|| custom_bad_request(format!("prior state at slot {} unknown", prior_slot)))?;
 
+    // This branch is reached from the HTTP API. We assume the user wants
+    // to cache states so that future calls are faster.
     let mut state = chain
-        .get_state(&state_root, Some(prior_slot))
+        .get_state(&state_root, Some(prior_slot), true)
         .and_then(|maybe_state| maybe_state.ok_or(BeaconChainError::MissingBeaconState(state_root)))
         .map_err(unhandled_error)?;
 
@@ -133,8 +135,10 @@ pub fn compute_block_rewards<T: BeaconChainTypes>(
                     ))
                 })?;
 
+            // This branch is reached from the HTTP API. We assume the user wants
+            // to cache states so that future calls are faster.
             let parent_state = chain
-                .get_state(&parent_block.state_root(), Some(parent_block.slot()))
+                .get_state(&parent_block.state_root(), Some(parent_block.slot()), true)
                 .map_err(unhandled_error)?
                 .ok_or_else(|| {
                     custom_bad_request(format!(

beacon_node/http_api/src/state_id.rs

@@ -189,8 +189,10 @@ impl StateId {
             _ => (self.root(chain)?, None),
         };
 
+        // This branch is reached from the HTTP API. We assume the user wants
+        // to cache states so that future calls are faster.
         let state = chain
-            .get_state(&state_root, slot_opt)
+            .get_state(&state_root, slot_opt, true)
             .map_err(warp_utils::reject::unhandled_error)
             .and_then(|opt| {
                 opt.ok_or_else(|| {

beacon_node/http_api/src/sync_committee_rewards.rs

@@ -58,8 +58,10 @@ pub fn get_state_before_applying_block<T: BeaconChainTypes>(
         })
         .map_err(|e| custom_not_found(format!("Parent block is not available! {:?}", e)))?;
 
+    // We are about to apply a new block to the chain. It's parent state
+    // is a useful/recent state, we elect to cache it.
     let parent_state = chain
-        .get_state(&parent_block.state_root(), Some(parent_block.slot()))
+        .get_state(&parent_block.state_root(), Some(parent_block.slot()), true)
         .and_then(|maybe_state| {
             maybe_state
                 .ok_or_else(|| BeaconChainError::MissingBeaconState(parent_block.state_root()))