Drop overflow cache (#5891)

* Drop overflow cache

* Update docs

* Update beacon_node/store/src/lib.rs

* Update data_availability_checker.rs

* Lint

beacon_node/beacon_chain/src/data_availability_checker.rs

@@ -2,7 +2,7 @@ use crate::blob_verification::{verify_kzg_for_blob_list, GossipVerifiedBlob, Kzg
 use crate::block_verification_types::{
     AvailabilityPendingExecutedBlock, AvailableExecutedBlock, RpcBlock,
 };
-use crate::data_availability_checker::overflow_lru_cache::OverflowLRUCache;
+use crate::data_availability_checker::overflow_lru_cache::DataAvailabilityCheckerInner;
 use crate::{BeaconChain, BeaconChainTypes, BeaconStore};
 use kzg::Kzg;
 use slog::{debug, error, Logger};
@@ -33,12 +33,32 @@ pub const OVERFLOW_LRU_CAPACITY: NonZeroUsize = new_non_zero_usize(1024);
 pub const STATE_LRU_CAPACITY_NON_ZERO: NonZeroUsize = new_non_zero_usize(2);
 pub const STATE_LRU_CAPACITY: usize = STATE_LRU_CAPACITY_NON_ZERO.get();
 
-/// This includes a cache for any blocks or blobs that have been received over gossip or RPC
-/// and are awaiting more components before they can be imported. Additionally the
-/// `DataAvailabilityChecker` is responsible for KZG verification of block components as well as
-/// checking whether a "availability check" is required at all.
+/// Cache to hold fully valid data that can't be imported to fork-choice yet. After Dencun hard-fork
+/// blocks have a sidecar of data that is received separately from the network. We call the concept
+/// of a block "becoming available" when all of its import dependencies are inserted into this
+/// cache.
+///
+/// Usually a block becomes available on its slot within a second of receiving its first component
+/// over gossip. However, a block may never become available if a malicious proposer does not
+/// publish its data, or there are network issues that prevent us from receiving it. If the block
+/// does not become available after some time we can safely forget about it. Consider these two
+/// cases:
+///
+/// - Global unavailability: If nobody has received the block components it's likely that the
+///   proposer never made the block available. So we can safely forget about the block as it will
+///   never become available.
+/// - Local unavailability: Some fraction of the network has received all block components, but not us.
+///   Some of our peers will eventually attest to a descendant of that block and lookup sync will
+///   fetch its components. Therefore it's not strictly necessary to hold to the partially available
+///   block for too long as we can recover from other peers.
+///
+/// Even in periods of non-finality, the proposer is expected to publish the block's data
+/// immediately. Because this cache only holds fully valid data, its capacity is bound to 1 block
+/// per slot and fork: before inserting into this cache we check the proposer signature and correct
+/// proposer. Having a capacity > 1 is an optimization to prevent sync lookup from having re-fetch
+/// data during moments of unstable network conditions.
 pub struct DataAvailabilityChecker<T: BeaconChainTypes> {
-    availability_cache: Arc<OverflowLRUCache<T>>,
+    availability_cache: Arc<DataAvailabilityCheckerInner<T>>,
     slot_clock: T::SlotClock,
     kzg: Option<Arc<Kzg>>,
     log: Logger,
@@ -74,7 +94,8 @@ impl<T: BeaconChainTypes> DataAvailabilityChecker<T> {
         log: &Logger,
         spec: ChainSpec,
     ) -> Result<Self, AvailabilityCheckError> {
-        let overflow_cache = OverflowLRUCache::new(OVERFLOW_LRU_CAPACITY, store, spec.clone())?;
+        let overflow_cache =
+            DataAvailabilityCheckerInner::new(OVERFLOW_LRU_CAPACITY, store, spec.clone())?;
         Ok(Self {
             availability_cache: Arc::new(overflow_cache),
             slot_clock,
@@ -329,15 +350,9 @@ impl<T: BeaconChainTypes> DataAvailabilityChecker<T> {
         })
     }
 
-    /// Persist all in memory components to disk
-    pub fn persist_all(&self) -> Result<(), AvailabilityCheckError> {
-        self.availability_cache.write_all_to_disk()
-    }
-
     /// Collects metrics from the data availability checker.
     pub fn metrics(&self) -> DataAvailabilityCheckerMetrics {
         DataAvailabilityCheckerMetrics {
-            num_store_entries: self.availability_cache.num_store_entries(),
             state_cache_size: self.availability_cache.state_cache_size(),
             block_cache_size: self.availability_cache.block_cache_size(),
         }
@@ -346,7 +361,6 @@ impl<T: BeaconChainTypes> DataAvailabilityChecker<T> {
 
 /// Helper struct to group data availability checker metrics.
 pub struct DataAvailabilityCheckerMetrics {
-    pub num_store_entries: usize,
     pub state_cache_size: usize,
     pub block_cache_size: usize,
 }
@@ -372,7 +386,7 @@ pub fn start_availability_cache_maintenance_service<T: BeaconChainTypes>(
 
 async fn availability_cache_maintenance_service<T: BeaconChainTypes>(
     chain: Arc<BeaconChain<T>>,
-    overflow_cache: Arc<OverflowLRUCache<T>>,
+    overflow_cache: Arc<DataAvailabilityCheckerInner<T>>,
 ) {
     let epoch_duration = chain.slot_clock.slot_duration() * T::EthSpec::slots_per_epoch() as u32;
     loop {