Use upgradable read lock for pubkey cache (sigp#6190)

* Use upgradable read for pubkey cache

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -3481,9 +3481,15 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         // would be difficult to check that they all lock fork choice first.
         let mut ops = {
             let _timer = metrics::start_timer(&metrics::BLOCK_PROCESSING_PUBKEY_CACHE_LOCK);
-            self.validator_pubkey_cache
-                .write()
-                .import_new_pubkeys(&state)?
+            let pubkey_cache = self.validator_pubkey_cache.upgradable_read();
+
+            // Only take a write lock if there are new keys to import.
+            if state.validators().len() > pubkey_cache.len() {
+                parking_lot::RwLockUpgradableReadGuard::upgrade(pubkey_cache)
+                    .import_new_pubkeys(&state)?
+            } else {
+                vec![]
+            }
         };
 
         // Apply the state to the attester cache, only if it is from the previous epoch or later.