Drop the block cache after genesis

beacon_node/client/src/builder.rs

@@ -591,6 +591,17 @@ where
 
         let backend = if let Some(eth1_service_from_genesis) = self.eth1_service {
             eth1_service_from_genesis.update_config(config.clone())?;
+
+            // This cache is not useful because it's first (earliest) block likely the block that
+            // triggered genesis.
+            //
+            // In order to vote we need to be able to go back at least 2 * `ETH1_FOLLOW_DISTANCE`
+            // from the genesis-triggering block.  Presently the block cache does not support
+            // importing blocks with decreasing block numbers, it only accepts them in increasing
+            // order. If this turns out to be a bottleneck we can update the block cache to allow
+            // adding earlier blocks too.
+            eth1_service_from_genesis.drop_block_cache();
+
             CachingEth1Backend::from_service(eth1_service_from_genesis, store)
         } else {
             CachingEth1Backend::new(config, context.log, store)

beacon_node/eth1/src/service.rs

@@ -163,6 +163,11 @@ impl Service {
         &self.inner.deposit_cache
     }
 
+    /// Drop the block cache, replacing it with an empty one.
+    pub fn drop_block_cache(&self) {
+        *(self.inner.block_cache.write()) = BlockCache::default();
+    }
+
     /// Returns the number of currently cached blocks.
     pub fn block_cache_len(&self) -> usize {
         self.blocks().read().len()