Refined payload pruning (sigp#3587)

## Proposed Changes

Improve the payload pruning feature in several ways:

- Payload pruning is now entirely optional. It is enabled by default but can be disabled with `--prune-payloads false`. The previous `--prune-payloads-on-startup` flag from sigp#3565 is removed.
- Initial payload pruning on startup now runs in a background thread. This thread will always load the split state, which is a small fraction of its total work (up to ~300ms) and then backtrack from that state. This pruning process ran in 2m5s on one Prater node with good I/O and 16m on a node with slower I/O.
- To work with the optional payload pruning the database function `try_load_full_block` will now attempt to load execution payloads for finalized slots _if_ pruning is currently disabled. This gives users an opt-out for the extensive traffic between the CL and EL for reconstructing payloads.

## Additional Info

If the `prune-payloads` flag is toggled on and off then the on-startup check may not see any payloads to delete and fail to clean them up. In this case the `lighthouse db prune_payloads` command should be used to force a manual sweep of the database.

beacon_node/beacon_chain/src/builder.rs

@@ -266,13 +266,6 @@ where
 
         self.genesis_time = Some(genesis_state.genesis_time());
 
-        // Prune finalized execution payloads.
-        if store.get_config().prune_payloads_on_init {
-            store
-                .try_prune_execution_payloads(false)
-                .map_err(|e| format!("Error pruning execution payloads: {e:?}"))?;
-        }
-
         self.op_pool = Some(
             store
                 .get_item::<PersistedOperationPool<TEthSpec>>(&OP_POOL_DB_KEY)
@@ -863,6 +856,20 @@ where
             beacon_chain.store_migrator.process_reconstruction();
         }
 
+        // Prune finalized execution payloads in the background.
+        if beacon_chain.store.get_config().prune_payloads {
+            let store = beacon_chain.store.clone();
+            let log = log.clone();
+            beacon_chain.task_executor.spawn_blocking(
+                move || {
+                    if let Err(e) = store.try_prune_execution_payloads(false) {
+                        error!(log, "Error pruning payloads in background"; "error" => ?e);
+                    }
+                },
+                "prune_payloads_background",
+            );
+        }
+
         Ok(beacon_chain)
     }
 }

beacon_node/src/cli.rs

@@ -516,9 +516,11 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .default_value("true")
         )
         .arg(
-            Arg::with_name("prune-payloads-on-startup")
-                .long("prune-payloads-on-startup")
-                .help("Check for execution payloads to prune on start-up.")
+            Arg::with_name("prune-payloads")
+                .long("prune-payloads")
+                .help("Prune execution payloads from Lighthouse's database. This saves space but \
+                       imposes load on the execution client, as payloads need to be \
+                       reconstructed and sent to syncing peers.")
                 .takes_value(true)
                 .default_value("true")
         )

beacon_node/src/config.rs

@@ -358,10 +358,8 @@ pub fn get_config<E: EthSpec>(
             .map_err(|_| "auto-compact-db takes a boolean".to_string())?;
     }
 
-    if let Some(prune_payloads_on_init) =
-        clap_utils::parse_optional(cli_args, "prune-payloads-on-startup")?
-    {
-        client_config.store.prune_payloads_on_init = prune_payloads_on_init;
+    if let Some(prune_payloads) = clap_utils::parse_optional(cli_args, "prune-payloads")? {
+        client_config.store.prune_payloads = prune_payloads;
     }
 
     /*

beacon_node/store/src/config.rs

@@ -21,8 +21,8 @@ pub struct StoreConfig {
     pub compact_on_init: bool,
     /// Whether to compact the database during database pruning.
     pub compact_on_prune: bool,
-    /// Whether to try pruning execution payloads on initialization.
-    pub prune_payloads_on_init: bool,
+    /// Whether to prune payloads on initialization and finalization.
+    pub prune_payloads: bool,
 }
 
 /// Variant of `StoreConfig` that gets written to disk. Contains immutable configuration params.
@@ -45,7 +45,7 @@ impl Default for StoreConfig {
             block_cache_size: DEFAULT_BLOCK_CACHE_SIZE,
             compact_on_init: false,
             compact_on_prune: true,
-            prune_payloads_on_init: true,
+            prune_payloads: true,
         }
     }
 }

beacon_node/store/src/hot_cold_store.rs

@@ -21,6 +21,7 @@ use crate::{
     get_key_for_col, DBColumn, DatabaseBlock, Error, ItemStore, KeyValueStoreOp,
     PartialBeaconState, StoreItem, StoreOp,
 };
+use itertools::process_results;
 use leveldb::iterator::LevelDBIterator;
 use lru::LruCache;
 use parking_lot::{Mutex, RwLock};
@@ -334,8 +335,10 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
         };
 
         // If the block is after the split point then we should have the full execution payload
-        // stored in the database. Otherwise, just return the blinded block.
-        // Hold the split lock so that it can't change.
+        // stored in the database. If it isn't but payload pruning is disabled, try to load it
+        // on-demand.
+        //
+        // Hold the split lock so that it can't change while loading the payload.
         let split = self.split.read_recursive();
 
         let block = if blinded_block.message().execution_payload().is_err()
@@ -348,6 +351,18 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
             self.block_cache.lock().put(*block_root, full_block.clone());
 
             DatabaseBlock::Full(full_block)
+        } else if !self.config.prune_payloads {
+            // If payload pruning is disabled there's a chance we may have the payload of
+            // this finalized block. Attempt to load it but don't error in case it's missing.
+            if let Some(payload) = self.get_execution_payload(block_root)? {
+                DatabaseBlock::Full(
+                    blinded_block
+                        .try_into_full_block(Some(payload))
+                        .ok_or(Error::AddPayloadLogicError)?,
+                )
+            } else {
+                DatabaseBlock::Blinded(blinded_block)
+            }
         } else {
             DatabaseBlock::Blinded(blinded_block)
         };
@@ -388,7 +403,9 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
         blinded_block: SignedBeaconBlock<E, BlindedPayload<E>>,
     ) -> Result<SignedBeaconBlock<E>, Error> {
         if blinded_block.message().execution_payload().is_ok() {
-            let execution_payload = self.get_execution_payload(block_root)?;
+            let execution_payload = self
+                .get_execution_payload(block_root)?
+                .ok_or(HotColdDBError::MissingExecutionPayload(*block_root))?;
             blinded_block.try_into_full_block(Some(execution_payload))
         } else {
             blinded_block.try_into_full_block(None)
@@ -433,9 +450,8 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
     pub fn get_execution_payload(
         &self,
         block_root: &Hash256,
-    ) -> Result<ExecutionPayload<E>, Error> {
-        self.get_item(block_root)?
-            .ok_or_else(|| HotColdDBError::MissingExecutionPayload(*block_root).into())
+    ) -> Result<Option<ExecutionPayload<E>>, Error> {
+        self.get_item(block_root)
     }
 
     /// Check if the execution payload for a block exists on disk.
@@ -1446,19 +1462,36 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
 
         // The finalized block may or may not have its execution payload stored, depending on
         // whether it was at a skipped slot. However for a fully pruned database its parent
-        // should *always* have been pruned.
-        let split_parent_block_root = split_state.get_block_root(split.slot - 1)?;
-        if !self.execution_payload_exists(split_parent_block_root)? && !force {
+        // should *always* have been pruned. In case of a long split (no parent found) we
+        // continue as if the payloads are pruned, as the node probably has other things to worry
+        // about.
+        let split_block_root = split_state.get_latest_block_root(split.state_root);
+
+        let already_pruned =
+            process_results(split_state.rev_iter_block_roots(&self.spec), |mut iter| {
+                iter.find(|(_, block_root)| *block_root != split_block_root)
+                    .map_or(Ok(true), |(_, split_parent_root)| {
+                        self.execution_payload_exists(&split_parent_root)
+                            .map(|exists| !exists)
+                    })
+            })??;
+
+        if already_pruned && !force {
             info!(self.log, "Execution payloads are pruned");
             return Ok(());
         }
 
         // Iterate block roots backwards to the Bellatrix fork or the anchor slot, whichever comes
         // first.
-        let split_block_root = split_state.get_latest_block_root(split.state_root);
+        warn!(
+            self.log,
+            "Pruning finalized payloads";
+            "info" => "you may notice degraded I/O performance while this runs"
+        );
         let anchor_slot = self.get_anchor_info().map(|info| info.anchor_slot);
 
         let mut ops = vec![];
+        let mut last_pruned_block_root = None;
 
         for res in std::iter::once(Ok((split_block_root, split.slot)))
             .chain(BlockRootsIterator::new(self, &split_state))
@@ -1468,7 +1501,7 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
                 Err(e) => {
                     warn!(
                         self.log,
-                        "Stopping backtrack early";
+                        "Stopping payload pruning early";
                         "error" => ?e,
                     );
                     break;
@@ -1478,25 +1511,28 @@ impl<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>> HotColdDB<E, Hot, Cold>
             if slot < bellatrix_fork_slot {
                 info!(
                     self.log,
-                    "Finished backtrack to Bellatrix fork";
+                    "Payload pruning reached Bellatrix boundary";
                 );
                 break;
             }
 
-            if self.execution_payload_exists(&block_root)? {
+            if Some(block_root) != last_pruned_block_root
+                && self.execution_payload_exists(&block_root)?
+            {
                 debug!(
                     self.log,
                     "Pruning execution payload";
                     "slot" => slot,
                     "block_root" => ?block_root,
                 );
+                last_pruned_block_root = Some(block_root);
                 ops.push(StoreOp::DeleteExecutionPayload(block_root));
             }
 
             if Some(slot) == anchor_slot {
                 info!(
                     self.log,
-                    "Finished backtrack to anchor state";
+                    "Payload pruning reached anchor state";
                     "slot" => slot
                 );
                 break;
@@ -1583,10 +1619,13 @@ pub fn migrate_database<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>>(
         // Delete the old summary, and the full state if we lie on an epoch boundary.
         hot_db_ops.push(StoreOp::DeleteState(state_root, Some(slot)));
 
-        // Delete the execution payload. Even if this execution payload is the payload of the
-        // new finalized block it is OK to delete it, as `try_get_full_block` looks at the split
-        // slot when determining whether to reconstruct payloads.
-        hot_db_ops.push(StoreOp::DeleteExecutionPayload(block_root));
+        // Delete the execution payload if payload pruning is enabled. At a skipped slot we may
+        // delete the payload for the finalized block itself, but that's OK as we only guarantee
+        // that payloads are present for slots >= the split slot. The payload fetching code is also
+        // forgiving of missing payloads.
+        if store.config.prune_payloads {
+            hot_db_ops.push(StoreOp::DeleteExecutionPayload(block_root));
+        }
     }
 
     // Warning: Critical section.  We have to take care not to put any of the two databases in an

lighthouse/tests/beacon_node.rs

@@ -1227,17 +1227,17 @@ fn compact_db_flag() {
         .with_config(|config| assert!(config.store.compact_on_init));
 }
 #[test]
-fn prune_payloads_on_startup_default() {
+fn prune_payloads_default() {
     CommandLineTest::new()
         .run_with_zero_port()
-        .with_config(|config| assert!(config.store.prune_payloads_on_init));
+        .with_config(|config| assert!(config.store.prune_payloads));
 }
 #[test]
 fn prune_payloads_on_startup_false() {
     CommandLineTest::new()
-        .flag("prune-payloads-on-startup", Some("false"))
+        .flag("prune-payloads", Some("false"))
         .run_with_zero_port()
-        .with_config(|config| assert!(!config.store.prune_payloads_on_init));
+        .with_config(|config| assert!(!config.store.prune_payloads));
 }
 #[test]
 fn reconstruct_historic_states_flag() {