Add a simple naive block cache in gossip sync lookups

Author: TheBlueMatt

lightning-block-sync/src/gossip.rs

@@ -4,6 +4,7 @@
 
 use crate::{AsyncBlockSourceResult, BlockData, BlockSource};
 
+use bitcoin::blockdata::block::Block;
 use bitcoin::blockdata::transaction::{TxOut, OutPoint};
 use bitcoin::hash_types::BlockHash;
 
@@ -17,7 +18,8 @@ use lightning::routing::utxo::{UtxoFuture, UtxoLookup, UtxoResult, UtxoLookupErr
 
 use lightning::util::logger::Logger;
 
-use std::sync::Arc;
+use std::sync::{Arc, Mutex};
+use std::collections::VecDeque;
 use std::future::Future;
 use std::ops::Deref;
 
@@ -27,9 +29,6 @@ use std::ops::Deref;
 /// Note that while this is implementable for a [`BlockSource`] which returns filtered block data
 /// (i.e. [`BlockData::HeaderOnly`] for [`BlockSource::get_block`] requests), such an
 /// implementation will reject all gossip as it is not fully able to verify the UTXOs referenced.
-///
-/// For effeciency, an implementation may consider caching some set of blocks, as many redundant
-/// calls may be made.
 pub trait UtxoSource : BlockSource + 'static {
 	/// Fetches the block hash of the block at the given height.
 	///
@@ -91,8 +90,11 @@ pub struct GossipVerifier<S: FutureSpawner,
 	peer_manager: Arc<PeerManager<Descriptor, CM, Arc<P2PGossipSync<Arc<NetworkGraph<L>>, Self, L>>, OM, L, CMH, NS>>,
 	gossiper: Arc<P2PGossipSync<Arc<NetworkGraph<L>>, Self, L>>,
 	spawn: S,
+	block_cache: Arc<Mutex<VecDeque<(u32, Block)>>>,
 }
 
+const BLOCK_CACHE_SIZE: usize = 5;
+
 impl<S: FutureSpawner,
 	Blocks: Deref + Send + Sync + Clone,
 	L: Deref + Send + Sync,
@@ -114,34 +116,68 @@ impl<S: FutureSpawner,
 	/// This is expected to be given to a [`P2PGossipSync`] (initially constructed with `None` for
 	/// the UTXO lookup) via [`P2PGossipSync::add_utxo_lookup`].
 	pub fn new(source: Blocks, spawn: S, gossiper: Arc<P2PGossipSync<Arc<NetworkGraph<L>>, Self, L>>, peer_manager: Arc<PeerManager<Descriptor, CM, Arc<P2PGossipSync<Arc<NetworkGraph<L>>, Self, L>>, OM, L, CMH, NS>>) -> Self {
-		Self { source, spawn, gossiper, peer_manager }
+		Self {
+			source, spawn, gossiper, peer_manager,
+			block_cache: Arc::new(Mutex::new(VecDeque::with_capacity(BLOCK_CACHE_SIZE))),
+		}
 	}
 
-	async fn retrieve_utxo(source: Blocks, short_channel_id: u64) -> Result<TxOut, UtxoLookupError> {
+	async fn retrieve_utxo(
+		source: Blocks, block_cache: Arc<Mutex<VecDeque<(u32, Block)>>>, short_channel_id: u64
+	) -> Result<TxOut, UtxoLookupError> {
 		let block_height = (short_channel_id >> 5 * 8) as u32; // block height is most significant three bytes
 		let transaction_index = ((short_channel_id >> 2 * 8) & 0xffffff) as u32;
 		let output_index = (short_channel_id & 0xffff) as u16;
 
-		let block_hash = source.get_block_hash_by_height(block_height).await
-			.map_err(|_| UtxoLookupError::UnknownTx)?;
-		let block_data = source.get_block(&block_hash).await
-			.map_err(|_| UtxoLookupError::UnknownTx)?;
-		let mut block = match block_data {
-			BlockData::HeaderOnly(_) => return Err(UtxoLookupError::UnknownTx),
-			BlockData::FullBlock(block) => block,
+		let (outpoint, output);
+
+		'tx_found: loop { // Used as a simple goto
+			macro_rules! process_block {
+				($block: expr) => { {
+					if transaction_index as usize >= $block.txdata.len() {
+						return Err(UtxoLookupError::UnknownTx);
+					}
+					let transaction = &$block.txdata[transaction_index as usize];
+					if output_index as usize >= transaction.output.len() {
+						return Err(UtxoLookupError::UnknownTx);
+					}
+
+					outpoint = OutPoint::new(transaction.txid(), output_index.into());
+					output = transaction.output[output_index as usize].clone();
+				} }
+			}
+			{
+				let recent_blocks = block_cache.lock().unwrap();
+				for (height, block) in recent_blocks.iter() {
+					if *height == block_height {
+						process_block!(block);
+						break 'tx_found;
+					}
+				}
+			}
+
+			let block_hash = source.get_block_hash_by_height(block_height).await
+				.map_err(|_| UtxoLookupError::UnknownTx)?;
+			let block_data = source.get_block(&block_hash).await
+				.map_err(|_| UtxoLookupError::UnknownTx)?;
+			let block = match block_data {
+				BlockData::HeaderOnly(_) => return Err(UtxoLookupError::UnknownTx),
+				BlockData::FullBlock(block) => block,
+			};
+			process_block!(block);
+			{
+				let mut recent_blocks = block_cache.lock().unwrap();
+				if recent_blocks.len() >= BLOCK_CACHE_SIZE {
+					recent_blocks.pop_front();
+				}
+				recent_blocks.push_back((block_height, block));
+			}
+			break 'tx_found;
 		};
-		if transaction_index as usize >= block.txdata.len() {
-			return Err(UtxoLookupError::UnknownTx);
-		}
-		let mut transaction = block.txdata.swap_remove(transaction_index as usize);
-		if output_index as usize >= transaction.output.len() {
-			return Err(UtxoLookupError::UnknownTx);
-		}
 		let outpoint_unspent =
-			source.is_output_unspent(OutPoint::new(transaction.txid(), output_index.into())).await
-				.map_err(|_| UtxoLookupError::UnknownTx)?;
+			source.is_output_unspent(outpoint).await.map_err(|_| UtxoLookupError::UnknownTx)?;
 		if outpoint_unspent {
-			Ok(transaction.output.swap_remove(output_index as usize))
+			Ok(output)
 		} else {
 			Err(UtxoLookupError::UnknownTx)
 		}
@@ -190,9 +226,10 @@ impl<S: FutureSpawner,
 		let fut = res.clone();
 		let source = self.source.clone();
 		let gossiper = Arc::clone(&self.gossiper);
+		let block_cache = Arc::clone(&self.block_cache);
 		let pm = Arc::clone(&self.peer_manager);
 		self.spawn.spawn(async move {
-			let res = Self::retrieve_utxo(source, short_channel_id).await;
+			let res = Self::retrieve_utxo(source, block_cache, short_channel_id).await;
 			fut.resolve(gossiper.network_graph(), &*gossiper, res);
 			pm.process_events();
 		});