From 60cbcbd96b08310a3bdbe93a9d77e66515eb267b Mon Sep 17 00:00:00 2001
From: Antioch Peverell <apeverell@protonmail.com>
Date: Wed, 28 Nov 2018 17:48:50 +0000
Subject: [PATCH] Get rid of block hashes cache (#2018)

check_known_head and check_known_orphans catches majority of cases
---
 chain/src/chain.rs | 18 ------------------
 chain/src/pipe.rs  | 20 ++------------------
 2 files changed, 2 insertions(+), 36 deletions(-)

diff --git a/chain/src/chain.rs b/chain/src/chain.rs
index 6bc5b2997..04fcbb991 100644
--- a/chain/src/chain.rs
+++ b/chain/src/chain.rs
@@ -23,7 +23,6 @@ use std::time::{Duration, Instant};
 use util::RwLock;
 
 use lmdb;
-use lru_cache::LruCache;
 
 use core::core::hash::{Hash, Hashed, ZERO_HASH};
 use core::core::merkle_proof::MerkleProof;
@@ -50,9 +49,6 @@ pub const MAX_ORPHAN_SIZE: usize = 200;
 /// When evicting, very old orphans are evicted first
 const MAX_ORPHAN_AGE_SECS: u64 = 300;
 
-/// Number of recent hashes we keep to de-duplicate block or header sends
-const HASHES_CACHE_SIZE: usize = 200;
-
 #[derive(Debug, Clone)]
 struct Orphan {
 	block: Block,
@@ -152,8 +148,6 @@ pub struct Chain {
 	adapter: Arc<ChainAdapter + Send + Sync>,
 	orphans: Arc<OrphanBlockPool>,
 	txhashset: Arc<RwLock<txhashset::TxHashSet>>,
-	// Recently processed blocks to avoid double-processing
-	block_hashes_cache: Arc<RwLock<LruCache<Hash, bool>>>,
 	verifier_cache: Arc<RwLock<VerifierCache>>,
 	// POW verification function
 	pow_verifier: fn(&BlockHeader) -> Result<(), pow::Error>,
@@ -221,7 +215,6 @@ impl Chain {
 			txhashset: Arc::new(RwLock::new(txhashset)),
 			pow_verifier,
 			verifier_cache,
-			block_hashes_cache: Arc::new(RwLock::new(LruCache::new(HASHES_CACHE_SIZE))),
 			archive_mode,
 			genesis: genesis.header.clone(),
 		})
@@ -276,17 +269,8 @@ impl Chain {
 			(maybe_new_head, prev_head)
 		};
 
-		let add_to_hash_cache = |hash: Hash| {
-			// only add to hash cache below if block is definitively accepted
-			// or rejected
-			let mut cache = self.block_hashes_cache.write();
-			cache.insert(hash, true);
-		};
-
 		match maybe_new_head {
 			Ok(head) => {
-				add_to_hash_cache(b.hash());
-
 				let status = self.determine_status(head.clone(), prev_head);
 
 				// notifying other parts of the system of the update
@@ -333,7 +317,6 @@ impl Chain {
 						b.header.height,
 						e
 					);
-					add_to_hash_cache(b.hash());
 					Err(ErrorKind::Other(format!("{:?}", e).to_owned()).into())
 				}
 			},
@@ -377,7 +360,6 @@ impl Chain {
 		Ok(pipe::BlockContext {
 			opts,
 			pow_verifier: self.pow_verifier,
-			block_hashes_cache: self.block_hashes_cache.clone(),
 			verifier_cache: self.verifier_cache.clone(),
 			txhashset,
 			batch,
diff --git a/chain/src/pipe.rs b/chain/src/pipe.rs
index 99eba55e6..47f8e5d60 100644
--- a/chain/src/pipe.rs
+++ b/chain/src/pipe.rs
@@ -20,11 +20,9 @@ use util::RwLock;
 use chrono::prelude::Utc;
 use chrono::Duration;
 
-use lru_cache::LruCache;
-
 use chain::OrphanBlockPool;
 use core::consensus;
-use core::core::hash::{Hash, Hashed};
+use core::core::hash::Hashed;
 use core::core::verifier_cache::VerifierCache;
 use core::core::Committed;
 use core::core::{Block, BlockHeader, BlockSums};
@@ -47,9 +45,6 @@ pub struct BlockContext<'a> {
 	pub txhashset: &'a mut txhashset::TxHashSet,
 	/// The active batch to use for block processing.
 	pub batch: store::Batch<'a>,
-
-	/// Recently processed blocks to avoid double-processing
-	pub block_hashes_cache: Arc<RwLock<LruCache<Hash, bool>>>,
 	/// The verifier cache (caching verifier for rangeproofs and kernel signatures)
 	pub verifier_cache: Arc<RwLock<VerifierCache>>,
 	/// Recent orphan blocks to avoid double-processing
@@ -84,7 +79,6 @@ fn process_header_for_block(
 // from cheapest to most expensive (delay hitting the db until last).
 fn check_known(block: &Block, ctx: &mut BlockContext) -> Result<(), Error> {
 	check_known_head(&block.header, ctx)?;
-	check_known_cache(&block.header, ctx)?;
 	check_known_orphans(&block.header, ctx)?;
 	check_known_store(&block.header, ctx)?;
 	Ok(())
@@ -106,6 +100,7 @@ pub fn process_block(b: &Block, ctx: &mut BlockContext) -> Result<Option<Tip>, E
 		b.kernels().len(),
 	);
 
+	// Check if we have already processed this block previously.
 	check_known(b, ctx)?;
 
 	// Delay hitting the db for current chain head until we know
@@ -291,17 +286,6 @@ fn check_known_head(header: &BlockHeader, ctx: &mut BlockContext) -> Result<(),
 	Ok(())
 }
 
-/// Quick in-memory check to fast-reject any block handled recently.
-/// Keeps duplicates from the network in check.
-/// Checks against the cache of recently processed block hashes.
-fn check_known_cache(header: &BlockHeader, ctx: &mut BlockContext) -> Result<(), Error> {
-	let mut cache = ctx.block_hashes_cache.write();
-	if cache.contains_key(&header.hash()) {
-		return Err(ErrorKind::Unfit("already known in cache".to_string()).into());
-	}
-	Ok(())
-}
-
 /// Check if this block is in the set of known orphans.
 fn check_known_orphans(header: &BlockHeader, ctx: &mut BlockContext) -> Result<(), Error> {
 	if ctx.orphans.contains(&header.hash()) {