From 15702a875c193359e6617c017dec3f1474a943d5 Mon Sep 17 00:00:00 2001
From: Arkadiy Paronyan <arkady.paronyan@gmail.com>
Date: Fri, 18 Nov 2016 17:45:00 +0100
Subject: [PATCH] Limit sync reorg (#3509)

---
 sync/src/block_sync.rs | 31 +++++++++++++++++++++----------
 sync/src/chain.rs      |  1 +
 2 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/sync/src/block_sync.rs b/sync/src/block_sync.rs
index 7c3cbf2d7..ff5411140 100644
--- a/sync/src/block_sync.rs
+++ b/sync/src/block_sync.rs
@@ -34,6 +34,7 @@ const MAX_RECEPITS_TO_REQUEST: usize = 128;
 const SUBCHAIN_SIZE: u64 = 256;
 const MAX_ROUND_PARENTS: usize = 32;
 const MAX_PARALLEL_SUBCHAIN_DOWNLOAD: usize = 5;
+const MAX_REORG_BLOCKS: u64 = 20;
 
 #[derive(Copy, Clone, Eq, PartialEq, Debug)]
 /// Downloader state
@@ -262,7 +263,8 @@ impl BlockDownloader {
 			State::Blocks => {
 				let count = headers.len();
 				// At least one of the heades must advance the subchain. Otherwise they are all useless.
-				if !any_known {
+				if count == 0 || !any_known {
+					trace!(target: "sync", "No useful headers");
 					return Err(BlockDownloaderImportError::Useless);
 				}
 				self.blocks.insert_headers(headers);
@@ -340,14 +342,21 @@ impl BlockDownloader {
 					self.last_imported_hash = p.clone();
 					trace!(target: "sync", "Searching common header from the last round {} ({})", self.last_imported_block, self.last_imported_hash);
 				} else {
-					match io.chain().block_hash(BlockID::Number(self.last_imported_block - 1)) {
-						Some(h) => {
-							self.last_imported_block -= 1;
-							self.last_imported_hash = h;
-							trace!(target: "sync", "Searching common header in the blockchain {} ({})", self.last_imported_block, self.last_imported_hash);
-						}
-						None => {
-							debug!(target: "sync", "Could not revert to previous block, last: {} ({})", self.last_imported_block, self.last_imported_hash);
+					let best = io.chain().chain_info().best_block_number;
+					if best > self.last_imported_block && best - self.last_imported_block > MAX_REORG_BLOCKS {
+						debug!(target: "sync", "Could not revert to previous ancient block, last: {} ({})", self.last_imported_block, self.last_imported_hash);
+						self.reset();
+					} else {
+						match io.chain().block_hash(BlockID::Number(self.last_imported_block - 1)) {
+							Some(h) => {
+								self.last_imported_block -= 1;
+								self.last_imported_hash = h;
+								trace!(target: "sync", "Searching common header in the blockchain {} ({})", self.last_imported_block, self.last_imported_hash);
+							}
+							None => {
+								debug!(target: "sync", "Could not revert to previous block, last: {} ({})", self.last_imported_block, self.last_imported_hash);
+								self.reset();
+							}
 						}
 					}
 				}
@@ -362,7 +371,9 @@ impl BlockDownloader {
 		match self.state {
 			State::Idle => {
 				self.start_sync_round(io);
-				return self.request_blocks(io, num_active_peers);
+				if self.state == State::ChainHead {
+					return self.request_blocks(io, num_active_peers);
+				}
 			},
 			State::ChainHead => {
 				if num_active_peers < MAX_PARALLEL_SUBCHAIN_DOWNLOAD {
diff --git a/sync/src/chain.rs b/sync/src/chain.rs
index d2939fbac..ffd89ecdd 100644
--- a/sync/src/chain.rs
+++ b/sync/src/chain.rs
@@ -1144,6 +1144,7 @@ impl ChainSync {
 					let have_latest = io.chain().block_status(BlockID::Hash(peer_latest)) != BlockStatus::Unknown;
 					if !have_latest && (higher_difficulty || force || self.state == SyncState::NewBlocks) {
 						// check if got new blocks to download
+						trace!(target: "sync", "Syncing with {}, force={}, td={:?}, our td={}, state={:?}", peer_id, force, peer_difficulty, syncing_difficulty, self.state);
 						if let Some(request) = self.new_blocks.request_blocks(io, num_active_peers) {
 							self.request_blocks(io, peer_id, request, BlockSet::NewBlocks);
 							if self.state == SyncState::Idle {