Fix ancient blocks sync

ethcore/sync/src/block_sync.rs

@@ -282,11 +282,24 @@ impl BlockDownloader {
 		match self.state {
 			State::ChainHead => {
 				if !headers.is_empty() {
-					// TODO: validate heads better. E.g. check that there is enough distance between blocks.
-					trace!(target: "sync", "Received {} subchain heads, proceeding to download", headers.len());
-					self.blocks.reset_to(hashes);
-					self.state = State::Blocks;
-					return Ok(DownloadAction::Reset);
+					// When the round starts, subchain headers are requested
+					// with gaps to be filled. However if the round is reset
+					// while in State::Blocks then its possible to receive
+					// stale responses for the subchain heads in the gap. In
+					// this case the headers will have consecutive numbers
+					// so can be ignored here.
+					let is_subchain_heads = headers.len() == 1
+						|| headers.len() > 1
+						&& headers[1].header.number() - headers[0].header.number() > 1;
+
+					if is_subchain_heads {
+						trace!(target: "sync", "Received {} subchain heads, proceeding to download", headers.len());
+						self.blocks.reset_to(hashes);
+						self.state = State::Blocks;
+						return Ok(DownloadAction::Reset);
+					} else {
+						debug!(target: "sync", "Ignoring consecutive headers: expected subchain headers with gap");
+					}
 				} else {
 					let best = io.chain().chain_info().best_block_number;
 					let oldest_reorg = io.chain().pruning_info().earliest_state;
@@ -515,6 +528,7 @@ impl BlockDownloader {
 				},
 				Err(BlockImportError(BlockImportErrorKind::Queue(QueueErrorKind::Full(limit)), _)) => {
 					debug!(target: "sync", "Block import queue full ({}), restarting sync", limit);
+					bad = true;
 					break;
 				},
 				Err(e) => {