Allow old blocks from peers with lower difficulty (#9226)

Previously we only allow downloading of old blocks if the peer
difficulty was greater than our syncing difficulty. This change allows
downloading of blocks from peers where the difficulty is greater then
the last downloaded old block.

ethcore/sync/src/chain/mod.rs

@@ -761,14 +761,24 @@ impl ChainSync {
 						}
 					}
 
-					// Only ask for old blocks if the peer has a higher difficulty
-					if force || higher_difficulty {
+					// Only ask for old blocks if the peer has a higher difficulty than the last imported old block
+					let last_imported_old_block_difficulty = self.old_blocks.as_mut().and_then(|d| {
+						io.chain().block_total_difficulty(BlockId::Number(d.last_imported_block_number()))
+					});
+
+					if force || last_imported_old_block_difficulty.map_or(true, |ld| peer_difficulty.map_or(true, |pd| pd > ld)) {
 						if let Some(request) = self.old_blocks.as_mut().and_then(|d| d.request_blocks(io, num_active_peers)) {
 							SyncRequester::request_blocks(self, io, peer_id, request, BlockSet::OldBlocks);
 							return;
 						}
 					} else {
-						trace!(target: "sync", "peer {} is not suitable for asking old blocks", peer_id);
+						trace!(
+							target: "sync",
+							"peer {:?} is not suitable for requesting old blocks, last_imported_old_block_difficulty={:?}, peer_difficulty={:?}",
+							peer_id,
+							last_imported_old_block_difficulty,
+							peer_difficulty
+						);
 						self.deactivate_peer(io, peer_id);
 					}
 				},