Only request old blocks from peers with >= difficulty

#9226 might be too
permissive and causing the behaviour of the retraction soon after the
fork block. With this change the peer difficulty has to be greater than
or euqal to our syncing difficulty, so should still fix
#9225

ethcore/sync/src/chain/mod.rs

@@ -762,22 +762,20 @@ impl ChainSync {
 						}
 					}
 
-					// 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()))
-					});
+					// Only ask for old blocks if the peer has an equal or higher difficulty
+					let equal_or_higher_difficulty = peer_difficulty.map_or(false, |pd| pd >= syncing_difficulty);
 
-					if force || last_imported_old_block_difficulty.map_or(true, |ld| peer_difficulty.map_or(true, |pd| pd > ld)) {
+					if force || equal_or_higher_difficulty {
 						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 requesting old blocks, last_imported_old_block_difficulty={:?}, peer_difficulty={:?}",
+							"peer {:?} is not suitable for requesting old blocks, syncing_difficulty={:?}, peer_difficulty={:?}",
 							peer_id,
-							last_imported_old_block_difficulty,
+							syncing_difficulty,
 							peer_difficulty
 						);
 						self.deactivate_peer(io, peer_id);