dispute-coordinator: fix underflow panic (paritytech#4557)

node/core/dispute-coordinator/src/real/ordering/mod.rs

@@ -256,12 +256,12 @@ impl OrderingProvider {
 	) -> Result<Vec<Hash>> {
 		let mut ancestors = Vec::new();
 
-		if self.last_observed_blocks.get(&head).is_some() {
+		let finalized_block_number = get_finalized_block_number(sender).await?;
+
+		if self.last_observed_blocks.get(&head).is_some() || head_number <= finalized_block_number {
 			return Ok(ancestors)
 		}
 
-		let finalized_block_number = get_finalized_block_number(sender).await?;
-
 		loop {
 			let (tx, rx) = oneshot::channel();
 			let hashes = {
@@ -279,7 +279,20 @@ impl OrderingProvider {
 				rx.await.or(Err(Fatal::ChainApiSenderDropped))?.map_err(Fatal::ChainApi)?
 			};
 
-			let earliest_block_number = head_number - hashes.len() as u32;
+			let earliest_block_number = match head_number.checked_sub(hashes.len() as u32) {
+				Some(number) => number,
+				None => {
+					// It's assumed that it's impossible to retrieve
+					// more than N ancestors for block number N.
+					tracing::error!(
+						target: LOG_TARGET,
+						"Received {} ancestors for block number {} from Chain API",
+						hashes.len(),
+						head_number,
+					);
+					return Ok(ancestors)
+				},
+			};
 			// The reversed order is parent, grandparent, etc. excluding the head.
 			let block_numbers = (earliest_block_number..head_number).rev();
 
@@ -295,6 +308,7 @@ impl OrderingProvider {
 
 				ancestors.push(*hash);
 			}
+
 			match hashes.last() {
 				Some(last_hash) => {
 					head = *last_hash;

node/core/dispute-coordinator/src/real/ordering/tests.rs

@@ -67,13 +67,11 @@ impl TestState {
 		let chain = vec![get_block_number_hash(0)];
 
 		let finalized_block_number = 0;
-		let expected_ancestry_len = 1;
-		let overseer_fut = overseer_process_active_leaves_update(
-			&mut ctx_handle,
-			&chain,
-			finalized_block_number,
-			expected_ancestry_len,
-		);
+		let overseer_fut = async {
+			assert_finalized_block_number_request(&mut ctx_handle, finalized_block_number).await;
+			// No requests for ancestors since the block is already finalized.
+			assert_candidate_events_request(&mut ctx_handle, &chain).await;
+		};
 
 		let ordering_provider =
 			join(OrderingProvider::new(ctx.sender(), leaf.clone()), overseer_fut)