fix(engine): request head block download when not buffered after backfill (#21150)

crates/engine/tree/src/tree/mod.rs

@@ -1676,6 +1676,18 @@ where
                 )));
                 return Ok(());
             }
+        } else {
+            // We don't have the head block or any of its ancestors buffered. Request
+            // a download for the head block which will then trigger further sync.
+            debug!(
+                target: "engine::tree",
+                head_hash = %sync_target_state.head_block_hash,
+                "Backfill complete but head block not buffered, requesting download"
+            );
+            self.emit_event(EngineApiEvent::Download(DownloadRequest::single_block(
+                sync_target_state.head_block_hash,
+            )));
+            return Ok(());
         }
 
         // try to close the gap by executing buffered blocks that are child blocks of the new head

crates/engine/tree/src/tree/tests.rs

@@ -1008,6 +1008,15 @@ async fn test_engine_tree_live_sync_transition_required_blocks_requested() {
         _ => panic!("Unexpected event: {event:#?}"),
     }
 
+    // After backfill completes with head not buffered, we also request head download
+    let event = test_harness.from_tree_rx.recv().await.unwrap();
+    match event {
+        EngineApiEvent::Download(DownloadRequest::BlockSet(hash_set)) => {
+            assert_eq!(hash_set, HashSet::from_iter([main_chain_last_hash]));
+        }
+        _ => panic!("Unexpected event: {event:#?}"),
+    }
+
     let _ = test_harness
         .tree
         .on_engine_message(FromEngine::DownloadedBlocks(vec![main_chain