[dag][bugfix] fix bitmask for incomplete first round

consensus/src/dag/dag_store.rs

@@ -272,6 +272,10 @@ impl Dag {
     }
 
     pub fn lowest_incomplete_round(&self) -> Option<Round> {
+        if self.nodes_by_round.is_empty() {
+            return Some(self.lowest_round());
+        }
+
         for (round, round_nodes) in &self.nodes_by_round {
             if round_nodes.iter().any(|node| node.is_none()) {
                 return Some(*round);

consensus/src/dag/tests/dag_test.rs

@@ -209,15 +209,14 @@ fn test_dag_recover_from_storage() {
 fn test_dag_bitmask() {
     let (signers, epoch_state, mut dag, _) = setup();
 
-    let mut metadatas = vec![];
+    assert_eq!(dag.bitmask(15), DagSnapshotBitmask::new(1, vec![]));
 
     for round in 1..5 {
         let parents = dag
             .get_strong_links_for_round(round, &epoch_state.verifier)
             .unwrap_or_default();
         for signer in &signers[0..3] {
             let node = new_certified_node(round, signer.author(), parents.clone());
-            metadatas.push(node.metadata().clone());
             assert!(dag.add_node(node).is_ok());
         }
     }
@@ -226,12 +225,12 @@ fn test_dag_bitmask() {
         DagSnapshotBitmask::new(1, vec![vec![true, true, true, false]; 4])
     );
 
+    // Populate the fourth author for all rounds
     for round in 1..5 {
         let parents = dag
             .get_strong_links_for_round(round, &epoch_state.verifier)
             .unwrap_or_default();
         let node = new_certified_node(round, signers[3].author(), parents.clone());
-        metadatas.push(node.metadata().clone());
         assert!(dag.add_node(node).is_ok());
     }
     assert_eq!(dag.bitmask(15), DagSnapshotBitmask::new(5, vec![]));