test(archival_mmr): Add panicking test for leaf-getter

Requested in #318.

Co-authored-by: Alan Szepieniec <[email protected]>

src/util_types/mutator_set/archival_mmr.rs

@@ -186,6 +186,13 @@ impl<Storage: StorageVec<Digest>> ArchivalMmr<Storage> {
 
     /// Get a leaf from the MMR, will panic if index is out of range.
     pub async fn get_leaf_async(&self, leaf_index: u64) -> Digest {
+        // Use debug-assert here to limit this lookup to *one* db-lookup in
+        // production. Otherwise, it would be two lookups.
+        debug_assert!(
+            leaf_index < self.num_leafs().await,
+            "Leaf index out-of-bounds. Got leaf index {leaf_index} but num_leafs was {}",
+            self.num_leafs().await
+        );
         let node_index = shared_advanced::leaf_index_to_node_index(leaf_index);
         self.digests.get(node_index).await
     }
@@ -1164,6 +1171,14 @@ pub(crate) mod mmr_test {
         }
     }
 
+    #[tokio::test]
+    #[should_panic(expected = "Leaf index out-of-bounds. Got leaf index 17 but num_leafs was 17")]
+    async fn get_panics_when_out_of_bounds() {
+        let digests = vec![Digest::default(); 17];
+        let ammr = mock::get_ammr_from_digests(digests.clone()).await;
+        ammr.get_leaf_async(17).await;
+    }
+
     #[tokio::test]
     async fn leveldb_persist_storage_schema_test() {
         let db = NeptuneLevelDb::open_new_test_database(true, None, None, None)