feat(RpcServer): Show block digests by height

Add an endpoint that returns known block digests by height. This can be
used to observe any reorganizations and block races happening.

src/models/blockchain/block/block_selector.rs

@@ -102,7 +102,9 @@ impl FromStr for BlockSelector {
 }
 
 impl BlockSelector {
-    /// returns Digest for this selector, if it exists.
+    /// returns Digest for this selector, if it exists. Returns the digest of
+    /// the block belonging to the canonical chain if multiple blocks with
+    /// same height is found.
     pub async fn as_digest(&self, state: &GlobalState) -> Option<Digest> {
         match self {
             BlockSelector::Digest(d) => Some(*d),

src/rpc_server.rs

@@ -190,6 +190,9 @@ pub trait RPC {
     /// Returns information about the specified block if found
     async fn block_info(block_selector: BlockSelector) -> Option<BlockInfo>;
 
+    /// Return the digests of known blocks with specified height.
+    async fn block_digests_by_height(height: BlockHeight) -> Vec<Digest>;
+
     /// Return the digest for the specified block if found
     async fn block_digest(block_selector: BlockSelector) -> Option<Digest>;
 
@@ -828,6 +831,23 @@ impl RPC for NeptuneRPCServer {
         ))
     }
 
+    // documented in trait. do not add doc-comment.
+    async fn block_digests_by_height(
+        self,
+        _: context::Context,
+        height: BlockHeight,
+    ) -> Vec<Digest> {
+        log_slow_scope!(fn_name!());
+
+        self.state
+            .lock_guard()
+            .await
+            .chain
+            .archival_state()
+            .block_height_to_block_digests(height)
+            .await
+    }
+
     // documented in trait. do not add doc-comment.
     async fn latest_tip_digests(self, _context: tarpc::context::Context, n: usize) -> Vec<Digest> {
         log_slow_scope!(fn_name!());
@@ -1646,6 +1666,14 @@ mod rpc_server_tests {
         let _ = rpc_server.clone().own_instance_id(ctx).await;
         let _ = rpc_server.clone().block_height(ctx).await;
         let _ = rpc_server.clone().peer_info(ctx).await;
+        let _ = rpc_server
+            .clone()
+            .block_digests_by_height(ctx, 42u64.into())
+            .await;
+        let _ = rpc_server
+            .clone()
+            .block_digests_by_height(ctx, 0u64.into())
+            .await;
         let _ = rpc_server.clone().all_punished_peers(ctx).await;
         let _ = rpc_server.clone().latest_tip_digests(ctx, 2).await;
         let _ = rpc_server