mtc_worker: Put landmark checkpoint in object storage

The landmark checkpoint is also part of the landmark bundle, but the
landmark bundle is not a standard API and is only intended to be
consumed by our internal infrastructure. Relying parties consume the
landmark checkpoint as a TLOG note.

Author: cjpatton

crates/mtc_api/src/landmark.rs

@@ -9,10 +9,13 @@ pub struct LandmarkSequence {
     pub landmarks: VecDeque<u64>,
 }
 
-/// The location in object storage for the landmark sequence
+/// The location in object storage for the landmark sequence.
 pub const LANDMARK_KEY: &str = "landmark";
 
-/// The location in object storage for the landmark bundle. Its serialized form is JSON
+/// The location in object storage for the landmark checkpoint.
+pub const LANDMARK_CHECKPOINT_KEY: &str = "landmark-checkpoint";
+
+/// The location in object storage for the landmark bundle. Its serialized form is JSON.
 pub const LANDMARK_BUNDLE_KEY: &str = "landmark-bundle";
 
 impl LandmarkSequence {

crates/mtc_worker/src/sequencer_do.rs

@@ -12,7 +12,10 @@ use generic_log_worker::{
     CachedRoObjectBucket, CheckpointCallbacker, GenericSequencer, ObjectBucket, SequencerConfig,
     SEQUENCER_BINDING,
 };
-use mtc_api::{BootstrapMtcLogEntry, LandmarkSequence, LANDMARK_BUNDLE_KEY, LANDMARK_KEY};
+use mtc_api::{
+    BootstrapMtcLogEntry, LandmarkSequence, LANDMARK_BUNDLE_KEY, LANDMARK_CHECKPOINT_KEY,
+    LANDMARK_KEY,
+};
 use serde::{Deserialize, Serialize};
 use serde_with::{base64::Base64, serde_as};
 use signed_note::Note;
@@ -139,6 +142,12 @@ fn checkpoint_callback(env: &Env, name: &str) -> CheckpointCallbacker {
                             .await?;
                     }
 
+                    // Update the landmark checkpoint.
+                    bucket_clone
+                        .put(LANDMARK_CHECKPOINT_KEY, new_checkpoint_str.clone())
+                        .execute()
+                        .await?;
+
                     // Compute the landmark bundle and save it
                     let subtrees =
                         get_landmark_subtrees(&seq, root_hash, tree_size, bucket_clone.clone())