1 files changed, 13 insertions, 6 deletions
diff --git a/src/swarm/sync.rs b/src/swarm/sync.rs
index 28f3ca0..394e589 100644
--- a/src/swarm/sync.rs
+++ b/src/swarm/sync.rs
@@ -239,14 +239,16 @@ async fn deliver_sync_to_peer(state: &AppState, peer: &str) -> Result<()> {
                     return (session, Ok(None));
                 };
                 // What the envelope adds to an Automerge message (`swarm::stats`), and of
-                // that, the epoch (msgpack: 36 bytes).
+                // that, the epoch's tag (msgpack: 12 bytes, up to 20 for generations past
+                // 127).
                 let inner = match &body {
                     transport::SyncBody::Message(payload) => Some(payload.0.len()),
                     transport::SyncBody::Delta(_) => None,
                 };
-                let ids = 36;
+                let ids = rmp_serde::encode::to_vec(&transport::EpochTag::of(epoch))
+                    .map_or(0, |tag| tag.len());
                 let envelope = transport::SyncEnvelope {
-                    epoch,
+                    epoch: transport::EpochTag::of(epoch),
                     epoch_seq,
                     body,
                     ready,
@@ -406,14 +408,20 @@ pub(crate) async fn handle_sync(
         return Ok(());
     }
 
-    let epoch_label = envelope.epoch;
+    // The tag names our epoch, or the message is of no epoch we sync on (the sender
+    // switched meanwhile, or we did).
+    let Some(epoch) = envelope.epoch.resolve([state.local_epoch()]) else {
+        debug!(peer = %peer, epoch = %envelope.epoch, "Dropped sync message from another epoch");
+        return Ok(());
+    };
+    let epoch_label = epoch;
     let from_ready = envelope.ready;
     let writer = envelope.writer;
     let delivered = envelope.delivered;
     let payload = match envelope.body {
         transport::SyncBody::Message(payload) => payload,
         transport::SyncBody::Delta(spec) => {
-            fetch_delta(state, peer, envelope.epoch, spec);
+            fetch_delta(state, peer, epoch, spec);
             return Ok(());
         }
     };
@@ -435,7 +443,6 @@ pub(crate) async fn handle_sync(
         let check_caught_up = !state.initial_sync_done.load(Ordering::Acquire);
         // While catching up, everything arrives late; that says nothing about propagation.
         let measure_propagation = !check_caught_up;
-        let epoch = envelope.epoch;
         let message = payload;
         let peer = peer.to_owned();
         let clock = Arc::clone(&state.clock);