don't resend sync messages to up-to-date peers

When an sync messages arrives and automerge has nothing to say about it,
then no reply is generated.  With c9279e4 it was assumed that every sync
message gets an answer, so missing replies caused unnecessary resends.

Now a reply is only awaited if the peer's last reported heads differ
from ours. If the peer's heads become equal through a third peer without
replying, the waiting will also abort if a ping or pong showss the same
heads digest on both sides.

1 files changed, 15 insertions, 6 deletions

diff --git a/src/swarm/peer.rs b/src/swarm/peer.rs
index d801b0f..bad8e24 100644
--- a/src/swarm/peer.rs
+++ b/src/swarm/peer.rs
@@ -133,12 +133,21 @@ async fn heal_if_diverged(
         return;
     };
 
-    let heal = peer_state_mutex.lock().await.needs_heal(
-        &local.heads_digest,
-        &remote.heads_digest,
-        &state.timing,
-        Instant::now(),
-    );
+    let heal = {
+        let mut peer_state = peer_state_mutex.lock().await;
+        if local.heads_digest == remote.heads_digest {
+            // Same document: whatever answer we were waiting for, there is nothing left
+            // to send. (Automerge answers only when it has something to say, so a sync
+            // message to a peer that already has everything gets no reply at all.)
+            peer_state.sync_in_step();
+        }
+        peer_state.needs_heal(
+            &local.heads_digest,
+            &remote.heads_digest,
+            &state.timing,
+            Instant::now(),
+        )
+    };
 
     if heal {
         warn!(peer = %peer, "Documents differ and sync is idle; re-handshaking to restart sync");