offer answers

3 files changed, 23 insertions, 1 deletions

diff --git a/docs/peers.md b/docs/peers.md
index 991bb40..5f5cfc9 100644
--- a/docs/peers.md
+++ b/docs/peers.md
@@ -178,7 +178,8 @@ as it knows them; of those, the ones it holds, and for any it lacks, the heads t
 share). The peer fetches them as a blob, in chunks (each deflated where that makes it
 at least a tenth smaller), from the sender and any other peer holding both sets of
 heads (every such peer produces the same bytes), loads them,
-and syncs on; the rest goes in ordinary messages. Until the peer answers, the sender
+and syncs on; the rest goes in ordinary messages. The offer is the sender's answer to
+what the peer sent: the peer waits for no other while it downloads. Until the peer answers, the sender
 sends it nothing; if the answer is overdue, it offers again (the same offer, without
 generating the message again, while neither side's heads moved). This is how a join without a
 recent remake, or a return after much was written, gets the bulk of its changes. A node
diff --git a/src/document/sync.rs b/src/document/sync.rs
index 7695b82..c703300 100644
--- a/src/document/sync.rs
+++ b/src/document/sync.rs
@@ -870,6 +870,16 @@ impl SyncSession {
         self.reply_backoff.reset();
     }
 
+    /// The peer answered our message with a delta offer (`SyncBody::Delta`): the wait
+    /// ends, and to Automerge no message of ours is in flight any more, as an answer of
+    /// its own would have told it. Else, with our heads unchanged (an offer we don't
+    /// fetch, or a download that fails), it would say nothing again, and the peer would
+    /// never hear what we have to offer afresh.
+    pub(crate) fn answered_by_offer(&mut self) {
+        self.reply_settled();
+        self.state.in_flight = false;
+    }
+
     /// If the answer is overdue: forgets that the unanswered changes were sent and that
     /// a message is in flight, so the next sync pass sends them again. Returns how many
     /// changes that concerns.
diff --git a/src/swarm/sync.rs b/src/swarm/sync.rs
index e551cb8..0edc277 100644
--- a/src/swarm/sync.rs
+++ b/src/swarm/sync.rs
@@ -487,6 +487,17 @@ pub(crate) async fn handle_sync(
             )
         }
         transport::SyncBody::Delta(spec) => {
+            // An offer answers what we sent as much as a message would: the wait for an
+            // answer ends here, not when the download does (its reply timeout would
+            // otherwise count against a peer that answered, suspect its path and give it
+            // up as the source to catch up from). What it offers is what it has.
+            {
+                let mut peer_state = peer_state_mutex.lock().await;
+                peer_state.reports_ready = Some(from_ready);
+                peer_state.last_heads = Some((epoch, transport::HeadsSummary::of(spec.to.clone())));
+                peer_state.last_sync_rx = Some(Instant::now());
+                peer_state.sync.answered_by_offer();
+            }
             fetch_delta(state, peer, epoch, spec);
             return Ok(());
         }