4 files changed, 55 insertions, 0 deletions
diff --git a/src/simtest/identity.rs b/src/simtest/identity.rs
index 7379048..154c72e 100644
--- a/src/simtest/identity.rs
+++ b/src/simtest/identity.rs
@@ -112,6 +112,7 @@ fn status(node: &AppState) -> ParsedMessage {
         leaving: false,
         announcing: false,
         eager: false,
+        reaches: None,
     })
 }
 
diff --git a/src/simtest/mod.rs b/src/simtest/mod.rs
index dc23440..e9d8c15 100644
--- a/src/simtest/mod.rs
+++ b/src/simtest/mod.rs
@@ -13,6 +13,7 @@ mod bench;
 mod deltas;
 mod identity;
 mod links;
+mod reach;
 mod relay;
 mod remakes;
 mod soak;
diff --git a/src/simtest/reach.rs b/src/simtest/reach.rs
new file mode 100644
index 0000000..b38db87
--- /dev/null
+++ b/src/simtest/reach.rs
@@ -0,0 +1,52 @@
+//! Whom neighbours reach (`swarm::reach`): told in STATUSes, as measured.
+
+use std::time::Duration;
+
+use super::Sim;
+use crate::state::AppState;
+use crate::swarm::bootstrap::Start;
+use crate::swarm::{reach, topology};
+
+const SOON: Duration = Duration::from_secs(30);
+
+/// Whether `node` reaches `member`, as `node` measured and told it.
+fn reaches(node: &AppState, member: &AppState) -> bool {
+    reach::published(node).contains(&topology::pair_key(member.local_id()))
+}
+
+/// Whether `node` heard from `neighbour` that it reaches `member`.
+async fn heard(node: &AppState, neighbour: &AppState, member: &AppState) -> bool {
+    let Some(peer_state) = node.get_peer_state(neighbour.local_id()).await else {
+        return false;
+    };
+    let said = peer_state.lock().await.reaches.clone();
+    reach::of(node, said.as_ref())
+        .is_some_and(|keys| keys.contains(&topology::pair_key(member.local_id())))
+}
+
+/// x, b and c on one net, all members of one full mesh, but nothing passes between x
+/// and c: b reaches both and says so; c learns from b that b reaches x, which c itself
+/// doesn't.
+#[tokio::test(flavor = "multi_thread", worker_threads = 4)]
+async fn neighbours_tell_whom_they_reach() {
+    let sim = Sim::with_identity(66);
+    let cut = crate::transport::sim::LinkParams {
+        loss: 1.0,
+        ..Default::default()
+    };
+    sim.net.set_link("x", "c", cut.clone());
+    sim.net.set_link("c", "x", cut);
+    let b = sim.start_on("b", &["sim"], &[], Start::Genesis);
+    let c = sim.start_on("c", &["sim"], &["b"], Start::Join);
+    let x = sim.start_on("x", &["sim"], &["b"], Start::Join);
+    sim.wait_ready(&[&b, &c, &x], SOON).await;
+    let (b, c, x) = (&b, &c, &x);
+    sim.wait_until(
+        "c heard from b that b reaches x, and c doesn't reach x",
+        SOON,
+        move || async move {
+            reaches(b, x) && reaches(b, c) && !reaches(c, x) && heard(c, b, x).await
+        },
+    )
+    .await;
+}
diff --git a/src/simtest/timing.rs b/src/simtest/timing.rs
index ea59689..d06e992 100644
--- a/src/simtest/timing.rs
+++ b/src/simtest/timing.rs
@@ -337,6 +337,7 @@ async fn late_epoch_does_not_flip_a_peers_epoch() {
             leaving: false,
             announcing: false,
             eager: false,
+            reaches: None,
         })
     };
     let a = &a;