7 files changed, 59 insertions, 7 deletions

diff --git a/src/swarm/peer/handshake.rs b/src/swarm/peer/handshake.rs
index c46df9c..038dac6 100644
--- a/src/swarm/peer/handshake.rs
+++ b/src/swarm/peer/handshake.rs
@@ -13,6 +13,12 @@ use crate::swarm::sync::SyncEvent;
 use crate::swarm::topology;
 use crate::transport::{self, ParsedMessage};
 
+/// Most peers a HELLO_REPLY lists, and most we take from one: each listed peer may get
+/// a HELLO, so a long list (a hub's hundreds of neighbours, or a peer making up keys)
+/// costs the recipient as much as it is long. A joining node needs only a few to find
+/// the swarm; the member list gives it the rest once it is ready.
+const MAX_LISTED_PEERS: usize = 64;
+
 pub(crate) async fn remember_peer(state: &AppState, peer: &str) {
     if peer.is_empty() {
         warn!("Ignoring empty peer public key");
@@ -183,6 +189,12 @@ pub(super) async fn send_hello_reply(
             peers.push(known);
         }
     }
+    // Those the recipient would most likely pick as neighbours itself (lowest pair
+    // score with it, as `topology` ranks them), if there are too many.
+    if peers.len() > MAX_LISTED_PEERS {
+        peers.sort_by_cached_key(|known| topology::pair_score(peer, known));
+        peers.truncate(MAX_LISTED_PEERS);
+    }
 
     let auth = state
         .swarm_key
@@ -215,18 +227,31 @@ pub(super) async fn send_hello_reply(
     Ok(())
 }
 
+/// Greets the peers a HELLO_REPLY listed: at most `MAX_LISTED_PEERS`, only valid
+/// identities, and, once the member list is known, only those that are our neighbours
+/// (it says whom we greet; a listed peer that isn't one would get nothing from us, so
+/// it isn't remembered either).
 pub(super) async fn discover_peers(state: &AppState, from_peer: &str, advertised_peers: &[String]) {
-    // TODO here we could add a trace t oshwo advertised_peers
+    let view_known = topology::view_known(state);
+    let mut taken = 0;
     for discovered_peer in advertised_peers {
-        if discovered_peer == from_peer {
+        if discovered_peer == from_peer || !state.transport.is_peer_id(discovered_peer) {
             continue;
         }
-
-        remember_peer(state, discovered_peer).await;
-
-        if !topology::is_neighbour(state, discovered_peer) {
+        if view_known && !topology::is_neighbour(state, discovered_peer) {
             continue;
         }
+        if taken == MAX_LISTED_PEERS {
+            debug!(
+                from = %from_peer,
+                listed = advertised_peers.len(),
+                "HELLO_REPLY listed more peers than we take; ignoring the rest"
+            );
+            break;
+        }
+        taken += 1;
+
+        remember_peer(state, discovered_peer).await;
         if let Err(error) = send_hello_once(state, discovered_peer).await {
             warn!(
                 peer = %discovered_peer,
diff --git a/src/swarm/topology.rs b/src/swarm/topology.rs
index 73c2f2c..e396a9d 100644
--- a/src/swarm/topology.rs
+++ b/src/swarm/topology.rs
@@ -39,6 +39,12 @@ pub(crate) struct NeighbourView {
     full_mesh: bool,
 }
 
+/// Whether the neighbours are computed from the member list yet (before, everyone
+/// counts as a neighbour).
+pub(crate) fn view_known(state: &AppState) -> bool {
+    state.neighbours.read_or_recover().is_some()
+}
+
 /// Whether every reachable member is our neighbour (also while the view is unknown).
 pub(crate) fn is_full_mesh(state: &AppState) -> bool {
     state
@@ -337,7 +343,7 @@ pub(crate) fn graph(
 }
 
 /// Symmetric: the same for (a, b) and (b, a).
-fn pair_score(a: &str, b: &str) -> [u8; 32] {
+pub(crate) fn pair_score(a: &str, b: &str) -> [u8; 32] {
     let (low, high) = if a <= b { (a, b) } else { (b, a) };
     hash(PAIR_LABEL, &[low, high])
 }
diff --git a/src/transport/firewall.rs b/src/transport/firewall.rs
index c8592bd..23d91fe 100644
--- a/src/transport/firewall.rs
+++ b/src/transport/firewall.rs
@@ -65,6 +65,10 @@ impl Transport for FirewallTransport {
         self.inner.local_id()
     }
 
+    fn is_peer_id(&self, id: &str) -> bool {
+        self.inner.is_peer_id(id)
+    }
+
     async fn resolve_seed(&self, seed: &str) -> Result<String> {
         self.inner.resolve_seed(seed).await
     }
diff --git a/src/transport/mod.rs b/src/transport/mod.rs
index 14c5ee4..b2ce63f 100644
--- a/src/transport/mod.rs
+++ b/src/transport/mod.rs
@@ -323,6 +323,10 @@ pub trait Transport: Send + Sync {
     /// This node's identity on the transport (for Mycelium: its public key).
     fn local_id(&self) -> &str;
 
+    /// Whether `id` can be a node's identity on this transport (for Mycelium: a public
+    /// key). Identities other peers name to us (in a HELLO_REPLY) are checked with it.
+    fn is_peer_id(&self, id: &str) -> bool;
+
     /// Resolves a seed (anything that leads to a node, e.g. an address) to that node's
     /// identity, as `local_id` gives it on that node.
     async fn resolve_seed(&self, seed: &str) -> Result<String>;
diff --git a/src/transport/mycelium/mod.rs b/src/transport/mycelium/mod.rs
index cbae567..0b74793 100644
--- a/src/transport/mycelium/mod.rs
+++ b/src/transport/mycelium/mod.rs
@@ -168,6 +168,10 @@ impl Transport for MyceliumTransport {
         &self.local_id
     }
 
+    fn is_peer_id(&self, id: &str) -> bool {
+        is_pubkey(id)
+    }
+
     async fn resolve_seed(&self, hint: &str) -> Result<String> {
         if is_pubkey(hint) {
             return Ok(hint.to_owned());
diff --git a/src/transport/sim.rs b/src/transport/sim.rs
index 675f382..05160f8 100644
--- a/src/transport/sim.rs
+++ b/src/transport/sim.rs
@@ -255,6 +255,11 @@ impl Transport for SimTransport {
         &self.id
     }
 
+    /// Any name will do.
+    fn is_peer_id(&self, id: &str) -> bool {
+        !id.is_empty()
+    }
+
     /// A seed is a node ID; one no node has doesn't resolve (yet).
     async fn resolve_seed(&self, seed: &str) -> Result<String> {
         if self.net.is_known(seed) {
diff --git a/src/transport/traffic.rs b/src/transport/traffic.rs
index 3d17860..78c5fee 100644
--- a/src/transport/traffic.rs
+++ b/src/transport/traffic.rs
@@ -138,6 +138,10 @@ impl Transport for CountingTransport {
         self.inner.local_id()
     }
 
+    fn is_peer_id(&self, id: &str) -> bool {
+        self.inner.is_peer_id(id)
+    }
+
     async fn resolve_seed(&self, seed: &str) -> Result<String> {
         self.inner.resolve_seed(seed).await
     }