Merge branch 'maint' into master

Forward-merge three maint fixes, hand-relocated into master's
post-sans-IO / discovery-to-lookup structure:
- drop the redundant TreeMetrics parent_switched counter (keep
  parent_switches), reconciled across the refactored tree/mmp sites and
  the spanning-tree test reads
- keep the tighter path_mtu when applying a LookupResponse, now in
  handlers/lookup.rs after the discovery module rename
- reuse one shared secp256k1 context in the identity module

src/node/handlers/lookup.rs

@@ -283,17 +283,33 @@ impl Node {
                     // map used by the TUN reader/writer at TCP MSS clamp time.
                     let fips_addr = crate::FipsAddress::from_node_addr(&target);
                     match self.path_mtu_lookup.write() {
-                        Ok(mut map) => {
-                            let prior = map.insert(fips_addr, path_mtu);
-                            debug!(
-                                target = %self.peer_display_name(&target),
-                                fips_addr = %fips_addr,
-                                path_mtu = path_mtu,
-                                prior = ?prior,
-                                map_len = map.len(),
-                                "Wrote path_mtu_lookup from discovery LookupResponse"
-                            );
-                        }
+                        Ok(mut map) => match map.get(&fips_addr).copied() {
+                            Some(existing) if existing <= path_mtu => {
+                                // Keep the tighter learned value; never loosen
+                                // the clamp. A reactive MtuExceeded or
+                                // PathMtuNotification tighten takes precedence
+                                // over a looser discovery estimate
+                                // (cross-carrier keep-tighter).
+                                debug!(
+                                    target = %self.peer_display_name(&target),
+                                    fips_addr = %fips_addr,
+                                    path_mtu = path_mtu,
+                                    existing = existing,
+                                    "LookupResponse: keeping tighter existing path_mtu_lookup value"
+                                );
+                            }
+                            other => {
+                                map.insert(fips_addr, path_mtu);
+                                debug!(
+                                    target = %self.peer_display_name(&target),
+                                    fips_addr = %fips_addr,
+                                    path_mtu = path_mtu,
+                                    prior = ?other,
+                                    map_len = map.len(),
+                                    "Wrote path_mtu_lookup from discovery LookupResponse"
+                                );
+                            }
+                        },
                         Err(e) => {
                             warn!(
                                 target = %self.peer_display_name(&target),

src/node/handlers/mmp.rs

@@ -233,7 +233,6 @@ impl Node {
                 self.coord_cache
                     .invalidate_via_node(our_identity.node_addr());
                 self.reset_lookup_backoff();
-                self.metrics().tree.parent_switched.inc();
                 self.metrics().tree.parent_switches.inc();
                 info!(
                     new_parent = %self.peer_display_name(&new_parent),
@@ -267,7 +266,6 @@ impl Node {
                 self.coord_cache
                     .invalidate_other_roots(our_identity.node_addr());
                 self.reset_lookup_backoff();
-                self.metrics().tree.parent_switched.inc();
                 self.metrics().tree.parent_switches.inc();
                 info!(
                     new_root = %self.tree_state.root(),

src/node/metrics.rs

@@ -284,7 +284,6 @@ pub struct TreeMetrics {
     pub stale: Counter,
     pub ancestry_invalid: Counter,
     pub accepted: Counter,
-    pub parent_switched: Counter,
     pub loop_detected: Counter,
     pub ancestry_changed: Counter,
     pub sent: Counter,
@@ -318,7 +317,6 @@ impl TreeMetrics {
             stale: self.stale.get(),
             ancestry_invalid: self.ancestry_invalid.get(),
             accepted: self.accepted.get(),
-            parent_switched: self.parent_switched.get(),
             loop_detected: self.loop_detected.get(),
             ancestry_changed: self.ancestry_changed.get(),
             sent: self.sent.get(),

src/node/stats.rs

@@ -273,7 +273,6 @@ pub struct TreeStatsSnapshot {
     pub stale: u64,
     pub ancestry_invalid: u64,
     pub accepted: u64,
-    pub parent_switched: u64,
     pub loop_detected: u64,
     pub ancestry_changed: u64,
     pub sent: u64,

src/node/tests/discovery.rs

@@ -914,6 +914,45 @@ async fn test_originator_stores_path_mtu_in_cache() {
     );
 }
 
+#[tokio::test]
+async fn test_originator_lookup_response_keeps_tighter_path_mtu_lookup() {
+    // Regression: a LookupResponse carrying a looser (larger) path_mtu must
+    // NOT clobber a tighter (smaller) value already in path_mtu_lookup that a
+    // reactive MtuExceeded or PathMtuNotification learned. Cross-carrier
+    // keep-tighter: the clamp must never loosen.
+    let mut node = make_node();
+    let from = make_node_addr(0xAA);
+
+    let target_identity = Identity::generate();
+    let target = *target_identity.node_addr();
+    let root = make_node_addr(0xF0);
+    let coords = TreeCoordinate::from_addrs(vec![target, root]).unwrap();
+
+    node.register_identity(target, target_identity.pubkey_full());
+
+    // Pre-seed a tighter value, as if a reactive signal already narrowed it.
+    let target_fips = crate::FipsAddress::from_node_addr(&target);
+    node.path_mtu_lookup_insert(target_fips, 1280);
+
+    let proof_data = LookupResponse::proof_bytes(800, &target, &coords);
+    let proof = target_identity.sign(&proof_data);
+
+    let mut response = LookupResponse::new(800, target, coords.clone(), proof);
+    // Looser discovery estimate that must be rejected in favor of the tighter
+    // existing entry.
+    response.path_mtu = 1500;
+
+    let payload = &response.encode()[1..];
+
+    node.handle_lookup_response(&from, payload).await;
+
+    assert_eq!(
+        node.path_mtu_lookup_get(&target_fips),
+        Some(1280),
+        "LookupResponse must not loosen a tighter existing path_mtu_lookup value"
+    );
+}
+
 // ============================================================================
 // Open-Discovery Sweep — cache-injection unit test
 // ============================================================================

src/node/tests/spanning_tree.rs

@@ -1228,7 +1228,7 @@ async fn test_tree_announce_self_root_promotion() {
     let announce = TreeAnnounce::new(decl, TreeCoordinate::from_addrs(vec![l_addr]).unwrap());
     let encoded = announce.encode().unwrap();
 
-    let switched_before = nodes[p_idx].node.metrics().tree.parent_switched.get();
+    let switched_before = nodes[p_idx].node.metrics().tree.parent_switches.get();
     nodes[p_idx]
         .node
         .handle_tree_announce(&l_addr, &encoded[1..])
@@ -1240,7 +1240,7 @@ async fn test_tree_announce_self_root_promotion() {
     );
     assert_eq!(*nodes[p_idx].node.tree_state().root(), p_addr);
     assert_eq!(
-        nodes[p_idx].node.metrics().tree.parent_switched.get(),
+        nodes[p_idx].node.metrics().tree.parent_switches.get(),
         switched_before + 1
     );
 

src/node/tree.rs

@@ -373,7 +373,6 @@ impl Node {
                     .invalidate_via_node(our_identity.node_addr());
                 self.reset_lookup_backoff();
 
-                self.metrics().tree.parent_switched.inc();
                 self.metrics().tree.parent_switches.inc();
 
                 info!(
@@ -418,7 +417,6 @@ impl Node {
                 self.coord_cache
                     .invalidate_other_roots(our_identity.node_addr());
                 self.reset_lookup_backoff();
-                self.metrics().tree.parent_switched.inc();
                 self.metrics().tree.parent_switches.inc();
                 info!(
                     new_root = %self.tree_state.root(),
@@ -627,7 +625,6 @@ impl Node {
                     .invalidate_via_node(our_identity.node_addr());
                 self.reset_lookup_backoff();
 
-                self.metrics().tree.parent_switched.inc();
                 self.metrics().tree.parent_switches.inc();
 
                 info!(
@@ -670,7 +667,6 @@ impl Node {
                 self.coord_cache
                     .invalidate_other_roots(our_identity.node_addr());
                 self.reset_lookup_backoff();
-                self.metrics().tree.parent_switched.inc();
                 self.metrics().tree.parent_switches.inc();
                 info!(
                     new_root = %self.tree_state.root(),