refactor(network): turn the AddressBook/CandidateSet into tower services

zebra-network/CHANGELOG.md

@@ -7,6 +7,23 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- The address book updater is now a `Buffer`-wrapped tower service that owns the address book
+  write path. Candidate selection and cache reads use its single ordered request queue instead
+  of locking the shared mutex, making pick-and-mark-`AttemptPending` atomic. Hot reads and
+  `init()` are unchanged, and there is no network-visible behavior change
+  ([#1976](https://github.com/ZcashFoundation/zebra/issues/1976)).
+- The manual `min_next_handshake`/`min_next_crawl` rate-limit timers in the candidate set are
+  replaced by small tower middlewares: outbound connection pacing is only charged when a
+  candidate is actually yielded, and crawls are still skipped while rate-limited. The intervals
+  and the existing rate-limit test assertions are unchanged
+  ([#1976](https://github.com/ZcashFoundation/zebra/issues/1976)).
+- The internal `CandidateSet` struct is dissolved into the peer crawler: candidate selection and
+  crawling are now plain cloneable service handles with shared rate limits, removing the async
+  mutex that used to serialize them
+  ([#1976](https://github.com/ZcashFoundation/zebra/issues/1976)).
+
 ## [12.0.0] - 2026-08-10
 
 ### Breaking Changes

zebra-network/proptest-regressions/peer_set/candidate_set/tests/prop.txt

@@ -6,3 +6,4 @@
 # everyone who runs the test benefits from these saved cases.
 cc e27ea62986a1ace4e16dde4b99d8ae8557a6b1e92e3f9a7c6aba0d7b179eb817 # shrinks to peers = [MetaAddr { addr: 0.0.0.0:0, services: NODE_NETWORK, last_seen: DateTime32 { timestamp: 1623572152, calendar: 2021-06-13T08:15:52Z }, last_connection_state: NeverAttemptedAlternate }, MetaAddr { addr: 0.0.0.0:0, services: NODE_NETWORK, last_seen: DateTime32 { timestamp: 1623572152, calendar: 2021-06-13T08:15:52Z }, last_connection_state: NeverAttemptedAlternate }, MetaAddr { addr: 0.0.0.0:1151, services: NODE_NETWORK, last_seen: DateTime32 { timestamp: 1623572152, calendar: 2021-06-13T08:15:52Z }, last_connection_state: NeverAttemptedAlternate }, MetaAddr { addr: 129.65.108.101:9019, services: NODE_NETWORK, last_seen: DateTime32 { timestamp: 1623572152, calendar: 2021-06-13T08:15:52Z }, last_connection_state: NeverAttemptedAlternate }, MetaAddr { addr: 95.68.199.21:46887, services: NODE_NETWORK, last_seen: DateTime32 { timestamp: 1623572152, calendar: 2021-06-13T08:15:52Z }, last_connection_state: NeverAttemptedAlternate }, MetaAddr { addr: 167.206.253.131:43927, services: NODE_NETWORK, last_seen: DateTime32 { timestamp: 1623572152, calendar: 2021-06-13T08:15:52Z }, last_connection_state: NeverAttemptedAlternate }, MetaAddr { addr: 160.138.155.239:63430, services: NODE_NETWORK, last_seen: DateTime32 { timestamp: 1623572152, calendar: 2021-06-13T08:15:52Z }, last_connection_state: NeverAttemptedAlternate }, MetaAddr { addr: [6a4e:a543:8c93:e074:8a2a:5cc4:96f8:ce95]:51071, services: NODE_NETWORK, last_seen: DateTime32 { timestamp: 1623572152, calendar: 2021-06-13T08:15:52Z }, last_connection_state: NeverAttemptedAlternate }], initial_candidates = 0, extra_candidates = 2
 cc 788fe2c47a4af559e9a921764d1ce8fcfd0743361c19e6e7a06adc093bc19078 # shrinks to peers = [MetaAddr { addr: [::e9:8050:dbf7:5665]:27188, services: NODE_NETWORK, last_seen: DateTime32 { timestamp: 1623573029, calendar: 2021-06-13T08:30:29Z }, last_connection_state: NeverAttemptedAlternate }, MetaAddr { addr: 106.27.232.177:51742, services: NODE_NETWORK, last_seen: DateTime32 { timestamp: 1623573029, calendar: 2021-06-13T08:30:29Z }, last_connection_state: NeverAttemptedAlternate }, MetaAddr { addr: 127.0.0.1:30827, services: NODE_NETWORK, last_seen: DateTime32 { timestamp: 1623573029, calendar: 2021-06-13T08:30:29Z }, last_connection_state: NeverAttemptedAlternate }, MetaAddr { addr: 0.0.0.0:21948, services: NODE_NETWORK, last_seen: DateTime32 { timestamp: 1623573029, calendar: 2021-06-13T08:30:29Z }, last_connection_state: NeverAttemptedAlternate }, MetaAddr { addr: 0.0.0.0:38186, services: NODE_NETWORK, last_seen: DateTime32 { timestamp: 1623573029, calendar: 2021-06-13T08:30:29Z }, last_connection_state: NeverAttemptedAlternate }, MetaAddr { addr: [4db2:96e6:38c5:fd62:e964:9338:4ce8:bfa0]:32686, services: NODE_NETWORK, last_seen: DateTime32 { timestamp: 1623573029, calendar: 2021-06-13T08:30:29Z }, last_connection_state: NeverAttemptedAlternate }, MetaAddr { addr: 0.0.0.0:22952, services: NODE_NETWORK, last_seen: DateTime32 { timestamp: 1623573029, calendar: 2021-06-13T08:30:29Z }, last_connection_state: NeverAttemptedAlternate }, MetaAddr { addr: 114.164.73.233:10203, services: NODE_NETWORK, last_seen: DateTime32 { timestamp: 1623573029, calendar: 2021-06-13T08:30:29Z }, last_connection_state: NeverAttemptedAlternate }], initial_candidates = 3, extra_candidates = 3
+cc b0ede440aa0549b3de2694c7e4c66cc61b2035ec4a4a707db9acc34a5482be99 # shrinks to peers = {127.0.0.1: 12457, ::ffff:127.0.0.1: 36973, 16.62.55.176: 2464, ::ffff:43.208.158.66: 38413, 18.125.234.138: 31209, f007:edbb:3024:14e5:d305:5a0e:c970:b82c: 2775, 83.130.58.195: 2456, 117.237.203.222: 55214}, initial_candidates = 3, extra_candidates = 3

zebra-network/src/address_book.rs

@@ -413,8 +413,25 @@ impl AddressBook {
     /// As an exception, this function can ignore all changes for specific
     /// [`PeerSocketAddr`]s. Ignored addresses will never be used to connect to
     /// peers.
-    #[allow(clippy::unwrap_in_result)]
     pub fn update(&mut self, change: MetaAddrChange) -> Option<MetaAddr> {
+        let updated = self.update_without_metrics(change);
+
+        if updated.is_some() {
+            self.update_metrics(Instant::now(), Utc::now());
+        }
+
+        updated
+    }
+
+    /// Applies `change`, without refreshing the address metrics.
+    ///
+    /// # Correctness
+    ///
+    /// Callers must call [`AddressBook::update_metrics`] once they have finished applying
+    /// changes, and only once: a refresh walks the address book several times, under the
+    /// mutex, so refreshing per change makes a batch quadratic.
+    #[allow(clippy::unwrap_in_result)]
+    fn update_without_metrics(&mut self, change: MetaAddrChange) -> Option<MetaAddr> {
         if self.bans_by_ip.contains_key(&change.addr().ip()) {
             // Remote peers control how often this fires, so keep it below `warn` (#11134).
             tracing::debug!(
@@ -554,9 +571,6 @@ impl AddressBook {
             }
 
             assert!(self.len() <= self.addr_limit);
-
-            std::mem::drop(_guard);
-            self.update_metrics(instant_now, chrono_now);
         }
 
         updated
@@ -858,8 +872,14 @@ impl Extend<MetaAddrChange> for AddressBook {
     where
         T: IntoIterator<Item = MetaAddrChange>,
     {
+        let mut any_updated = false;
+
         for change in iter.into_iter() {
-            self.update(change);
+            any_updated |= self.update_without_metrics(change).is_some();
+        }
+
+        if any_updated {
+            self.update_metrics(Instant::now(), Utc::now());
         }
     }
 }