node: introduce Reloadable trait and migrate host map to a lock-free snapshot

Add a `Reloadable` trait that normalizes the node's reloadable
configuration/resource pattern onto a single contract built around an
`arc_swap::ArcSwap` snapshot: a lock-free `load()` for the hot read path
and an async `reload()` that re-reads the backing source and atomically
swaps in a fresh snapshot. The trait carries the canonical Arc-wrapper
template documentation (single-writer node tick, many-reader hot path,
whole-snapshot swap so readers never observe a partial update).

Migrate the host map to this trait via a new `HostMapReloadable` that
reuses the existing load/merge/mtime helpers in upper::hosts. The Node
`host_map` field changes from `Arc<HostMap>` to `HostMapReloadable`, and
`peer_display_name` reads through a lock-free guard. The initial snapshot
is byte-identical to the previous construction, so behavior is unchanged.

The host map is still snapshotted once at construction and not polled;
`reload()` is exercised only by unit tests for now. Wiring the periodic
poll into the node tick, and deduplicating the hosts-file stat against
the ACL reloader's embedded copy, is left as a follow-up.

Add `arc-swap` as a dependency. Unit tests cover initial load (base +
file, base only), change/no-change/deletion/creation detection,
base-preserved-on-reload, and equivalence of the initial snapshot to the
pre-migration construction.

Cargo.lock

@@ -98,6 +98,15 @@ version = "1.0.102"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7f202df86484c868dbad7eaa557ef785d5c66295e41b460ef922eca0723b842c"
 
+[[package]]
+name = "arc-swap"
+version = "1.9.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6a3a1fd6f75306b68087b831f025c712524bcb19aad54e557b1129cfa0a2b207"
+dependencies = [
+ "rustversion",
+]
+
 [[package]]
 name = "arrayvec"
 version = "0.7.6"
@@ -1063,6 +1072,7 @@ checksum = "9844ddc3a6e533d62bba727eb6c28b5d360921d5175e9ff0f1e621a5c590a4d5"
 name = "fips"
 version = "0.4.0-dev"
 dependencies = [
+ "arc-swap",
  "bech32",
  "bluer",
  "clap",

Cargo.toml

@@ -35,6 +35,7 @@ portable-atomic = { version = "1", features = ["std"] }
 
 nostr = { version = "0.44", features = ["std", "nip59"] }
 nostr-sdk = "0.44"
+arc-swap = "1"
 
 [target.'cfg(unix)'.dependencies]
 tun = { version = "0.8.7", features = ["async"] }

src/node/mod.rs

@@ -15,6 +15,7 @@ mod handlers;
 mod lifecycle;
 mod rate_limit;
 pub(crate) mod reject;
+mod reloadable;
 mod retry;
 mod routing_error_rate_limit;
 pub(crate) mod session;
@@ -28,6 +29,7 @@ pub(crate) mod wire;
 
 use self::discovery_rate_limit::{DiscoveryBackoff, DiscoveryForwardRateLimiter};
 use self::rate_limit::HandshakeRateLimiter;
+use self::reloadable::Reloadable;
 use self::routing_error_rate_limit::RoutingErrorRateLimiter;
 
 /// Half-range of the symmetric jitter applied to the per-session rekey timer.
@@ -501,8 +503,9 @@ pub struct Node {
 
     // === Host Map ===
     /// Static hostname → npub mapping for DNS resolution.
-    /// Built at construction from peer aliases and /etc/fips/hosts.
+    /// Built at construction from peer aliases and /etc/fips/hosts, and
-    host_map: Arc<HostMap>,
+    /// published through a lock-free snapshot for the display path.
+    host_map: reloadable::HostMapReloadable,
 
     /// Off-task FMP-encrypt + UDP-send worker pool. Unix-only —
     /// the worker issues direct sendmmsg(2) / sendmsg+UDP_GSO calls
@@ -595,13 +598,9 @@ impl Node {
         let forward_min_interval_secs = config.node.discovery.forward_min_interval_secs;
 
         let base_host_map = HostMap::from_peer_configs(config.peers());
-        let mut host_map = base_host_map.clone();
         let hosts_path = std::path::PathBuf::from(crate::upper::hosts::DEFAULT_HOSTS_PATH);
-        let hosts_file = HostMap::load_hosts_file(std::path::Path::new(
+        let host_map =
-            crate::upper::hosts::DEFAULT_HOSTS_PATH,
+            reloadable::HostMapReloadable::new(base_host_map.clone(), hosts_path.clone());
-        ));
-        host_map.merge(hosts_file);
-        let host_map = Arc::new(host_map);
         let peer_acl = acl::PeerAclReloader::with_alias_sources(
             std::path::PathBuf::from(acl::DEFAULT_PEERS_ALLOW_PATH),
             std::path::PathBuf::from(acl::DEFAULT_PEERS_DENY_PATH),
@@ -744,16 +743,14 @@ impl Node {
         let coords_response_interval_ms = config.node.session.coords_response_interval_ms;
 
         let base_host_map = HostMap::from_peer_configs(config.peers());
-        let mut host_map = base_host_map.clone();
+        let hosts_path = std::path::PathBuf::from(crate::upper::hosts::DEFAULT_HOSTS_PATH);
-        host_map.merge(HostMap::load_hosts_file(std::path::Path::new(
+        let host_map =
-            crate::upper::hosts::DEFAULT_HOSTS_PATH,
+            reloadable::HostMapReloadable::new(base_host_map.clone(), hosts_path.clone());
-        )));
-        let host_map = Arc::new(host_map);
         let peer_acl = acl::PeerAclReloader::with_alias_sources(
             std::path::PathBuf::from(acl::DEFAULT_PEERS_ALLOW_PATH),
             std::path::PathBuf::from(acl::DEFAULT_PEERS_DENY_PATH),
             base_host_map,
-            std::path::PathBuf::from(crate::upper::hosts::DEFAULT_HOSTS_PATH),
+            hosts_path,
         );
 
         #[cfg(unix)]
@@ -1084,7 +1081,8 @@ impl Node {
     /// 4. Session endpoint's short npub (end-to-end, may not be direct peer)
     /// 5. Truncated NodeAddr hex (unknown address)
     pub(crate) fn peer_display_name(&self, addr: &NodeAddr) -> String {
-        if let Some(hostname) = self.host_map.lookup_hostname(addr) {
+        let hosts = self.host_map.load();
+        if let Some(hostname) = hosts.lookup_hostname(addr) {
             return hostname.to_string();
         }
         if let Some(name) = self.peer_aliases.get(addr) {

src/node/reloadable.rs

@@ -0,0 +1,308 @@
+//! Lock-free reloadable configuration / resource snapshots.
+//!
+//! Several node-owned resources are loaded from disk at startup and may be
+//! re-read when the backing file changes (for example the `/etc/fips/hosts`
+//! map). Historically each one carried its own ad-hoc reloader with a
+//! slightly different shape. The [`Reloadable`] trait normalizes them onto a
+//! single contract built around an [`arc_swap::ArcSwap`] snapshot.
+//!
+//! # Canonical Arc-wrapper template
+//!
+//! These resources follow a single-writer / many-reader pattern: the node
+//! tick task is the only writer, while the hot path reads the current value
+//! frequently and must never block.
+//!
+//! - The reader-facing immutable snapshot lives in an
+//!   [`arc_swap::ArcSwap<T>`]. Readers call [`Reloadable::load`], which yields
+//!   a lock-free [`arc_swap::Guard<Arc<T>>`] that derefs straight to the
+//!   snapshot — no mutex, no clone on the read path.
+//! - The owning struct also holds the change-detection state (file mtime,
+//!   immutable base data, source path). That state is touched only by
+//!   [`Reloadable::reload`], which runs on the single writer task.
+//! - `reload` builds a brand-new `T` and then stores `Arc::new(new)` into the
+//!   `ArcSwap`, so a reader either sees the entire old snapshot or the entire
+//!   new one — never a partial update.
+//! - Construction performs the initial synchronous load so the snapshot is
+//!   valid before the node starts serving reads.
+//!
+//! `reload` returns `bool` rather than a value or `Result`: the underlying
+//! loaders already absorb I/O errors internally (a missing or unreadable
+//! source degrades to an empty/base snapshot plus a warning), so callers have
+//! nothing to handle. The return flag reports only whether the snapshot was
+//! replaced, which is all the tick loop needs for logging.
+
+use std::sync::Arc;
+
+use crate::upper::hosts::{HostMap, file_mtime};
+
+/// A resource backed by a lock-free [`arc_swap::ArcSwap`] snapshot that can be
+/// re-read from its source on demand.
+///
+/// See the [module documentation](self) for the canonical Arc-wrapper
+/// template that implementors follow.
+pub trait Reloadable: Send {
+    /// The immutable snapshot type readers observe.
+    type Snapshot;
+
+    /// Re-read the backing source and replace the snapshot if it changed.
+    ///
+    /// Returns `true` if a new snapshot was stored, `false` if nothing
+    /// changed. I/O errors are absorbed internally (degrading to an
+    /// empty/base snapshot with a warning) rather than surfaced.
+    ///
+    /// Not yet driven from the node tick — the host map snapshot is still
+    /// taken once at construction and not polled. Wiring the periodic poll
+    /// into the tick is a follow-up.
+    #[cfg_attr(not(test), allow(dead_code))]
+    async fn reload(&mut self) -> bool;
+
+    /// Acquire a lock-free guard over the current snapshot.
+    ///
+    /// This is the hot-path read: it performs no locking and no allocation.
+    fn load(&self) -> arc_swap::Guard<Arc<Self::Snapshot>>;
+}
+
+/// Reloadable hostname → npub map (base peer aliases merged with the operator
+/// hosts file).
+///
+/// Holds the immutable base map (from peer-config aliases) plus the
+/// change-detection state for the hosts file. The effective map (base merged
+/// with the hosts file) is published through an [`arc_swap::ArcSwap`] so the
+/// display path can read it without locking.
+pub struct HostMapReloadable {
+    /// Reader-facing effective snapshot (base merged with hosts file).
+    snapshot: arc_swap::ArcSwap<HostMap>,
+    /// Base map from peer-config aliases (never changes). Read only by
+    /// `reload`, which is not yet driven from the node tick.
+    #[cfg_attr(not(test), allow(dead_code))]
+    base: HostMap,
+    /// Path to the operator hosts file. Read only by `reload`.
+    #[cfg_attr(not(test), allow(dead_code))]
+    path: std::path::PathBuf,
+    /// Last observed modification time of the hosts file (`None` if absent).
+    /// Read only by `reload`.
+    #[cfg_attr(not(test), allow(dead_code))]
+    last_mtime: Option<std::time::SystemTime>,
+}
+
+impl HostMapReloadable {
+    /// Create a reloadable host map.
+    ///
+    /// Performs the initial load of the hosts file and merges it over the
+    /// base map so the published snapshot is valid immediately.
+    pub fn new(base: HostMap, path: std::path::PathBuf) -> Self {
+        let last_mtime = file_mtime(&path);
+        let hosts_file = HostMap::load_hosts_file(&path);
+        let mut effective = base.clone();
+        effective.merge(hosts_file);
+
+        Self {
+            snapshot: arc_swap::ArcSwap::from(Arc::new(effective)),
+            base,
+            path,
+            last_mtime,
+        }
+    }
+}
+
+impl Reloadable for HostMapReloadable {
+    type Snapshot = HostMap;
+
+    async fn reload(&mut self) -> bool {
+        let current_mtime = file_mtime(&self.path);
+
+        if current_mtime == self.last_mtime {
+            return false;
+        }
+
+        // File appeared, disappeared, or was modified.
+        self.last_mtime = current_mtime;
+        let hosts_file = HostMap::load_hosts_file(&self.path);
+        let mut new_effective = self.base.clone();
+        new_effective.merge(hosts_file);
+
+        let count = new_effective.len();
+        self.snapshot.store(Arc::new(new_effective));
+
+        tracing::info!(
+            path = %self.path.display(),
+            entries = count,
+            "Reloaded hosts file"
+        );
+        true
+    }
+
+    fn load(&self) -> arc_swap::Guard<Arc<HostMap>> {
+        self.snapshot.load()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::Identity;
+
+    #[tokio::test]
+    async fn test_initial_load_base_and_file() {
+        let id_base = Identity::generate();
+        let id_file = Identity::generate();
+
+        let mut base = HostMap::new();
+        base.insert("core", &id_base.npub()).unwrap();
+
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("hosts");
+        std::fs::write(&path, format!("gateway   {}\n", id_file.npub())).unwrap();
+
+        let reloadable = HostMapReloadable::new(base, path);
+        let snapshot = reloadable.load();
+        assert_eq!(snapshot.len(), 2);
+        assert!(snapshot.lookup_npub("core").is_some());
+        assert!(snapshot.lookup_npub("gateway").is_some());
+    }
+
+    #[tokio::test]
+    async fn test_initial_load_no_file_base_only() {
+        let id = Identity::generate();
+        let mut base = HostMap::new();
+        base.insert("core", &id.npub()).unwrap();
+
+        let reloadable =
+            HostMapReloadable::new(base, std::path::PathBuf::from("/nonexistent/hosts"));
+        let snapshot = reloadable.load();
+        assert_eq!(snapshot.len(), 1);
+        assert!(snapshot.lookup_npub("core").is_some());
+    }
+
+    #[tokio::test]
+    async fn test_reload_detects_file_change() {
+        let id1 = Identity::generate();
+        let id2 = Identity::generate();
+
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("hosts");
+        std::fs::write(&path, format!("gateway   {}\n", id1.npub())).unwrap();
+
+        let mut reloadable = HostMapReloadable::new(HostMap::new(), path.clone());
+        assert_eq!(reloadable.load().len(), 1);
+        assert_eq!(
+            reloadable.load().lookup_npub("gateway"),
+            Some(id1.npub().as_str())
+        );
+
+        // No change yet.
+        assert!(!reloadable.reload().await);
+
+        // Bump mtime by rewriting; sleep for filesystem mtime granularity.
+        std::thread::sleep(std::time::Duration::from_millis(50));
+        std::fs::write(
+            &path,
+            format!("gateway   {}\nnew-host   {}\n", id1.npub(), id2.npub()),
+        )
+        .unwrap();
+
+        assert!(reloadable.reload().await);
+        let snapshot = reloadable.load();
+        assert_eq!(snapshot.len(), 2);
+        assert!(snapshot.lookup_npub("new-host").is_some());
+    }
+
+    #[tokio::test]
+    async fn test_reload_no_change_returns_false() {
+        let id = Identity::generate();
+
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("hosts");
+        std::fs::write(&path, format!("gateway   {}\n", id.npub())).unwrap();
+
+        let mut reloadable = HostMapReloadable::new(HostMap::new(), path);
+        assert!(!reloadable.reload().await);
+        assert!(!reloadable.reload().await);
+    }
+
+    #[tokio::test]
+    async fn test_reload_detects_file_deletion() {
+        let id = Identity::generate();
+
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("hosts");
+        std::fs::write(&path, format!("gateway   {}\n", id.npub())).unwrap();
+
+        let mut reloadable = HostMapReloadable::new(HostMap::new(), path.clone());
+        assert_eq!(reloadable.load().len(), 1);
+
+        std::fs::remove_file(&path).unwrap();
+
+        assert!(reloadable.reload().await);
+        assert!(reloadable.load().is_empty());
+    }
+
+    #[tokio::test]
+    async fn test_reload_detects_file_creation() {
+        let id = Identity::generate();
+
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("hosts");
+
+        let mut reloadable = HostMapReloadable::new(HostMap::new(), path.clone());
+        assert!(reloadable.load().is_empty());
+
+        std::fs::write(&path, format!("gateway   {}\n", id.npub())).unwrap();
+
+        assert!(reloadable.reload().await);
+        let snapshot = reloadable.load();
+        assert_eq!(snapshot.len(), 1);
+        assert!(snapshot.lookup_npub("gateway").is_some());
+    }
+
+    #[tokio::test]
+    async fn test_reload_preserves_base() {
+        let id_base = Identity::generate();
+        let id_file = Identity::generate();
+
+        let mut base = HostMap::new();
+        base.insert("core", &id_base.npub()).unwrap();
+
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("hosts");
+        std::fs::write(&path, format!("gateway   {}\n", id_file.npub())).unwrap();
+
+        let mut reloadable = HostMapReloadable::new(base, path.clone());
+        assert_eq!(reloadable.load().len(), 2);
+
+        std::fs::remove_file(&path).unwrap();
+        assert!(reloadable.reload().await);
+        let snapshot = reloadable.load();
+        assert_eq!(snapshot.len(), 1);
+        assert!(snapshot.lookup_npub("core").is_some());
+        assert!(snapshot.lookup_npub("gateway").is_none());
+    }
+
+    /// The initial published snapshot must be byte-for-byte equivalent to the
+    /// pre-migration `Arc<HostMap>` built by `base.clone()` + `merge(file)`.
+    #[tokio::test]
+    async fn test_initial_snapshot_matches_pre_migration_construction() {
+        let id_base = Identity::generate();
+        let id_file = Identity::generate();
+
+        let mut base = HostMap::new();
+        base.insert("core", &id_base.npub()).unwrap();
+
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("hosts");
+        std::fs::write(&path, format!("gateway   {}\n", id_file.npub())).unwrap();
+
+        // Pre-migration construction.
+        let mut expected = base.clone();
+        expected.merge(HostMap::load_hosts_file(&path));
+
+        // Post-migration construction.
+        let reloadable = HostMapReloadable::new(base, path);
+        let snapshot = reloadable.load();
+
+        assert_eq!(snapshot.len(), expected.len());
+        for key in ["core", "gateway"] {
+            assert_eq!(snapshot.lookup_npub(key), expected.lookup_npub(key));
+        }
+    }
+}