Coordinator: join the cluster when NATS comes up, report the connection

A node that started while NATS was down never got a coordinator. The
connect failed at boot, bootstrap recorded a build error and the node ran
with Coordinator::None until restarted. It had no broadcast subscriber
or publisher, so cross-node push and cache invalidation to it stayed
broken, and its healthcheck said nothing about it. Losing NATS after
startup was silent too.

- The NATS client now connects in the background
  (retry_on_initial_connect): startup never waits on NATS or fails over
  it, the node gets its coordinator, subscriber and publisher at once,
  and the client keeps trying (async-nats's backoff, at most 4 s apart)
  until NATS answers. Subscriptions made meanwhile start delivering when
  it does. A configured maxReconnects still ends the attempts.
- Three new events report the connection: cluster.coordinator-connected
  (info), cluster.coordinator-disconnected (warn: lost, closed, gave up,
  or not connected within the connection timeout at startup) and
  cluster.coordinator-error (warn: a failed attempt, reported once per
  outage rather than every retry, and server errors, slow consumers and
  lame duck mode). They are in the packaged schema, ids 644 to 646.
- GET /healthz/cluster reports the coordinator: 200
  {"coordinator":"connected"}, 503 {"coordinator":"disconnected"}, or
  200 with "none" (no coordinator) or "unknown" (a backend that doesn't
  track its connection). /healthz/live and /healthz/ready are unchanged
  on purpose: a node without its coordinator still serves mail, and
  failing those would have orchestrators restart, or pull out of
  service, every node at once whenever NATS is down.

Only NATS connects lazily; the other coordinator backends still fail at
boot as before.

cluster::coordinator::coordinator_reconnect_tests starts a node against a
NATS port with nothing behind it, checks it boots with a coordinator and
reports it disconnected, subscribes, then starts NATS on that port: the
node connects on its own and the subscription receives a message from a
second client. Stopping and restarting NATS shows disconnected, then
connected, and the same subscription keeps working.

tests/src/cluster/coordinator.rs

@@ -0,0 +1,145 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Coffey Labs
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+//! A node that starts while its NATS coordinator is down joins the cluster
+//! once NATS comes up, without a restart, and reports the coordinator's
+//! connection on `/healthz/cluster` as it goes and comes back.
+
+use crate::utils::server::TestServerBuilder;
+use coordinator::Coordinator;
+use registry::{
+    schema::{
+        enums::NetworkListenerProtocol,
+        structs::{Coordinator as CoordinatorSetting, NatsCoordinator},
+    },
+    types::map::Map,
+};
+use serde_json::{Value, json};
+use std::time::{Duration, Instant};
+use testcontainers::{
+    GenericImage, ImageExt, core::IntoContainerPort, core::WaitFor, runners::AsyncRunner,
+};
+
+const HTTP_PORT: u16 = 11_310;
+const TOPIC: &str = "inbuxa-coordinator-test";
+
+#[tokio::test(flavor = "multi_thread")]
+pub async fn coordinator_reconnect_tests() {
+    println!("Running coordinator reconnect tests...");
+
+    // A port with no NATS server behind it, yet
+    let nats_port = std::net::TcpListener::bind("127.0.0.1:0")
+        .unwrap()
+        .local_addr()
+        .unwrap()
+        .port();
+    let config = NatsCoordinator {
+        addresses: Map::new(vec![format!("127.0.0.1:{nats_port}")]),
+        use_tls: false,
+        timeout_connection: 1_000u64.into(),
+        ..Default::default()
+    };
+
+    // 1. The node starts, without a build error, while NATS is down, and
+    // says so
+    let test = TestServerBuilder::new("coordinator_reconnect_tests")
+        .await
+        .with_object(CoordinatorSetting::Nats(config.clone()))
+        .await
+        .with_listener(NetworkListenerProtocol::Http, "http", HTTP_PORT, true)
+        .await
+        .build()
+        .await;
+    let coordinator = test.server.core.storage.coordinator.clone();
+    assert!(
+        coordinator.is_enabled(),
+        "a coordinator, though not connected"
+    );
+    assert_eq!(coordinator.is_connected(), Some(false));
+    assert_eq!(
+        cluster_health().await,
+        (503, json!({"coordinator": "disconnected"}))
+    );
+
+    // A subscription made now, as the broadcast subscriber makes it at
+    // startup, has to work once NATS is up
+    let mut stream = coordinator.subscribe(TOPIC).await.unwrap();
+
+    // 2. NATS comes up: the node connects on its own
+    let nats = GenericImage::new("nats", "latest")
+        .with_wait_for(WaitFor::message_on_stderr("Server is ready"))
+        .with_mapped_port(nats_port, 4222.tcp())
+        .start()
+        .await
+        .expect("Failed to start NATS container");
+    wait_for_health(200, "connected").await;
+    let other_node = coordinator::backend::nats::NatsPubSub::open(config.clone())
+        .await
+        .unwrap();
+    wait_until_connected(&other_node).await;
+    round_trip(&other_node, &mut stream, b"after startup").await;
+
+    // 3. NATS goes away: the node reports it; and it comes back: the node
+    // reconnects and the same subscription carries on
+    nats.stop().await.unwrap();
+    wait_for_health(503, "disconnected").await;
+    nats.start().await.unwrap();
+    wait_for_health(200, "connected").await;
+    wait_until_connected(&other_node).await;
+    round_trip(&other_node, &mut stream, b"after reconnect").await;
+
+    drop(nats);
+    if test.is_reset() {
+        test.temp_dir.delete();
+    }
+}
+
+async fn cluster_health() -> (u16, Value) {
+    let response = reqwest::Client::builder()
+        .danger_accept_invalid_certs(true)
+        .timeout(Duration::from_secs(5))
+        .build()
+        .unwrap()
+        .get(format!("https://127.0.0.1:{HTTP_PORT}/healthz/cluster"))
+        .send()
+        .await
+        .unwrap();
+    let status = response.status().as_u16();
+    (status, response.json().await.unwrap())
+}
+
+async fn wait_for_health(status: u16, state: &str) {
+    let started = Instant::now();
+    loop {
+        let health = cluster_health().await;
+        if health == (status, json!({"coordinator": state})) {
+            return;
+        }
+        assert!(
+            started.elapsed() < Duration::from_secs(30),
+            "expected {status} {state}, still {health:?}"
+        );
+        tokio::time::sleep(Duration::from_millis(250)).await;
+    }
+}
+
+async fn wait_until_connected(coordinator: &Coordinator) {
+    let started = Instant::now();
+    while coordinator.is_connected() != Some(true) {
+        assert!(started.elapsed() < Duration::from_secs(30), "not connected");
+        tokio::time::sleep(Duration::from_millis(100)).await;
+    }
+}
+
+/// Another node publishes; this one's subscription receives it.
+async fn round_trip(from: &Coordinator, stream: &mut coordinator::PubSubStream, payload: &[u8]) {
+    from.publish(TOPIC, payload.to_vec()).await.unwrap();
+    let message = tokio::time::timeout(Duration::from_secs(10), stream.next())
+        .await
+        .expect("no message within 10 seconds")
+        .expect("subscription ended");
+    assert_eq!(message.payload(), payload);
+}

tests/src/cluster/mod.rs

@@ -2,7 +2,11 @@
  * SPDX-FileCopyrightText: 2020 Stalwart Labs LLC <[email protected]>
  *
  * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-SEL
+ *
+ * Modified by Coffey Labs in 2026 for INBUXA.
  */
 
 pub mod broadcast;
+#[cfg(feature = "nats")]
+pub mod coordinator; // inbuxa: coordinator reconnects
 pub mod stress;