test(health): pin the readiness default at compile time

Readiness reported the KMS ready on the service status bit alone, which says
the manager started, not that the backend can still serve a request. Consume
the background probe snapshot instead: a running service is withdrawn only
when a fresh snapshot shows failures at the threshold.

The readiness path stays free of backend calls — it reads the lock-free
snapshot — so a struggling KMS cannot be amplified into probe traffic of its
own. Everything the probe cannot speak to (no worker, no completed round,
a snapshot older than three probe intervals) leaves the previous status-bit
verdict standing, and the check remains off by default.

Refs rustfs/backlog#1584 (part of rustfs/backlog#1562)

rustfs/src/server/health.rs

@@ -18,7 +18,10 @@ use super::{
     collect_cluster_read_health_report, collect_cluster_write_health_report, collect_node_readiness_report,
 };
 use http::{Method, StatusCode};
+use rustfs_kms::ProbeStatus;
+use rustfs_kms::probe::{DEFAULT_PROBE_INTERVAL, ENV_KMS_PROBE_INTERVAL_SECS, MIN_PROBE_INTERVAL};
 use serde_json::{Value, json};
+use std::time::Duration;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub(crate) struct HealthCheckState {
@@ -122,6 +125,86 @@ pub(crate) fn health_minimal_response_enabled() -> bool {
     )
 }
 
+/// Consecutive failed probe rounds tolerated before readiness withdraws the KMS.
+///
+/// One failed round is routinely an external hiccup — a Vault leader election, a
+/// dropped connection, a rotated token — and reporting not ready on the first one
+/// would turn that hiccup into a rolling restart of every node that shares the
+/// backend. Requiring the failure to survive three rounds keeps the signal while
+/// costing at most three intervals of detection latency.
+const KMS_PROBE_FAILURE_THRESHOLD: u32 = 3;
+
+/// Probe rounds a snapshot may age before readiness stops acting on it.
+///
+/// Expressed in rounds rather than seconds so the bound follows the configured
+/// interval: a deployment probing every ten minutes must not read every snapshot
+/// as stale. Three rounds absorbs a round that overruns its slot or a worker that
+/// misses a tick without discarding a signal that is merely late.
+const KMS_PROBE_STALENESS_ROUNDS: u32 = 3;
+
+/// Upper bound on the age of a probe snapshot that readiness is willing to act on.
+pub(crate) fn kms_probe_staleness_limit() -> Duration {
+    configured_kms_probe_interval().saturating_mul(KMS_PROBE_STALENESS_ROUNDS)
+}
+
+/// Probe interval currently in force.
+///
+/// Readiness reads the same variable as the probe worker because it only needs
+/// the value to scale [`kms_probe_staleness_limit`]; the worker owns the interval
+/// itself and keeps its parsing private.
+fn configured_kms_probe_interval() -> Duration {
+    kms_probe_interval_from_env(std::env::var(ENV_KMS_PROBE_INTERVAL_SECS).ok().as_deref())
+}
+
+/// Interval implied by a raw environment value, mirroring the worker's own rules:
+/// unset or unparsable means the default, and anything below the floor is raised
+/// to it. `0` disables the probe, which publishes no snapshot at all — the bound
+/// is then never consulted, so the default stands in.
+fn kms_probe_interval_from_env(value: Option<&str>) -> Duration {
+    let Some(seconds) = value.and_then(|value| value.trim().parse::<u64>().ok()) else {
+        return DEFAULT_PROBE_INTERVAL;
+    };
+    if seconds == 0 {
+        return DEFAULT_PROBE_INTERVAL;
+    }
+    Duration::from_secs(seconds).max(MIN_PROBE_INTERVAL)
+}
+
+/// KMS readiness verdict from the service status bit and the background probe.
+///
+/// Reads only the published snapshot and never calls the backend: a readiness
+/// request must not become load on a KMS that is already struggling, and a
+/// Kubernetes probe timing out on an external dependency is how one slow Vault
+/// becomes a fleet-wide restart.
+///
+/// The probe can only ever withdraw readiness from a service that is otherwise
+/// running, and only on evidence that is both fresh and repeated. Everything the
+/// probe cannot speak to — no worker (a backend without a data key round trip, or
+/// the probe disabled), no completed round yet, or a snapshot older than
+/// `staleness_limit` — leaves the status-bit verdict standing rather than
+/// reporting a failure nobody observed.
+pub(crate) fn kms_ready_from_probe(service_running: bool, probe: Option<&ProbeStatus>, staleness_limit: Duration) -> bool {
+    if !service_running {
+        return false;
+    }
+
+    let Some(probe) = probe else {
+        return true;
+    };
+
+    let Some(age) = probe.last_round_age() else {
+        return true;
+    };
+
+    if age > staleness_limit {
+        return true;
+    }
+
+    // Every non-failure result resets the counter, so an unsupported backend or a
+    // recovered round can never reach the threshold.
+    probe.consecutive_failures < KMS_PROBE_FAILURE_THRESHOLD
+}
+
 pub(crate) fn build_component_details(
     storage_ready: bool,
     iam_ready: bool,
@@ -277,3 +360,177 @@ pub(crate) fn build_health_payload(ctx: HealthPayloadContext<'_>) -> Value {
 
     payload
 }
+
+#[cfg(test)]
+mod tests {
+    use super::super::readiness::DependencyReadiness;
+    use super::*;
+    use rustfs_kms::{ProbeFailureKind, ProbeResult};
+    use serial_test::serial;
+    use temp_env::with_var;
+    use tokio::time::Instant;
+
+    const STALENESS_LIMIT: Duration = Duration::from_secs(180);
+
+    fn snapshot(result: ProbeResult, last_round_at: Option<Instant>, consecutive_failures: u32) -> ProbeStatus {
+        ProbeStatus {
+            result,
+            last_round_at,
+            last_success_at: last_round_at.filter(|_| result == ProbeResult::Success),
+            last_success_unix_secs: None,
+            consecutive_failures,
+        }
+    }
+
+    fn ready_report() -> DependencyReadinessReport {
+        DependencyReadinessReport {
+            readiness: DependencyReadiness {
+                storage_ready: true,
+                iam_ready: true,
+                lock_quorum_ready: true,
+                peer_health_ready: true,
+            },
+            degraded_reasons: Vec::new(),
+        }
+    }
+
+    #[tokio::test(start_paused = true)]
+    async fn a_fresh_successful_round_keeps_the_service_ready() {
+        let round_at = Instant::now();
+        tokio::time::advance(Duration::from_secs(1)).await;
+
+        let status = snapshot(ProbeResult::Success, Some(round_at), 0);
+        assert!(kms_ready_from_probe(true, Some(&status), STALENESS_LIMIT));
+    }
+
+    #[tokio::test(start_paused = true)]
+    async fn a_single_failed_round_does_not_withdraw_readiness() {
+        let round_at = Instant::now();
+        tokio::time::advance(Duration::from_secs(1)).await;
+
+        let status = snapshot(
+            ProbeResult::Failure(ProbeFailureKind::Decrypt),
+            Some(round_at),
+            KMS_PROBE_FAILURE_THRESHOLD - 1,
+        );
+        assert!(kms_ready_from_probe(true, Some(&status), STALENESS_LIMIT));
+    }
+
+    #[tokio::test(start_paused = true)]
+    async fn failures_reaching_the_threshold_withdraw_readiness() {
+        let round_at = Instant::now();
+        tokio::time::advance(Duration::from_secs(1)).await;
+
+        let status = snapshot(
+            ProbeResult::Failure(ProbeFailureKind::Generate),
+            Some(round_at),
+            KMS_PROBE_FAILURE_THRESHOLD,
+        );
+        assert!(!kms_ready_from_probe(true, Some(&status), STALENESS_LIMIT));
+    }
+
+    #[tokio::test(start_paused = true)]
+    async fn a_stale_snapshot_falls_back_to_the_status_bit() {
+        let round_at = Instant::now();
+        tokio::time::advance(STALENESS_LIMIT + Duration::from_secs(1)).await;
+
+        let status = snapshot(
+            ProbeResult::Failure(ProbeFailureKind::Mismatch),
+            Some(round_at),
+            KMS_PROBE_FAILURE_THRESHOLD * 10,
+        );
+        assert!(
+            kms_ready_from_probe(true, Some(&status), STALENESS_LIMIT),
+            "a snapshot nobody refreshed is unknown, not a failure"
+        );
+    }
+
+    #[tokio::test(start_paused = true)]
+    async fn an_unsupported_backend_is_not_a_failure() {
+        let round_at = Instant::now();
+        tokio::time::advance(Duration::from_secs(1)).await;
+
+        let status = snapshot(ProbeResult::Unsupported, Some(round_at), 0);
+        assert!(kms_ready_from_probe(true, Some(&status), STALENESS_LIMIT));
+    }
+
+    #[tokio::test(start_paused = true)]
+    async fn a_stopped_service_stays_not_ready_despite_a_successful_round() {
+        let round_at = Instant::now();
+        tokio::time::advance(Duration::from_secs(1)).await;
+
+        let status = snapshot(ProbeResult::Success, Some(round_at), 0);
+        assert!(!kms_ready_from_probe(false, Some(&status), STALENESS_LIMIT));
+    }
+
+    #[test]
+    fn a_service_without_a_probe_keeps_the_status_bit_verdict() {
+        assert!(kms_ready_from_probe(true, None, STALENESS_LIMIT));
+        assert!(!kms_ready_from_probe(false, None, STALENESS_LIMIT));
+    }
+
+    #[test]
+    fn a_probe_with_no_completed_round_keeps_the_service_ready() {
+        let status = snapshot(ProbeResult::Pending, None, 0);
+        assert!(kms_ready_from_probe(true, Some(&status), STALENESS_LIMIT));
+    }
+
+    #[test]
+    fn the_probe_interval_follows_the_worker_parsing_rules() {
+        assert_eq!(kms_probe_interval_from_env(None), DEFAULT_PROBE_INTERVAL);
+        assert_eq!(kms_probe_interval_from_env(Some("not-a-number")), DEFAULT_PROBE_INTERVAL);
+        assert_eq!(kms_probe_interval_from_env(Some("0")), DEFAULT_PROBE_INTERVAL);
+        assert_eq!(kms_probe_interval_from_env(Some(" 600 ")), Duration::from_secs(600));
+        assert_eq!(kms_probe_interval_from_env(Some("1")), MIN_PROBE_INTERVAL);
+    }
+
+    #[test]
+    #[serial]
+    fn the_staleness_bound_spans_three_probe_rounds() {
+        with_var(ENV_KMS_PROBE_INTERVAL_SECS, Some("30"), || {
+            assert_eq!(kms_probe_staleness_limit(), Duration::from_secs(90));
+        });
+    }
+
+    /// Flipping this default is a deployment behaviour change, not a code
+    /// change: it would start failing readiness on clusters that never asked
+    /// for the KMS to gate it. Pinned at compile time so the constant cannot
+    /// drift without this line being edited too.
+    const _: () = assert!(!rustfs_config::DEFAULT_HEALTH_COMPAT_KMS_READY_CHECK_ENABLE);
+
+    #[test]
+    #[serial]
+    fn a_withdrawn_kms_degrades_the_readiness_response() {
+        with_var(rustfs_config::ENV_HEALTH_MINIMAL_RESPONSE_ENABLE, Some("false"), || {
+            let report = ready_report();
+            let parts = build_health_response_parts(
+                Method::GET,
+                HealthProbe::Readiness,
+                Some(&report),
+                "rustfs-endpoint",
+                None,
+                Some(false),
+            );
+
+            assert_eq!(parts.status_code, StatusCode::SERVICE_UNAVAILABLE);
+            let payload = parts.payload.expect("GET should include payload");
+            assert_eq!(payload["details"]["kms"]["ready"], false);
+            assert_eq!(payload["degradedReasons"], json!(["kms_not_ready"]));
+        });
+    }
+
+    #[test]
+    #[serial]
+    fn a_disabled_check_leaves_the_readiness_response_untouched() {
+        with_var(rustfs_config::ENV_HEALTH_MINIMAL_RESPONSE_ENABLE, Some("false"), || {
+            let report = ready_report();
+            let parts =
+                build_health_response_parts(Method::GET, HealthProbe::Readiness, Some(&report), "rustfs-endpoint", None, None);
+
+            assert_eq!(parts.status_code, StatusCode::OK);
+            let payload = parts.payload.expect("GET should include payload");
+            assert!(payload["details"].get("kms").is_none());
+            assert_eq!(payload["degradedReasons"], json!([]));
+        });
+    }
+}

rustfs/src/server/layer.rs

@@ -23,6 +23,7 @@ use crate::server::{
     MINIO_ADMIN_V3_PREFIX, MINIO_HEALTH_CLUSTER_PATH, MINIO_HEALTH_CLUSTER_READ_PATH, MINIO_HEALTH_LIVE_PATH,
     MINIO_HEALTH_READY_PATH, PROFILE_CPU_PATH, PROFILE_MEMORY_PATH, RPC_PREFIX, RUSTFS_ADMIN_PREFIX, active_http_requests,
     build_health_response_parts, collect_probe_readiness, has_path_prefix, is_admin_path, is_table_catalog_path,
+    kms_probe_staleness_limit, kms_ready_from_probe,
 };
 use crate::storage_api::server::layer::apply_cors_headers;
 use crate::storage_api::server::layer::request_context::{RequestContext, extract_request_id_from_headers, spawn_traced};
@@ -1281,7 +1282,9 @@ async fn health_kms_ready() -> bool {
         return true;
     };
 
-    matches!(service_manager.get_status().await, rustfs_kms::KmsServiceStatus::Running)
+    let service_running = matches!(service_manager.get_status().await, rustfs_kms::KmsServiceStatus::Running);
+    let probe_status = service_manager.probe_status();
+    kms_ready_from_probe(service_running, probe_status.as_deref(), kms_probe_staleness_limit())
 }
 
 async fn build_public_health_http_response<RestBody, GrpcBody>(

rustfs/src/server/mod.rs

@@ -55,7 +55,10 @@ pub(crate) use health::{
     HealthPayloadContext, HealthReadinessSource, build_component_details, build_health_payload, health_check_state,
     readiness_source_for_probe,
 };
-pub(crate) use health::{HealthProbe, build_health_response_parts, collect_probe_readiness, probe_from_path};
+pub(crate) use health::{
+    HealthProbe, build_health_response_parts, collect_probe_readiness, kms_probe_staleness_limit, kms_ready_from_probe,
+    probe_from_path,
+};
 pub(crate) use http::HeaderMapCarrier;
 pub(crate) use http::active_http_requests;
 pub(crate) use layer::{RequestContextLayer, is_sts_query_request};