liminal_server/health/

endpoint.rs

use std::io::{Read, Write};
use std::net::{Shutdown, SocketAddr, TcpListener, TcpStream};
use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
use std::sync::{Arc, Mutex, PoisonError};
use std::thread::{self, JoinHandle};
use std::time::Duration;

use crate::ServerError;
use crate::server::listener::{loopback_interrupt_target, shed_on_fd_exhaustion};

use super::checks::{SharedReadinessState, health_check, readiness_check};

use super::metrics_route;

const HEALTH_PATH: &str = "/health";
const READY_PATH: &str = "/ready";
const METRICS_PATH: &str = "/metrics";
const APPLICATION_JSON: &str = "application/json";
const READ_BUFFER_BYTES: usize = 2048;

/// Handle for a running health endpoint server.
///
/// W4 leg 2 (§4.2): the health accept worker BLOCKS in `accept` (kernel-parked,
/// zero idle wakes) rather than spinning a non-blocking poll with a backoff
/// sleep. Shutdown wakes the blocked `accept` with an explicit self-connect
/// interrupt to `interrupt_target` (the bound address, loopback-normalised),
/// mirroring the leg-1 listeners. Shutdown also interrupts an in-flight request
/// read directly via `active_stream`, so a silent client parked in
/// `handle_connection`'s read cannot defer shutdown by its admitted deadline.
#[derive(Debug)]
pub struct HealthServerHandle {
    local_addr: SocketAddr,
    /// Loopback-normalised self-connect target used to interrupt the blocking
    /// `accept` at shutdown.
    interrupt_target: SocketAddr,
    shutdown: Arc<AtomicBool>,
    /// Slot holding a `try_clone` of the request stream the worker is currently
    /// reading, if any. The worker registers it under the lock (with a shutdown
    /// recheck) before blocking on the request read and clears it on completion;
    /// `stop_worker` takes it and `shutdown(Both)`s it to interrupt an in-flight
    /// blocking read directly (TOLD) rather than waiting out the read's admitted
    /// deadline. At most one stream exists at a time (the worker is serial), so
    /// one slot suffices.
    active_stream: Arc<Mutex<Option<TcpStream>>>,
    worker: Option<JoinHandle<Result<(), ServerError>>>,
    /// Count of `accept` calls issued by the worker (test observability for the
    /// zero-idle-wakes oracle: on a silent listener this stays at the single
    /// parked call). The worker always maintains the counter; only the host-side
    /// handle for reading it is test-scoped.
    #[cfg(test)]
    accept_attempts: Arc<AtomicU64>,
    /// Count of connections shed under fd exhaustion via the reserve descriptor
    /// (test observability for the shed helper reused from leg 1).
    #[cfg(test)]
    shed_count: Arc<AtomicU64>,
    /// Count of accepted requests the worker has entered (incremented just
    /// before it blocks reading the request). Test observability so a race can
    /// deterministically catch the worker mid-request rather than parked in
    /// `accept`.
    #[cfg(test)]
    requests_entered: Arc<AtomicU64>,
}

impl HealthServerHandle {
    /// Returns the bound address for the health endpoint server.
    #[must_use]
    pub const fn local_addr(&self) -> SocketAddr {
        self.local_addr
    }

    /// Stops the health endpoint server and waits for its worker thread to exit.
    ///
    /// # Errors
    ///
    /// Returns [`ServerError::HealthEndpoint`] if the worker thread cannot be
    /// joined cleanly or if the server loop recorded a serving error.
    pub fn shutdown(mut self) -> Result<(), ServerError> {
        self.stop_worker()
    }

    fn stop_worker(&mut self) -> Result<(), ServerError> {
        self.shutdown.store(true, Ordering::SeqCst);
        let Some(worker) = self.worker.take() else {
            return Ok(());
        };
        // Interrupt an in-flight request read directly (TOLD): the flag store
        // above happens-before this lock acquire, so any stream the worker
        // registered before observing the flag is taken here and shut down,
        // waking its blocked read at once. `shutdown(Both)` on the clone reaches
        // the same underlying socket the worker is reading (a dup'd fd). If the
        // worker is instead parked in `accept`, the slot is empty and the
        // self-connect below wakes it. The guard is released (statement end)
        // before the socket call, so no lock is held across `shutdown`.
        let in_flight = self
            .active_stream
            .lock()
            .unwrap_or_else(PoisonError::into_inner)
            .take();
        if let Some(stream) = in_flight {
            let _ = stream.shutdown(Shutdown::Both);
        }
        // Explicit cross-platform interrupt (mirrors the leg-1 listeners): a
        // single self-connect wakes a blocked `accept`. The worker sees the
        // shutdown flag it observed under the same ordering and sheds the woken
        // (spurious) socket rather than serving it — at most one spurious accept
        // per interrupt. If the worker already exited (a real accept then flag
        // check), the listener is gone and this connect fails fast; the join then
        // returns immediately.
        if let Ok(waker) = TcpStream::connect(self.interrupt_target) {
            drop(waker);
        }

        worker.join().map_err(|_| ServerError::HealthEndpoint {
            message: "health endpoint worker thread terminated unexpectedly".to_owned(),
        })?
    }

    /// `accept` calls issued by the worker (test observability, oracle 8).
    #[cfg(test)]
    fn accept_attempts(&self) -> u64 {
        self.accept_attempts.load(Ordering::SeqCst)
    }

    /// Connections shed under fd exhaustion (test observability).
    #[cfg(test)]
    fn shed_count(&self) -> u64 {
        self.shed_count.load(Ordering::SeqCst)
    }

    /// Accepted requests the worker has entered (test observability).
    #[cfg(test)]
    fn requests_entered(&self) -> u64 {
        self.requests_entered.load(Ordering::SeqCst)
    }
}

impl Drop for HealthServerHandle {
    fn drop(&mut self) {
        if let Err(error) = self.stop_worker() {
            tracing::debug!(%error, "health endpoint shutdown during drop failed");
        }
    }
}

/// Starts the health endpoint HTTP server on a distinct health bind address.
///
/// The returned server handle is independent from the main wire protocol
/// listener. Binding the health endpoint does not mark the main listener ready.
///
/// # Errors
///
/// Returns [`ServerError::HealthEndpoint`] when the health listener cannot bind
/// or cannot report its local address. The listener stays BLOCKING (W4 leg 2):
/// the accept worker kernel-parks in `accept` with zero idle wakes; shutdown
/// wakes it via the self-connect interrupt.
pub fn start_health_server(
    bind_address: SocketAddr,
    readiness: SharedReadinessState,
) -> Result<HealthServerHandle, ServerError> {
    let listener =
        TcpListener::bind(bind_address).map_err(|error| ServerError::HealthEndpoint {
            message: format!("failed to bind health endpoint at {bind_address}: {error}"),
        })?;
    // The listener stays BLOCKING (W4 leg 2): the accept worker kernel-parks in
    // `accept` with zero idle wakes; shutdown wakes it via the self-connect
    // interrupt below.
    let local_addr = listener
        .local_addr()
        .map_err(|error| ServerError::HealthEndpoint {
            message: format!("failed to inspect health endpoint listener address: {error}"),
        })?;
    let interrupt_target = loopback_interrupt_target(local_addr);
    let shutdown = Arc::new(AtomicBool::new(false));
    let active_stream = Arc::new(Mutex::new(None));
    let accept_attempts = Arc::new(AtomicU64::new(0));
    let shed_count = Arc::new(AtomicU64::new(0));
    let requests_entered = Arc::new(AtomicU64::new(0));
    let worker_shutdown = Arc::clone(&shutdown);
    let worker_stream = Arc::clone(&active_stream);
    let worker_attempts = Arc::clone(&accept_attempts);
    let worker_shed = Arc::clone(&shed_count);
    let worker_entered = Arc::clone(&requests_entered);
    let worker = thread::spawn(move || {
        serve(
            &listener,
            &readiness,
            &worker_shutdown,
            &worker_stream,
            &worker_attempts,
            &worker_shed,
            &worker_entered,
        )
    });

    Ok(HealthServerHandle {
        local_addr,
        interrupt_target,
        shutdown,
        active_stream,
        worker: Some(worker),
        #[cfg(test)]
        accept_attempts,
        #[cfg(test)]
        shed_count,
        #[cfg(test)]
        requests_entered,
    })
}

fn serve(
    listener: &TcpListener,
    readiness: &SharedReadinessState,
    shutdown: &AtomicBool,
    active_stream: &Mutex<Option<TcpStream>>,
    accept_attempts: &AtomicU64,
    shed_count: &AtomicU64,
    requests_entered: &AtomicU64,
) -> Result<(), ServerError> {
    // One reserve descriptor held for the shed-with-spare-fd EMFILE policy,
    // reusing the leg-1 helper.
    let mut reserve = listener.try_clone().ok();
    while !shutdown.load(Ordering::SeqCst) {
        accept_attempts.fetch_add(1, Ordering::SeqCst);
        match listener.accept() {
            Ok((stream, ..)) => {
                // Register the in-flight stream and re-check shutdown atomically
                // under the slot lock. If shutdown already fired, shed without
                // serving (no request slips past the broadcast); otherwise the
                // registered clone lets `stop_worker` interrupt this request's
                // blocking read directly (TOLD). Pairing the flag load here with
                // `stop_worker`'s flag-store-then-lock means a registration can
                // never be missed: either the worker sees the flag and sheds, or
                // shutdown finds the clone in the slot.
                let admitted = {
                    let mut slot = active_stream.lock().unwrap_or_else(PoisonError::into_inner);
                    if shutdown.load(Ordering::SeqCst) {
                        false
                    } else {
                        *slot = stream.try_clone().ok();
                        true
                    }
                };
                if !admitted {
                    drop(stream);
                    continue;
                }
                requests_entered.fetch_add(1, Ordering::SeqCst);
                // A per-connection error (e.g. a TCP probe that connects but sends no HTTP
                // data within the read timeout) must NOT terminate the serve loop — otherwise
                // a single port probe kills the health server for the process lifetime and
                // subsequent liveness/readiness probes get connection-refused. Only fatal
                // listener-level accept errors (below) terminate serving.
                let result = handle_connection(stream, readiness);
                // The request is done: clear the slot so a later shutdown finds
                // nothing to interrupt (and never shuts down an unrelated stream).
                // A no-op if `stop_worker` already took the clone.
                *active_stream.lock().unwrap_or_else(PoisonError::into_inner) = None;
                if let Err(error) = result {
                    tracing::debug!(%error, "health endpoint connection error");
                }
            }
            Err(error) if error.kind() == std::io::ErrorKind::Interrupted => {}
            Err(error) if is_transient_accept_error(&error) => {
                shed_on_fd_exhaustion(listener, &mut reserve, shed_count, &error);
            }
            Err(error) => {
                return Err(ServerError::HealthEndpoint {
                    message: format!("health endpoint accept failed: {error}"),
                });
            }
        }
    }

    Ok(())
}

/// EMFILE/ENFILE resource exhaustion is transient, exactly as the leg-1 accept
/// loops treat it (mirrors the sibling WebSocket listener's local predicate).
fn is_transient_accept_error(error: &std::io::Error) -> bool {
    matches!(error.raw_os_error(), Some(code) if code == 24 || code == 23)
}

fn handle_connection(
    mut stream: TcpStream,
    readiness: &SharedReadinessState,
) -> Result<(), ServerError> {
    stream
        .set_nonblocking(false)
        .map_err(|error| ServerError::HealthEndpoint {
            message: format!("failed to configure health request stream: {error}"),
        })?;
    stream
        .set_read_timeout(Some(Duration::from_secs(2)))
        .map_err(|error| ServerError::HealthEndpoint {
            message: format!("failed to set health request read timeout: {error}"),
        })?;

    let mut buffer = [0_u8; READ_BUFFER_BYTES];
    let bytes_read = stream
        .read(&mut buffer)
        .map_err(|error| ServerError::HealthEndpoint {
            message: format!("failed to read health request: {error}"),
        })?;

    if bytes_read == 0 {
        return Ok(());
    }

    let response = response_for_request(&buffer[..bytes_read], readiness)?;
    stream
        .write_all(&response)
        .map_err(|error| ServerError::HealthEndpoint {
            message: format!("failed to write health response: {error}"),
        })?;
    stream.flush().map_err(|error| ServerError::HealthEndpoint {
        message: format!("failed to flush health response: {error}"),
    })
}

fn response_for_request(
    request: &[u8],
    readiness: &SharedReadinessState,
) -> Result<Vec<u8>, ServerError> {
    let Ok(request) = std::str::from_utf8(request) else {
        return Ok(empty_response(StatusCode::BadRequest));
    };
    let Some((method, path)) = parse_request_line(request) else {
        return Ok(empty_response(StatusCode::BadRequest));
    };

    match (method, path) {
        ("GET", HEALTH_PATH) => json_response(StatusCode::Ok, &health_check()),
        ("GET", READY_PATH) => {
            let status = readiness_check(&readiness.snapshot());
            let status_code = if status.ready {
                StatusCode::Ok
            } else {
                StatusCode::ServiceUnavailable
            };
            json_response(status_code, &status)
        }
        ("GET", METRICS_PATH) => Ok(response(
            StatusCode::Ok,
            Some(metrics_route::CONTENT_TYPE),
            metrics_route::render_body().as_bytes(),
        )),
        (_, HEALTH_PATH | READY_PATH | METRICS_PATH) => {
            Ok(empty_response(StatusCode::MethodNotAllowed))
        }
        _ => Ok(empty_response(StatusCode::NotFound)),
    }
}

fn parse_request_line(request: &str) -> Option<(&str, &str)> {
    let request_line = request.lines().next()?;
    let mut parts = request_line.split_whitespace();
    let method = parts.next()?;
    let path = parts.next()?;
    parts.next()?;

    Some((method, path))
}

fn json_response<T>(status: StatusCode, value: &T) -> Result<Vec<u8>, ServerError>
where
    T: serde::Serialize,
{
    let body = serde_json::to_vec(value).map_err(|error| ServerError::HealthEndpoint {
        message: format!("failed to serialize health response: {error}"),
    })?;
    Ok(response(status, Some(APPLICATION_JSON), &body))
}

fn empty_response(status: StatusCode) -> Vec<u8> {
    response(status, None, &[])
}

fn response(status: StatusCode, content_type: Option<&str>, body: &[u8]) -> Vec<u8> {
    let mut response = Vec::new();
    let status_line = format!("HTTP/1.1 {} {}\r\n", status.code(), status.reason());
    response.extend_from_slice(status_line.as_bytes());
    response.extend_from_slice(format!("Content-Length: {}\r\n", body.len()).as_bytes());
    response.extend_from_slice(b"Connection: close\r\n");
    if let Some(content_type) = content_type {
        response.extend_from_slice(format!("Content-Type: {content_type}\r\n").as_bytes());
    }
    response.extend_from_slice(b"\r\n");
    response.extend_from_slice(body);
    response
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum StatusCode {
    Ok,
    BadRequest,
    NotFound,
    MethodNotAllowed,
    ServiceUnavailable,
}

impl StatusCode {
    const fn code(self) -> u16 {
        match self {
            Self::Ok => 200,
            Self::BadRequest => 400,
            Self::NotFound => 404,
            Self::MethodNotAllowed => 405,
            Self::ServiceUnavailable => 503,
        }
    }

    const fn reason(self) -> &'static str {
        match self {
            Self::Ok => "OK",
            Self::BadRequest => "Bad Request",
            Self::NotFound => "Not Found",
            Self::MethodNotAllowed => "Method Not Allowed",
            Self::ServiceUnavailable => "Service Unavailable",
        }
    }
}

#[cfg(test)]
mod tests {
    use std::io::{Read, Write};
    use std::net::{SocketAddr, TcpStream};
    use std::thread;
    use std::time::{Duration, Instant};

    use serde_json::Value;

    use super::{response_for_request, start_health_server};
    use crate::health::checks::{
        ClusterReadiness, ReadinessCondition, ReadinessState, SharedReadinessState,
    };

    fn loopback_ephemeral() -> Result<SocketAddr, Box<dyn std::error::Error>> {
        Ok("127.0.0.1:0".parse()?)
    }

    fn get(address: SocketAddr, path: &str) -> Result<String, Box<dyn std::error::Error>> {
        let mut stream = TcpStream::connect(address)?;
        stream.set_read_timeout(Some(Duration::from_secs(2)))?;
        let request = format!("GET {path} HTTP/1.1\r\nHost: localhost\r\n\r\n");
        stream.write_all(request.as_bytes())?;

        let mut response = String::new();
        stream.read_to_string(&mut response)?;
        Ok(response)
    }

    fn assert_status(response: &str, status: u16) {
        let expected = format!("HTTP/1.1 {status} ");
        assert!(
            response.starts_with(&expected),
            "response status did not start with {expected}: {response}"
        );
    }

    fn body(response: &str) -> Result<&str, Box<dyn std::error::Error>> {
        let Some((_headers, body)) = response.split_once("\r\n\r\n") else {
            return Err("response did not contain a header/body separator".into());
        };
        Ok(body)
    }

    fn json_body(response: &str) -> Result<Value, Box<dyn std::error::Error>> {
        Ok(serde_json::from_str(body(response)?)?)
    }

    #[test]
    fn health_endpoint_returns_json_200_regardless_of_readiness()
    -> Result<(), Box<dyn std::error::Error>> {
        let readiness = SharedReadinessState::new(ReadinessState::default());
        let server = start_health_server(loopback_ephemeral()?, readiness)?;

        let response = get(server.local_addr(), "/health")?;
        server.shutdown()?;

        assert_status(&response, 200);
        assert!(response.contains("Content-Type: application/json\r\n"));
        let body = json_body(&response)?;
        assert_eq!(body["status"], "healthy");

        Ok(())
    }

    #[test]
    fn ready_endpoint_returns_503_before_main_listener_binds()
    -> Result<(), Box<dyn std::error::Error>> {
        let readiness = SharedReadinessState::new(ReadinessState::new(
            true,
            false,
            ClusterReadiness::NotConfigured,
        ));
        let server = start_health_server(loopback_ephemeral()?, readiness)?;

        let response = get(server.local_addr(), "/ready")?;
        server.shutdown()?;

        assert_status(&response, 503);
        assert!(response.contains("Content-Type: application/json\r\n"));
        let body = json_body(&response)?;
        assert_eq!(body["ready"], false);
        assert_eq!(body["unmet_conditions"][0], "listener_bound");

        Ok(())
    }

    #[test]
    fn ready_endpoint_returns_200_after_all_startup_gates() -> Result<(), Box<dyn std::error::Error>>
    {
        let readiness = SharedReadinessState::new(ReadinessState::ready_without_cluster());
        let server = start_health_server(loopback_ephemeral()?, readiness)?;

        let response = get(server.local_addr(), "/ready")?;
        server.shutdown()?;

        assert_status(&response, 200);
        let body = json_body(&response)?;
        assert_eq!(body["ready"], true);
        let Some(unmet_conditions) = body["unmet_conditions"].as_array() else {
            return Err("unmet_conditions should be an array".into());
        };
        assert!(unmet_conditions.is_empty());

        Ok(())
    }

    #[test]
    fn ready_endpoint_updates_from_shared_readiness_state() -> Result<(), Box<dyn std::error::Error>>
    {
        let readiness = SharedReadinessState::new(ReadinessState::default());
        let server = start_health_server(loopback_ephemeral()?, readiness.clone())?;

        let response = get(server.local_addr(), "/ready")?;
        assert_status(&response, 503);

        readiness.set_config_loaded(true);
        readiness.set_listener_bound(true);
        let response = get(server.local_addr(), "/ready")?;
        server.shutdown()?;

        assert_status(&response, 200);

        Ok(())
    }

    #[test]
    fn clustered_ready_transitions_503_to_200_when_membership_established()
    -> Result<(), Box<dyn std::error::Error>> {
        // A clustered server starts with the cluster gate unmet: config loaded and
        // listener bound, but membership not yet established (G2). /ready is 503.
        let readiness = SharedReadinessState::new(ReadinessState::new(
            true,
            true,
            ClusterReadiness::Configured {
                membership_established: false,
            },
        ));
        let server = start_health_server(loopback_ephemeral()?, readiness.clone())?;

        let response = get(server.local_addr(), "/ready")?;
        assert_status(&response, 503);
        let body = json_body(&response)?;
        assert_eq!(body["ready"], false);
        assert_eq!(
            body["unmet_conditions"][0],
            serde_json::to_value(ReadinessCondition::ClusterMembershipEstablished)?
        );

        // The cluster start's on_established hook flips exactly this flag; once set,
        // /ready must transition to 200 with no unmet conditions.
        readiness.set_cluster_membership_established(true);
        let response = get(server.local_addr(), "/ready")?;
        server.shutdown()?;

        assert_status(&response, 200);
        let body = json_body(&response)?;
        assert_eq!(body["ready"], true);
        let Some(unmet_conditions) = body["unmet_conditions"].as_array() else {
            return Err("unmet_conditions should be an array".into());
        };
        assert!(unmet_conditions.is_empty());

        Ok(())
    }

    #[test]
    fn cluster_readiness_is_listed_when_configured_but_not_joined()
    -> Result<(), Box<dyn std::error::Error>> {
        let readiness = SharedReadinessState::new(ReadinessState::new(
            true,
            true,
            ClusterReadiness::Configured {
                membership_established: false,
            },
        ));
        let response = response_for_request(b"GET /ready HTTP/1.1\r\n\r\n", &readiness)?;
        let response = String::from_utf8(response)?;

        assert_status(&response, 503);
        let body = json_body(&response)?;
        assert_eq!(
            body["unmet_conditions"][0],
            serde_json::to_value(ReadinessCondition::ClusterMembershipEstablished)?
        );

        Ok(())
    }

    #[test]
    fn unsupported_paths_are_not_served() -> Result<(), Box<dyn std::error::Error>> {
        let readiness = SharedReadinessState::default();
        let response = response_for_request(b"GET /unknown HTTP/1.1\r\n\r\n", &readiness)?;
        let response = String::from_utf8(response)?;

        assert_status(&response, 404);

        Ok(())
    }

    #[test]
    fn unsupported_methods_on_health_paths_are_rejected() -> Result<(), Box<dyn std::error::Error>>
    {
        let readiness = SharedReadinessState::default();
        let response = response_for_request(b"POST /health HTTP/1.1\r\n\r\n", &readiness)?;
        let response = String::from_utf8(response)?;

        assert_status(&response, 405);

        Ok(())
    }

    /// Oracle 8 (W4 leg 2) — on a quiet health listener the blocking accept is
    /// issued exactly once (the parked call) and never again: zero repeated
    /// accepts, zero application wakes after arming, with route behaviour
    /// unchanged (a real request is still served afterwards).
    #[test]
    fn silent_health_listener_has_zero_application_wakes() -> Result<(), Box<dyn std::error::Error>>
    {
        let server = start_health_server(loopback_ephemeral()?, SharedReadinessState::default())?;

        let deadline = Instant::now() + Duration::from_secs(2);
        while server.accept_attempts() < 1 && Instant::now() < deadline {
            thread::sleep(Duration::from_millis(5));
        }
        let armed = server.accept_attempts();
        assert_eq!(
            armed, 1,
            "the blocking accept is issued exactly once when parked"
        );

        thread::sleep(Duration::from_millis(200));
        assert_eq!(
            server.accept_attempts(),
            armed,
            "a silent health listener must not wake or re-accept"
        );
        assert_eq!(
            server.shed_count(),
            0,
            "a silent health listener sheds nothing"
        );

        // Route behaviour unchanged: a real request is still served after silence.
        let response = get(server.local_addr(), "/health")?;
        assert_status(&response, 200);
        let body = json_body(&response)?;
        assert_eq!(body["status"], "healthy");

        server.shutdown()?;
        Ok(())
    }

    /// Oracle 9 (W4 leg 2) — absence proof over this module's production source:
    /// the retired non-blocking flip and its `WouldBlock` + sleep poll must not
    /// appear in the health accept path.
    #[test]
    fn health_accept_source_has_no_wouldblock_sleep_poll() {
        const SOURCE: &str = include_str!("endpoint.rs");
        let production = SOURCE.split("mod tests").next().unwrap_or(SOURCE);
        for forbidden in [
            "set_nonblocking(true)",
            "ErrorKind::WouldBlock",
            "thread::sleep",
        ] {
            assert!(
                !production.contains(forbidden),
                "retired health accept-path source `{forbidden}` reappeared"
            );
        }
    }

    /// Oracle 10 (W4 leg 2) — shutdown interrupts the blocking accept wait at
    /// every race point: before the worker arms, after it parks, concurrent with
    /// a pending connection, and after an accept returns. Each shutdown returns
    /// promptly (no sleep-poll) and joins cleanly (no worker leak); the released
    /// listener refuses further connects (no descriptor leak).
    #[test]
    fn health_shutdown_interrupts_accept_wait() -> Result<(), Box<dyn std::error::Error>> {
        // (a) shutdown immediately after start — possibly before the worker arms.
        let server = start_health_server(loopback_ephemeral()?, SharedReadinessState::default())?;
        let start = Instant::now();
        server.shutdown()?;
        assert!(
            start.elapsed() < Duration::from_secs(2),
            "shutdown before arming must interrupt promptly, not sleep-poll"
        );

        // (b) shutdown after the worker has parked the blocking accept.
        let server = start_health_server(loopback_ephemeral()?, SharedReadinessState::default())?;
        let deadline = Instant::now() + Duration::from_secs(2);
        while server.accept_attempts() < 1 && Instant::now() < deadline {
            thread::sleep(Duration::from_millis(5));
        }
        assert_eq!(
            server.accept_attempts(),
            1,
            "the worker parked before shutdown"
        );
        let parked_addr = server.local_addr();
        let start = Instant::now();
        server.shutdown()?;
        assert!(
            start.elapsed() < Duration::from_secs(2),
            "shutdown of a parked accept must interrupt promptly"
        );
        // No descriptor leak: the released listener refuses further connects.
        assert!(
            TcpStream::connect(parked_addr).is_err(),
            "the listener descriptor was released; further connects are refused"
        );

        // (c) shutdown concurrent with accept readiness: a client is pending.
        // Whether the worker is still parked in `accept` or has already accepted
        // and is blocked reading the silent client, shutdown interrupts promptly
        // (self-connect for the parked case, in-flight stream shutdown for the
        // reading case) — deterministically under the 2s read deadline. The bound
        // is tightened to 500 ms to reflect the TOLD interrupt: it must not
        // approach the read window that the pre-fix bytes deferred to (see
        // `shutdown_interrupts_in_flight_silent_request_read` for the dedicated
        // mid-read regression).
        let server = start_health_server(loopback_ephemeral()?, SharedReadinessState::default())?;
        let deadline = Instant::now() + Duration::from_secs(2);
        while server.accept_attempts() < 1 && Instant::now() < deadline {
            thread::sleep(Duration::from_millis(5));
        }
        let _pending = TcpStream::connect(server.local_addr())?;
        let start = Instant::now();
        server.shutdown()?;
        assert!(
            start.elapsed() < Duration::from_millis(500),
            "shutdown concurrent with a pending accept must interrupt promptly (TOLD), \
             not defer by a request read deadline: elapsed {:?}",
            start.elapsed()
        );

        // (d) shutdown after an accept returns and a request is served.
        let server = start_health_server(loopback_ephemeral()?, SharedReadinessState::default())?;
        let response = get(server.local_addr(), "/health")?;
        assert_status(&response, 200);
        let start = Instant::now();
        server.shutdown()?;
        assert!(
            start.elapsed() < Duration::from_secs(2),
            "shutdown after a served request must interrupt the next parked accept promptly"
        );

        Ok(())
    }

    /// Regression for oracle 10 case (c) (W4 leg 2 — tear-seat BOUNCE): an
    /// in-flight SILENT request must not defer shutdown by its read window. A
    /// client that connects and sends nothing parks the worker inside
    /// `handle_connection`'s blocking read (the admitted 2s slow-client
    /// deadline). Shutdown must interrupt that read directly (TOLD stream
    /// interrupt), NOT wait for the deadline to expire.
    ///
    /// The promptness bound is inherently a timing assertion: it is set to
    /// 500 ms — comfortably under the 2s read deadline (so the pre-fix bytes,
    /// where the self-connect interrupt merely queues behind the blocked read,
    /// red deterministically) and comfortably over scheduler-wakeup noise even
    /// under full-workspace parallel load (so the post-fix stream interrupt
    /// greens deterministically). The worker's entry into the read is observed
    /// via the `requests_entered` counter, not a sleep, so the race is caught
    /// deterministically mid-request.
    #[test]
    fn shutdown_interrupts_in_flight_silent_request_read() -> Result<(), Box<dyn std::error::Error>>
    {
        let server = start_health_server(loopback_ephemeral()?, SharedReadinessState::default())?;

        // Connect but send NOTHING and keep the socket open: the worker accepts
        // and blocks in handle_connection's read on the silent stream. The named
        // binding keeps the client alive (a bare `_` would drop it and end the
        // read early with EOF).
        let _silent = TcpStream::connect(server.local_addr())?;

        // Observe the worker has entered the in-flight request read (counter, not
        // a sleep) so shutdown is fired while it is genuinely blocked on the read.
        let deadline = Instant::now() + Duration::from_secs(2);
        while server.requests_entered() < 1 && Instant::now() < deadline {
            thread::sleep(Duration::from_millis(5));
        }
        assert_eq!(
            server.requests_entered(),
            1,
            "the worker entered the in-flight silent request read"
        );

        let start = Instant::now();
        server.shutdown()?;
        assert!(
            start.elapsed() < Duration::from_millis(500),
            "shutdown must interrupt an in-flight silent request read promptly (TOLD), \
             not defer by the read's admitted 2s deadline: elapsed {:?}",
            start.elapsed()
        );

        Ok(())
    }

    /// Oracle 11 (W4 leg 2, idle-honesty both-sides) — an unrelated served
    /// request grows the BUSY listener's accept-attempt counter while the silent
    /// listener's accept-attempt counter stays FLAT during the workload. The
    /// growing side proves the fixture cannot pass by hiding the workload (a
    /// frozen harness would leave the busy counter flat and fail); the flat side
    /// proves genuine silence rather than a global freeze.
    #[test]
    fn health_idle_grows_unrelated_counters_while_accept_stays_flat()
    -> Result<(), Box<dyn std::error::Error>> {
        let idle = start_health_server(loopback_ephemeral()?, SharedReadinessState::default())?;
        let busy = start_health_server(loopback_ephemeral()?, SharedReadinessState::default())?;

        let deadline = Instant::now() + Duration::from_secs(2);
        while (idle.accept_attempts() < 1 || busy.accept_attempts() < 1)
            && Instant::now() < deadline
        {
            thread::sleep(Duration::from_millis(5));
        }
        let idle_armed = idle.accept_attempts();
        assert_eq!(idle_armed, 1, "the idle listener parks exactly one accept");
        let busy_before = busy.accept_attempts();

        // Unrelated served workload on the BUSY listener: each served request
        // returns the parked accept and re-parks a fresh one, growing its counter.
        for _ in 0..5 {
            let response = get(busy.local_addr(), "/health")?;
            assert_status(&response, 200);
        }
        let deadline = Instant::now() + Duration::from_secs(2);
        while busy.accept_attempts() <= busy_before && Instant::now() < deadline {
            thread::sleep(Duration::from_millis(5));
        }

        assert!(
            busy.accept_attempts() > busy_before,
            "an unrelated served request grows the busy listener's accept counter"
        );
        assert_eq!(
            idle.accept_attempts(),
            idle_armed,
            "the silent listener's accept counter stays flat during the workload"
        );
        assert_eq!(idle.shed_count(), 0, "the silent listener sheds nothing");

        idle.shutdown()?;
        busy.shutdown()?;
        Ok(())
    }
}