//! Application-owned QUIC endpoint and connection lifecycles.
//!
//! s2n-quic's default Tokio IO adapter discards the endpoint task handle. This
//! module keeps that handle and gives the application an explicit stop signal,
//! so a peer runtime does not report itself stopped while its QUIC endpoint is
//! still running.

use std::{
    future::Future as _,
    io,
    net::SocketAddr,
    ops::{Deref, DerefMut},
    pin::Pin,
    sync::{Arc, Mutex},
    task::{Context, Poll},
};

use lanspread_proto::{MAX_CONTROL_FRAME_BYTES, PeerEndpoint};
use s2n_quic::{
    Client as QuicClient,
    Connection,
    client::Connect,
    provider::{
        congestion_controller,
        io::{Provider as IoProvider, tokio::Provider as TokioIo},
        limits::Limits,
    },
};
use s2n_quic_core::{
    endpoint::{self, CloseError, Endpoint},
    inet::SocketAddress,
    io::{rx, tx},
    path::mtu,
    time::{Clock, Timestamp},
};
use tokio::task::{JoinError, JoinHandle};
use tokio_util::sync::{CancellationToken, WaitForCancellationFutureOwned};

use crate::{
    config::{
        QUIC_CONNECTION_DATA_WINDOW,
        QUIC_ENDPOINT_SHUTDOWN_GRACE,
        QUIC_HANDSHAKE_TIMEOUT,
        QUIC_IDLE_TIMEOUT,
        QUIC_INITIAL_CONGESTION_WINDOW,
        QUIC_MAX_SEND_BUFFER_SIZE,
        QUIC_SOCKET_BUFFER_SIZE,
        QUIC_STREAM_DATA_WINDOW,
    },
    tls,
};

/// Transport-level bound on simultaneously open bidirectional streams in
/// either direction. The server independently enforces the same application
/// control-stream task bound before spawning work.
pub(crate) const MAX_OPEN_BIDIRECTIONAL_STREAMS: u64 = 32;
/// One server connection exposes only enough aggregate receive credit for one
/// maximal length-delimited control frame at a time.
pub(crate) const SERVER_CONTROL_RECEIVE_WINDOW: u64 = MAX_CONTROL_FRAME_BYTES as u64 + 4;

pub(crate) fn quic_client_limits() -> eyre::Result<Limits> {
    Ok(Limits::default()
        .with_data_window(QUIC_CONNECTION_DATA_WINDOW)?
        .with_bidirectional_local_data_window(QUIC_STREAM_DATA_WINDOW)?
        .with_bidirectional_remote_data_window(0)?
        // The application protocol never opens or accepts unidirectional
        // streams, so expose neither stream IDs nor receive-window state.
        .with_unidirectional_data_window(0)?
        .with_max_open_local_bidirectional_streams(MAX_OPEN_BIDIRECTIONAL_STREAMS)?
        .with_max_open_remote_bidirectional_streams(0)?
        .with_max_open_local_unidirectional_streams(0)?
        .with_max_open_remote_unidirectional_streams(0)?
        .with_max_send_buffer_size(QUIC_MAX_SEND_BUFFER_SIZE)?
        .with_max_handshake_duration(QUIC_HANDSHAKE_TIMEOUT)?
        .with_max_idle_timeout(QUIC_IDLE_TIMEOUT)?)
}

/// Server-specific transport limits for public remote-initiated control
/// streams. Large client receive windows remain available only on the
/// separately configured outbound connector.
pub(crate) fn quic_server_limits() -> eyre::Result<Limits> {
    Ok(Limits::default()
        .with_data_window(SERVER_CONTROL_RECEIVE_WINDOW)?
        .with_bidirectional_local_data_window(0)?
        .with_bidirectional_remote_data_window(SERVER_CONTROL_RECEIVE_WINDOW)?
        .with_unidirectional_data_window(0)?
        .with_max_open_local_bidirectional_streams(0)?
        .with_max_open_remote_bidirectional_streams(MAX_OPEN_BIDIRECTIONAL_STREAMS)?
        .with_max_open_local_unidirectional_streams(0)?
        .with_max_open_remote_unidirectional_streams(0)?
        .with_max_send_buffer_size(
            u32::try_from(MAX_CONTROL_FRAME_BYTES)
                .map_err(|_| eyre::eyre!("control frame bound exceeds u32"))?,
        )?
        .with_max_handshake_duration(QUIC_HANDSHAKE_TIMEOUT)?
        .with_max_idle_timeout(QUIC_IDLE_TIMEOUT)?)
}

pub(crate) fn quic_congestion_controller() -> congestion_controller::Bbr {
    congestion_controller::bbr::Builder::default()
        .with_initial_congestion_window(QUIC_INITIAL_CONGESTION_WINDOW)
        .build()
}

/// Builds an IO provider together with the owner used to retrieve its endpoint
/// task after s2n-quic has started it.
pub(crate) fn tracked_quic_io(
    addr: SocketAddr,
) -> eyre::Result<(TrackedIoProvider, EndpointTaskControl)> {
    let inner = s2n_quic::provider::io::tokio::Builder::default()
        .with_receive_address(addr)?
        .with_send_buffer_size(QUIC_SOCKET_BUFFER_SIZE)?
        .with_recv_buffer_size(QUIC_SOCKET_BUFFER_SIZE)?
        .build()?;
    let control = EndpointTaskControl::new();
    Ok((
        TrackedIoProvider {
            inner,
            control: control.clone(),
        },
        control,
    ))
}

/// Tokio IO provider that retains the s2n endpoint task instead of discarding
/// its handle like the default adapter does.
pub(crate) struct TrackedIoProvider {
    inner: TokioIo,
    control: EndpointTaskControl,
}

impl IoProvider for TrackedIoProvider {
    type PathHandle = <TokioIo as IoProvider>::PathHandle;
    type Error = io::Error;

    fn start<E: Endpoint<PathHandle = Self::PathHandle>>(
        self,
        endpoint: E,
    ) -> Result<SocketAddress, Self::Error> {
        // Lock before starting the endpoint. That way a poisoned/invalid slot
        // cannot cause a newly spawned endpoint task to escape unowned.
        let mut task_slot = self.control.lock_task_slot()?;
        if task_slot.is_some() {
            return Err(io::Error::other("QUIC endpoint task was already started"));
        }

        let endpoint = ControlledEndpoint::new(endpoint, self.control.stop.clone());
        let (task, local_addr) = self.inner.start(endpoint)?;
        *task_slot = Some(task);
        Ok(local_addr)
    }
}

#[derive(Clone)]
pub(crate) struct EndpointTaskControl {
    stop: CancellationToken,
    task: Arc<EndpointTaskSlot>,
}

struct EndpointTaskSlot(Mutex<Option<JoinHandle<()>>>);

impl Drop for EndpointTaskSlot {
    fn drop(&mut self) {
        let task = self
            .0
            .get_mut()
            .unwrap_or_else(std::sync::PoisonError::into_inner)
            .take();
        if let Some(task) = task {
            // A successful pinned s2n builder returns immediately after IO
            // start, so normal code always transfers this handle into
            // EndpointTask. This is only a synchronous panic-safety fallback;
            // the peer supervisor destroys and joins the isolated Tokio
            // runtime before its public owner may return.
            log::error!("QUIC endpoint started but ownership was not transferred; aborting it");
            task.abort();
        }
    }
}

impl EndpointTaskControl {
    fn new() -> Self {
        Self {
            stop: CancellationToken::new(),
            task: Arc::new(EndpointTaskSlot(Mutex::new(None))),
        }
    }

    fn lock_task_slot(&self) -> io::Result<std::sync::MutexGuard<'_, Option<JoinHandle<()>>>> {
        self.task
            .0
            .lock()
            .map_err(|_| io::Error::other("QUIC endpoint task slot was poisoned"))
    }

    /// Takes ownership of the endpoint task after a successful s2n-quic start.
    pub(crate) fn take_started(&self) -> eyre::Result<EndpointTask> {
        let task = self
            .lock_task_slot()?
            .take()
            .ok_or_else(|| eyre::eyre!("QUIC IO provider did not start an endpoint task"))?;
        Ok(EndpointTask {
            stop: self.stop.clone(),
            task: Some(task),
            abort_requested: false,
        })
    }
}

struct ControlledEndpoint<E> {
    inner: E,
    stop: Pin<Box<WaitForCancellationFutureOwned>>,
}

impl<E> ControlledEndpoint<E> {
    fn new(inner: E, stop: CancellationToken) -> Self {
        Self {
            inner,
            stop: Box::pin(stop.cancelled_owned()),
        }
    }
}

impl<E: Endpoint> Endpoint for ControlledEndpoint<E> {
    type PathHandle = E::PathHandle;
    type Subscriber = E::Subscriber;

    const ENDPOINT_TYPE: endpoint::Type = E::ENDPOINT_TYPE;

    fn receive<Rx, C>(&mut self, queue: &mut Rx, clock: &C)
    where
        Rx: rx::Queue<Handle = Self::PathHandle>,
        C: Clock,
    {
        self.inner.receive(queue, clock);
    }

    fn transmit<Tx, C>(&mut self, queue: &mut Tx, clock: &C)
    where
        Tx: tx::Queue<Handle = Self::PathHandle>,
        C: Clock,
    {
        self.inner.transmit(queue, clock);
    }

    fn poll_wakeups<C: Clock>(
        &mut self,
        cx: &mut Context<'_>,
        clock: &C,
    ) -> Poll<Result<usize, CloseError>> {
        if self.stop.as_mut().poll(cx).is_ready() {
            return Poll::Ready(Err(CloseError));
        }
        self.inner.poll_wakeups(cx, clock)
    }

    fn timeout(&self) -> Option<Timestamp> {
        self.inner.timeout()
    }

    fn set_mtu_config(&mut self, config: mtu::Config) {
        self.inner.set_mtu_config(config);
    }

    fn subscriber(&mut self) -> &mut Self::Subscriber {
        self.inner.subscriber()
    }
}

/// Application-owned endpoint task.
///
/// Every normal owner path consumes this value through [`Self::shutdown_and_join`].
/// Drop is only a panic-safety fallback: it prevents continued endpoint work,
/// while the peer supervisor's final runtime teardown bounds and destroys the
/// aborted task before `PeerRuntimeHandle` may finish joining.
#[must_use = "the QUIC endpoint task must be explicitly stopped and joined"]
pub(crate) struct EndpointTask {
    stop: CancellationToken,
    task: Option<JoinHandle<()>>,
    abort_requested: bool,
}

impl EndpointTask {
    pub(crate) async fn shutdown_and_join(mut self) -> eyre::Result<()> {
        self.shutdown_and_join_with_grace(QUIC_ENDPOINT_SHUTDOWN_GRACE)
            .await
    }

    async fn shutdown_and_join_with_grace(
        &mut self,
        grace: std::time::Duration,
    ) -> eyre::Result<()> {
        self.stop.cancel();
        let Some(task) = self.task.as_mut() else {
            eyre::bail!("QUIC endpoint task was already joined");
        };

        // Keep the handle in `self` until every await has completed. If the
        // caller cancels this cleanup future, dropping `EndpointTask` can then
        // still abort the endpoint instead of silently detaching a handle that
        // was moved into the cancelled future's local state.
        let result = if let Ok(result) = tokio::time::timeout(grace, &mut *task).await {
            join_result(result, self.abort_requested)
        } else {
            log::warn!("QUIC endpoint did not stop within {grace:?}; aborting and joining it");
            self.abort_requested = true;
            task.abort();
            join_result(task.await, true)
        };

        // A completed Tokio JoinHandle is inert. Remove it only after the
        // terminal await so `Drop` remains a cancellation-safety guard for the
        // whole shutdown operation.
        self.task = None;
        result
    }
}

impl Drop for EndpointTask {
    fn drop(&mut self) {
        if let Some(task) = self.task.take() {
            log::error!(
                "QUIC endpoint owner was dropped before join; aborting as panic-safety fallback"
            );
            self.stop.cancel();
            // The task remains owned by the isolated peer Tokio runtime. Its
            // supervisor thread cannot finish (and its public handle cannot
            // return from Drop/wait) until that runtime has been destroyed.
            task.abort();
        }
    }
}

fn join_result(result: Result<(), JoinError>, expected_abort: bool) -> eyre::Result<()> {
    match result {
        Ok(()) => Ok(()),
        Err(error) if expected_abort && error.is_cancelled() => Ok(()),
        Err(error) => Err(eyre::eyre!("QUIC endpoint task failed to join: {error}")),
    }
}

/// Cloneable capability for opening connections on the runtime-owned client
/// endpoint.
#[derive(Clone, Debug)]
pub(crate) struct QuicConnector {
    client: Option<QuicClient>,
}

impl QuicConnector {
    pub(crate) async fn connect(&self, endpoint: &PeerEndpoint) -> eyre::Result<PeerConnection> {
        let client = self
            .client
            .as_ref()
            .ok_or_else(|| eyre::eyre!("QUIC connector is unavailable"))?;
        let server_name = tls::sni_for_peer(endpoint.peer_id)?;
        let connect = Connect::new(endpoint.addr).with_server_name(server_name);
        Ok(PeerConnection::new(client.connect(connect).await?))
    }

    #[cfg(test)]
    pub(crate) fn unavailable() -> Self {
        Self { client: None }
    }
}

/// Runtime owner for the single outgoing client endpoint.
pub(crate) struct QuicClientRuntime {
    client: QuicClient,
    endpoint: EndpointTask,
}

impl QuicClientRuntime {
    /// Waits for outstanding connections to settle, then always stops and
    /// joins the endpoint. A stuck transport can consume the grace period, but
    /// cannot make peer shutdown unbounded.
    pub(crate) async fn shutdown(mut self) -> eyre::Result<()> {
        let idle_result =
            match tokio::time::timeout(QUIC_ENDPOINT_SHUTDOWN_GRACE, self.client.wait_idle()).await
            {
                Ok(result) => result.map_err(eyre::Report::from),
                Err(_) => Err(eyre::eyre!(
                    "QUIC client did not become idle within {QUIC_ENDPOINT_SHUTDOWN_GRACE:?}"
                )),
            };

        // No application connector remains after the runtime's child scopes
        // have drained. Drop this final handle before forcing endpoint stop.
        drop(self.client);
        let endpoint_result = self.endpoint.shutdown_and_join().await;

        match (idle_result, endpoint_result) {
            (Ok(()), Ok(())) => Ok(()),
            (Err(error), Ok(())) | (Ok(()), Err(error)) => Err(error),
            (Err(idle_error), Err(endpoint_error)) => Err(eyre::eyre!(
                "QUIC client shutdown failed: {idle_error:#}; endpoint shutdown also failed: {endpoint_error:#}"
            )),
        }
    }

    /// Settles the test-only hostile-handshake fixture after the verifier has
    /// already proved that connection establishment was rejected.
    ///
    /// A failed TLS `CertificateVerify` can remain in s2n-quic's endpoint-owned
    /// closing state beyond the normal wait-idle grace. This seam is not a
    /// general timeout bypass: it drops the final client handle, then still
    /// explicitly stops and joins the endpoint task.
    #[cfg(test)]
    pub(crate) async fn shutdown_rejected_handshake_fixture(self) -> eyre::Result<()> {
        drop(self.client);
        self.endpoint.shutdown_and_join().await
    }
}

pub(crate) fn start_quic_client() -> eyre::Result<(QuicClientRuntime, QuicConnector)> {
    let (io, endpoint_control) = tracked_quic_io(SocketAddr::from(([0, 0, 0, 0], 0)))?;
    let client = QuicClient::builder()
        .with_tls(tls::client_provider()?)?
        .with_io(io)?
        .with_limits(quic_client_limits()?)?
        .with_congestion_controller(quic_congestion_controller())?
        .start()?;
    let endpoint = endpoint_control.take_started()?;
    let connector = QuicConnector {
        client: Some(client.clone()),
    };

    Ok((QuicClientRuntime { client, endpoint }, connector))
}

/// Connection whose lexical owner always initiates QUIC close before dropping
/// the handle, including when a request future is cancelled or times out.
pub(crate) struct PeerConnection {
    connection: Connection,
}

impl PeerConnection {
    fn new(connection: Connection) -> Self {
        Self { connection }
    }
}

impl Deref for PeerConnection {
    type Target = Connection;

    fn deref(&self) -> &Self::Target {
        &self.connection
    }
}

impl DerefMut for PeerConnection {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.connection
    }
}

impl Drop for PeerConnection {
    fn drop(&mut self) {
        self.connection.close(0u32.into());
    }
}

#[cfg(test)]
mod tests {
    use std::{sync::Arc, time::Duration};

    use tokio::sync::Notify;
    use tokio_util::sync::CancellationToken;

    use super::{EndpointTask, EndpointTaskControl, start_quic_client};

    #[test]
    fn endpoint_control_rejects_take_before_provider_start() {
        let control = EndpointTaskControl::new();
        assert!(control.take_started().is_err());
    }

    #[tokio::test]
    async fn client_start_captures_an_endpoint_that_can_be_joined() {
        let (runtime, connector) = start_quic_client().expect("client endpoint should start");
        drop(connector);

        tokio::time::timeout(Duration::from_secs(2), runtime.shutdown())
            .await
            .expect("client endpoint shutdown should be bounded")
            .expect("client endpoint should join cleanly");
    }

    #[tokio::test]
    async fn endpoint_owner_signals_and_joins_its_task() {
        let stop = CancellationToken::new();
        let stopped = Arc::new(Notify::new());
        let task_stop = stop.clone();
        let task_stopped = stopped.clone();
        let task = tokio::spawn(async move {
            task_stop.cancelled().await;
            task_stopped.notify_one();
        });
        let owner = EndpointTask {
            stop,
            task: Some(task),
            abort_requested: false,
        };

        owner
            .shutdown_and_join()
            .await
            .expect("synthetic endpoint should join");
        tokio::time::timeout(Duration::from_secs(1), stopped.notified())
            .await
            .expect("endpoint cleanup should happen before owner returns");
    }

    #[tokio::test]
    async fn endpoint_owner_aborts_but_still_joins_uncooperative_task() {
        let stop = CancellationToken::new();
        let dropped = Arc::new(Notify::new());
        let task_dropped = dropped.clone();
        let task = tokio::spawn(async move {
            struct DropNotice(Arc<Notify>);
            impl Drop for DropNotice {
                fn drop(&mut self) {
                    self.0.notify_one();
                }
            }
            let _drop_notice = DropNotice(task_dropped);
            std::future::pending::<()>().await;
        });
        let owner = EndpointTask {
            stop,
            task: Some(task),
            abort_requested: false,
        };

        let mut owner = owner;
        owner
            .shutdown_and_join_with_grace(Duration::from_millis(10))
            .await
            .expect("aborted endpoint should still join");
        tokio::time::timeout(Duration::from_secs(1), dropped.notified())
            .await
            .expect("aborted endpoint should finish unwinding before owner returns");
    }

    #[tokio::test]
    async fn cancelling_endpoint_cleanup_retains_task_for_retry() {
        let stop = CancellationToken::new();
        let started = Arc::new(Notify::new());
        let dropped = Arc::new(Notify::new());
        let task_started = started.clone();
        let task_dropped = dropped.clone();
        let task = tokio::spawn(async move {
            struct DropNotice(Arc<Notify>);
            impl Drop for DropNotice {
                fn drop(&mut self) {
                    self.0.notify_one();
                }
            }

            let _drop_notice = DropNotice(task_dropped);
            task_started.notify_one();
            std::future::pending::<()>().await;
        });
        let mut owner = EndpointTask {
            stop,
            task: Some(task),
            abort_requested: false,
        };

        started.notified().await;
        let mut cleanup = Box::pin(owner.shutdown_and_join_with_grace(Duration::from_secs(10)));
        assert!(
            tokio::time::timeout(Duration::from_millis(10), cleanup.as_mut())
                .await
                .is_err(),
            "synthetic endpoint should still be awaiting its grace period"
        );

        drop(cleanup);
        assert!(
            owner.task.is_some(),
            "cancelling cleanup must leave the JoinHandle with its owner"
        );
        owner
            .shutdown_and_join_with_grace(Duration::from_millis(10))
            .await
            .expect("the owner should be able to retry and join after cancellation");

        tokio::time::timeout(Duration::from_secs(1), dropped.notified())
            .await
            .expect("retried cleanup must join the task retained by its owner");
    }
}