betex 0.13.0

use super::error::EngineError;

use crate::{
    book::{
        BookEvent, BookEventEnvelope,
        protocol::{
            command::{Command, CommandKind},
            reject::RejectReason,
            response::Response,
        },
    },
    config::{Config, ThreadPinning},
    disruptor::{Envelope, ResponseCallback, RingSlot},
    engine::{
        journaler::JournalHandler,
        root::EngineRoot,
        state::{EngineState, Market, MarketConfig},
    },
    error::RuntimeError,
    types::{MarketId, MarketKind},
};

use std::{
    collections::VecDeque,
    marker::PhantomData,
    path::{Path, PathBuf},
    sync::{
        Arc,
        atomic::{AtomicBool, Ordering},
    },
    thread,
    time::{Duration, Instant},
};

use disrupt_rs::{
    BusySpin, EventPoller, GatedSequence, MultiConsumerDependentsBarrier, Producer,
    SingleProducerBarrier, build_single_producer,
    builder::{MC, NC, SC, single::SPBuilder},
    wait_strategies::WaitStrategy,
};
use tracing::{error, info, trace, warn};

mod tuple;

pub use tuple::HandlerRecovery;
#[doc(hidden)]
pub use tuple::RegisteredHandler;

/// Type alias for engine events (what consumers receive)
pub type EngineEvent = RingSlot<BookEventEnvelope>;
type NcBuilder<W> = SPBuilder<NC, EngineEvent, W, MultiConsumerDependentsBarrier>;
type ScBuilder<W> = SPBuilder<SC, EngineEvent, W, MultiConsumerDependentsBarrier>;
type McBuilder<W> = SPBuilder<MC, EngineEvent, W, MultiConsumerDependentsBarrier>;

/// Builder for constructing an Engine with explicit startup recovery policy for every handler.
pub struct EngineBuilder<B, W = BusySpin, Hs = ()>
where
    W: WaitStrategy + Clone + Send + 'static,
{
    config: Config,
    root: EngineRoot,
    wal_path: PathBuf,
    registered_handlers: Hs,
    shutdown: Arc<AtomicBool>,
    metrics: crate::metrics::Metrics,
    pinning: ThreadPinning,
    wait_strategy: W,
    _builder_state: PhantomData<fn() -> B>,
}

impl EngineBuilder<NcBuilder<BusySpin>, BusySpin> {
    pub fn with_markets(
        config: Config,
        markets: impl IntoIterator<Item = MarketConfig>,
        wal_path: impl AsRef<Path>,
    ) -> anyhow::Result<Self> {
        let mut state = EngineState::new(config.clone());
        for market_config in markets {
            let market_id = market_config.market_id();
            state.markets.insert(
                market_id,
                Market::from_config_with_cfg(&config, market_config),
            );
        }
        let root = EngineRoot::new_with(state, 0);
        Self::build_inner(root, config, wal_path)
    }

    pub fn new(
        config: Config,
        market_ids: impl IntoIterator<Item = MarketId>,
        wal_path: impl AsRef<Path>,
    ) -> anyhow::Result<Self> {
        let markets = market_ids
            .into_iter()
            .map(|mid| MarketConfig::multi_runner_dynamic(mid, MarketKind::InPlayCapable));
        Self::with_markets(config, markets, wal_path)
    }

    pub fn from_root(
        initial_root: EngineRoot,
        config: Config,
        wal_path: impl AsRef<Path>,
    ) -> anyhow::Result<Self> {
        Self::build_inner(initial_root, config, wal_path)
    }

    fn build_inner(
        root: EngineRoot,
        config: Config,
        wal_path: impl AsRef<Path>,
    ) -> anyhow::Result<Self> {
        crate::btx_metrics_init!(
            "engine",
            "wal_poller",
            "projection_apply",
            "projection_live",
            "projection_snapshot",
            "projection_disk",
        );
        let metrics = crate::metrics::globals::metrics();
        let pinning = config.pinning.clone().unwrap_or_default();

        Ok(EngineBuilder {
            config,
            root,
            wal_path: wal_path.as_ref().to_path_buf(),
            registered_handlers: (),
            shutdown: Arc::new(AtomicBool::new(false)),
            metrics,
            pinning,
            wait_strategy: BusySpin,
            _builder_state: PhantomData,
        })
    }
}

impl<B, W, R> EngineBuilder<B, W, R>
where
    W: WaitStrategy + Clone + Send + 'static,
{
    fn map_handlers<B2, R2>(self, map: impl FnOnce(R) -> R2) -> EngineBuilder<B2, W, R2> {
        let EngineBuilder {
            config,
            root,
            wal_path,
            registered_handlers,
            shutdown,
            metrics,
            pinning,
            wait_strategy,
            _builder_state: _,
        } = self;

        EngineBuilder {
            config,
            root,
            wal_path,
            registered_handlers: map(registered_handlers),
            shutdown,
            metrics,
            pinning,
            wait_strategy,
            _builder_state: PhantomData,
        }
    }

    fn set_wait_strategy<B2, W2>(self, wait_strategy: W2) -> EngineBuilder<B2, W2, R>
    where
        W2: WaitStrategy + Clone + Send + 'static,
    {
        EngineBuilder {
            config: self.config,
            root: self.root,
            wal_path: self.wal_path,
            registered_handlers: self.registered_handlers,
            shutdown: self.shutdown,
            metrics: self.metrics,
            pinning: self.pinning,
            wait_strategy,
            _builder_state: PhantomData,
        }
    }
}

impl<W, R> EngineBuilder<NcBuilder<W>, W, R>
where
    W: WaitStrategy + Clone + Send + 'static,
{
    pub fn with_wait_strategy<W2>(self, wait_strategy: W2) -> EngineBuilder<NcBuilder<W2>, W2, R>
    where
        W2: WaitStrategy + Clone + Send + 'static,
    {
        self.set_wait_strategy(wait_strategy)
    }
}

#[allow(clippy::type_complexity)]
fn build_disruptor_pipeline<W>(
    config: &Config,
    wait_strategy: W,
) -> (
    EventPoller<EngineEvent, SingleProducerBarrier, W::Notifier>,
    NcBuilder<W>,
    GatedSequence<W::Notifier>,
)
where
    W: WaitStrategy,
{
    let (wal_poller, builder) = build_single_producer(
        config.ring_size_pow2,
        || {
            RingSlot::new(Envelope {
                seq: 0,
                payload: BookEventEnvelope {
                    market_id: MarketId(0),
                    market_name: String::new(),
                    market_seq: 0,
                    timestamp: crate::types::unix_epoch(),
                    metadata: crate::book::EventMetadata::default(),
                    event: BookEvent::MarketStateChanged {
                        to: crate::book::common::types::BookMarketState::Open,
                        reason: String::new(),
                    },
                },
                response_cb: None,
                tx_id: 0,
                tx_len: 1,
                tx_ix: 0,
            })
        },
        wait_strategy,
    )
    .event_poller();

    let (builder, gates) = builder.new_gates::<1>();
    let durability_gate = gates[0].clone();
    let builder = builder.and_then_with_dependents(&gates);
    (wal_poller, builder, durability_gate)
}

struct EngineBuildCtx<W: WaitStrategy> {
    root: EngineRoot,
    wal_poller: EventPoller<EngineEvent, SingleProducerBarrier, W::Notifier>,
    journaler: JournalHandler,
    durability_gate: GatedSequence<W::Notifier>,
    shutdown: Arc<AtomicBool>,
    metrics: crate::metrics::Metrics,
    pinning: ThreadPinning,
    wait_strategy: W,
}

/// Shared engine construction logic.
fn build_engine<B, W>(
    producer: disrupt_rs::SingleProducer<EngineEvent, B, W>,
    ctx: EngineBuildCtx<W>,
) -> anyhow::Result<Engine>
where
    B: disrupt_rs::Barrier + 'static,
    W: WaitStrategy + Clone + Send + 'static,
{
    let EngineBuildCtx {
        root,
        wal_poller,
        journaler,
        durability_gate,
        shutdown,
        metrics,
        pinning,
        wait_strategy,
    } = ctx;

    let wal_thread = journaler
        .into_poller(
            wal_poller,
            wait_strategy.clone(),
            durability_gate,
            Arc::clone(&shutdown),
        )
        .poll_pinned(pinning.wal_poller_core);

    let (cmd_tx, cmd_rx) =
        flume::bounded::<EngineMsg>(crate::config::DEFAULT_ENGINE_CHANNEL_CAPACITY);
    let handler = EngineHandler { tx: cmd_tx };

    let engine_thread = {
        let shutdown = Arc::clone(&shutdown);
        let core_id = pinning.engine_core;
        thread::spawn(move || {
            if let Some(id) = core_id
                && !core_affinity::set_for_current(core_affinity::CoreId { id })
            {
                warn!(
                    core_id = id,
                    "failed to pin engine thread to requested core"
                );
            }
            crate::metrics_stage!("engine");
            run_engine_loop(root, producer, cmd_rx, shutdown);
        })
    };

    info!("engine threads started");
    Ok(Engine {
        handler,
        metrics,
        shutdown,
        engine_thread: Some(engine_thread),
        wal_thread: Some(wal_thread),
    })
}

pub struct Engine {
    handler: EngineHandler,
    metrics: crate::metrics::Metrics,
    shutdown: Arc<AtomicBool>,
    engine_thread: Option<thread::JoinHandle<()>>,
    wal_thread: Option<thread::JoinHandle<anyhow::Result<()>>>,
}

impl Engine {
    pub fn handle(&self) -> EngineHandler {
        self.handler.clone()
    }

    pub fn metrics_snapshot(&self) -> crate::metrics::MetricsSnapshot {
        self.metrics.snapshot()
    }

    pub fn metrics(&self) -> crate::metrics::Metrics {
        self.metrics.clone()
    }

    pub fn stop(&mut self) -> anyhow::Result<()> {
        // Ensure all background threads observe shutdown even if senders/producers are still alive.
        self.shutdown.store(true, Ordering::Release);

        let Some(engine_thread) = self.engine_thread.take() else {
            if let Some(wal_thread) = self.wal_thread.take()
                && wal_thread.join().is_err()
            {
                error!("wal thread panicked");
                return Err(RuntimeError::WalThreadPanic.into());
            }
            return Ok(());
        };

        if engine_thread.join().is_err() {
            error!("engine thread panicked");
        }
        if let Some(wal_thread) = self.wal_thread.take() {
            match wal_thread.join() {
                Ok(Ok(())) => {}
                Ok(Err(e)) => {
                    error!(error = ?e, "wal thread returned error");
                    return Err(e);
                }
                Err(_) => {
                    error!("wal thread panicked");
                    return Err(RuntimeError::WalThreadPanic.into());
                }
            }
        }

        Ok(())
    }
}

fn run_engine_loop<B, W>(
    mut root: EngineRoot,
    mut producer: disrupt_rs::SingleProducer<EngineEvent, B, W>,
    cmd_rx: flume::Receiver<EngineMsg>,
    shutdown: Arc<AtomicBool>,
) where
    B: disrupt_rs::Barrier,
    W: WaitStrategy,
{
    // Round-robin cursor so multiple in-progress batch processes make progress.
    let mut batch_process_rr_cursor: Option<MarketId> = None;
    // Internal commands (e.g. ContinueCloseMarket) to run even under sustained external load.
    let mut internal_queue: VecDeque<EngineMsg> = VecDeque::new();

    while !shutdown.load(Ordering::Acquire) {
        crate::metric!({ inflight: 0 });
        crate::metric!({ queue_len: cmd_rx.len().saturating_add(internal_queue.len()) as i64 });
        let msg = if let Some(m) = internal_queue.pop_front() {
            m
        } else {
            match cmd_rx.recv_timeout(Duration::from_millis(
                crate::config::DEFAULT_ENGINE_POLL_TIMEOUT_MS,
            )) {
                Ok(m) => m,
                Err(flume::RecvTimeoutError::Timeout) => {
                    // Drive in-progress internal processes even if no external commands arrive.
                    if let Some(msg) =
                        pick_next_batch_process_command(&root, &mut batch_process_rr_cursor)
                    {
                        msg
                    } else {
                        continue;
                    }
                }
                Err(flume::RecvTimeoutError::Disconnected) => break,
            }
        };

        crate::metric!({ inflight: 1, in_total: 1 });
        crate::metric!({ queue_len: cmd_rx.len().saturating_add(internal_queue.len()) as i64 });
        let t0 = Instant::now();

        let EngineMsg::Command { cmd, resp_tx } = msg;
        let had_response_channel = resp_tx.is_some();
        let was_internal_continue = matches!(
            &cmd.kind,
            CommandKind::ContinueCloseMarket
                | CommandKind::ContinueBatchCancelOrders
                | CommandKind::ContinueLapseOrders
                | CommandKind::ContinueVoidOrders
        );

        trace!(
            market_id = ?cmd.market_id,
            correlation_id = ?cmd.correlation_id,
            command = ?cmd.kind,
            "engine command"
        );

        let start_seq = root.version().saturating_add(1);
        if let Err(err) = root.handle(cmd) {
            crate::metric!({
                duration_ns: t0.elapsed().as_nanos() as u64,
                out_total: 0,
                inflight: 0
            });
            if let Some(tx) = resp_tx
                && tx
                    .send(Ok(Response::Rejected {
                        reason: err.into_reason(),
                        tx_start_seq: None,
                        tx_len: 0,
                    }))
                    .is_err()
            {
                trace!("response receiver dropped before sending rejection");
            }
            if !was_internal_continue
                && let Some(msg) =
                    pick_next_batch_process_command(&root, &mut batch_process_rr_cursor)
            {
                internal_queue.push_back(msg);
            }
            continue;
        }

        let events = root.take_changes();
        let events_len = events.len();
        debug_assert!(
            events_len > 0,
            "accepted commands must emit at least one event"
        );

        let tx_id = root.take_tx_id();
        let tx_len = match u16::try_from(events_len) {
            Ok(len) => len,
            Err(_) => {
                error!(
                    events_len,
                    "tx_len overflow: too many events for single transaction"
                );
                if let Some(tx) = resp_tx.as_ref()
                    && tx
                        .send(Ok(Response::Rejected {
                            reason: RejectReason::InternalError,
                            tx_start_seq: None,
                            tx_len: 0,
                        }))
                        .is_err()
                {
                    trace!("response receiver dropped before sending internal rejection");
                }
                continue;
            }
        };

        // Apply first so the engine always holds the latest state when handling commands.
        // Invariant checks happen during handle; apply is pure state transition.
        root.apply(&events);

        let resp_tx_for_error = resp_tx.clone();
        let callback = resp_tx.map(|tx| ResponseCallback { tx });
        if events_len == 1 {
            let Some(payload) = events.into_iter().next() else {
                error!("event batch empty despite events_len == 1");
                if let Some(tx) = resp_tx_for_error.as_ref()
                    && tx
                        .send(Ok(Response::Rejected {
                            reason: RejectReason::InternalError,
                            tx_start_seq: None,
                            tx_len: 0,
                        }))
                        .is_err()
                {
                    trace!("response receiver dropped before sending internal rejection");
                }
                shutdown.store(true, Ordering::Release);
                break;
            };
            producer.publish(move |slot| {
                slot.seq = start_seq;
                slot.payload = payload;
                slot.response_cb = callback;
                slot.tx_id = tx_id;
                slot.tx_len = 1;
                slot.tx_ix = 0;
            });
        } else {
            let mut payloads = events.into_iter();
            let mut callback = callback;
            let mut publish_error = None;
            producer.batch_publish(events_len, |iter| {
                publish_error =
                    fill_batch_slots(iter, &mut payloads, start_seq, tx_id, tx_len, &mut callback)
                        .err();
            });
            if let Some(err_kind) = publish_error {
                error!(?err_kind, "batch_publish payload/slot length mismatch");
                if let Some(tx) = resp_tx_for_error.as_ref()
                    && tx
                        .send(Ok(Response::Rejected {
                            reason: RejectReason::InternalError,
                            tx_start_seq: None,
                            tx_len: 0,
                        }))
                        .is_err()
                {
                    trace!("response receiver dropped before sending internal rejection");
                }
                shutdown.store(true, Ordering::Release);
                break;
            }
        }

        crate::metric!({
            duration_ns: t0.elapsed().as_nanos() as u64,
            out_total: events_len as u64,
            inflight: 0
        });

        // Ensure internal processes progress even when other commands are flowing.
        if (!was_internal_continue || had_response_channel)
            && let Some(msg) = pick_next_batch_process_command(&root, &mut batch_process_rr_cursor)
        {
            internal_queue.push_back(msg);
        }
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum BatchFillError {
    IteratorLongerThanPayloads,
    IteratorShorterThanPayloads,
}

fn fill_batch_slots<'a, I>(
    iter: I,
    payloads: &mut std::vec::IntoIter<BookEventEnvelope>,
    start_seq: u64,
    tx_id: u64,
    tx_len: u16,
    callback: &mut Option<ResponseCallback>,
) -> Result<(), BatchFillError>
where
    I: Iterator<Item = &'a mut EngineEvent>,
{
    let mut last_slot: Option<&'a mut EngineEvent> = None;

    for (ix, slot) in iter.enumerate() {
        let Some(payload) = payloads.next() else {
            return Err(BatchFillError::IteratorLongerThanPayloads);
        };
        slot.seq = start_seq.saturating_add(ix as u64);
        slot.payload = payload;
        slot.response_cb = None;
        slot.tx_id = tx_id;
        slot.tx_len = tx_len;
        slot.tx_ix = ix as u16;
        last_slot = Some(slot);
    }

    if payloads.next().is_some() {
        return Err(BatchFillError::IteratorShorterThanPayloads);
    }

    if let Some(slot) = last_slot {
        slot.response_cb = callback.take();
    }

    Ok(())
}

fn continue_close_command(market_id: MarketId) -> Command {
    Command {
        correlation_id: None,
        metadata: None,
        market_id,
        kind: CommandKind::ContinueCloseMarket,
    }
}

fn continue_batch_cancel_command(market_id: MarketId) -> Command {
    Command {
        correlation_id: None,
        metadata: None,
        market_id,
        kind: CommandKind::ContinueBatchCancelOrders,
    }
}

fn continue_lapse_command(market_id: MarketId) -> Command {
    Command {
        correlation_id: None,
        metadata: None,
        market_id,
        kind: CommandKind::ContinueLapseOrders,
    }
}

fn continue_void_command(market_id: MarketId) -> Command {
    Command {
        correlation_id: None,
        metadata: None,
        market_id,
        kind: CommandKind::ContinueVoidOrders,
    }
}

impl Drop for Engine {
    fn drop(&mut self) {
        if let Err(err) = self.stop() {
            error!(error = ?err, "failed to stop engine cleanly during drop");
        }
    }
}

fn pick_next_batch_process_command(
    root: &EngineRoot,
    cursor: &mut Option<MarketId>,
) -> Option<EngineMsg> {
    fn classify_market(m: &crate::engine::state::Market) -> Option<CommandKind> {
        if m.book.close_process_state().is_some() {
            return Some(CommandKind::ContinueCloseMarket);
        }
        if m.book.cancel_process_state().is_some() {
            return Some(CommandKind::ContinueBatchCancelOrders);
        }
        match m.book.batch_process_state() {
            Some(crate::book::BatchProcessState::Lapse(_)) => {
                Some(CommandKind::ContinueLapseOrders)
            }
            Some(crate::book::BatchProcessState::Void(_)) => Some(CommandKind::ContinueVoidOrders),
            _ => None,
        }
    }

    // Round-robin: find the smallest MarketId after cursor, or wrap to the global minimum.
    // Two-pass over the HashMap avoids allocating a Vec and sorting.
    let mut best_after_cursor: Option<(MarketId, CommandKind)> = None;
    let mut global_min: Option<(MarketId, CommandKind)> = None;

    for (&mid, m) in root.state().markets.iter() {
        let Some(kind) = classify_market(m) else {
            continue;
        };
        // Track smallest MarketId overall (for wrap-around).
        if global_min.as_ref().is_none_or(|(best, _)| mid.0 < best.0) {
            global_min = Some((mid, kind.clone()));
        }
        // Track smallest MarketId strictly after cursor.
        if let Some(cur) = *cursor
            && mid.0 > cur.0
            && best_after_cursor
                .as_ref()
                .is_none_or(|(best, _)| mid.0 < best.0)
        {
            best_after_cursor = Some((mid, kind));
        }
    }

    let next = best_after_cursor.or(global_min)?;
    *cursor = Some(next.0);

    let cmd = match next.1 {
        CommandKind::ContinueCloseMarket => continue_close_command(next.0),
        CommandKind::ContinueBatchCancelOrders => continue_batch_cancel_command(next.0),
        CommandKind::ContinueLapseOrders => continue_lapse_command(next.0),
        CommandKind::ContinueVoidOrders => continue_void_command(next.0),
        _ => return None,
    };
    Some(EngineMsg::Command { cmd, resp_tx: None })
}

#[derive(Clone)]
pub struct EngineHandler {
    pub(crate) tx: flume::Sender<EngineMsg>,
}

pub(crate) enum EngineMsg {
    Command {
        cmd: Command,
        resp_tx:
            Option<flume::Sender<Result<crate::book::protocol::response::Response, EngineError>>>,
    },
}

impl EngineHandler {
    pub fn submit(&self, cmd: Command) -> Result<(), EngineError> {
        self.tx
            .send(EngineMsg::Command { cmd, resp_tx: None })
            .map_err(|_| EngineError::DisruptorStopped)?;
        Ok(())
    }

    /// Submit a command and receive a response only if the command emits no events.
    ///
    /// For commands that emit events, the response is expected to be produced by a downstream
    /// consumer (e.g. `engine::consumers::ResponseCallbackHandler`), so the returned receiver may not resolve if no
    /// such handler is registered.
    pub fn submit_with_response(
        &self,
        cmd: Command,
    ) -> Result<
        flume::Receiver<Result<crate::book::protocol::response::Response, EngineError>>,
        EngineError,
    > {
        let (resp_tx, resp_rx) = flume::bounded(1);
        self.tx
            .send(EngineMsg::Command {
                cmd,
                resp_tx: Some(resp_tx),
            })
            .map_err(|_| EngineError::DisruptorStopped)?;
        Ok(resp_rx)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::book::{BookEvent, BookMarketState};

    fn mk_env(market_seq: u64) -> BookEventEnvelope {
        BookEventEnvelope {
            market_id: MarketId(7),
            market_name: String::new(),
            market_seq,
            timestamp: crate::types::unix_epoch(),
            metadata: None,
            event: BookEvent::MarketStateChanged {
                to: BookMarketState::Open,
                reason: "T".to_string(),
            },
        }
    }

    fn mk_slot() -> EngineEvent {
        RingSlot::new(Envelope {
            seq: 0,
            payload: mk_env(0),
            response_cb: None,
            tx_id: 0,
            tx_len: 1,
            tx_ix: 0,
        })
    }

    #[test]
    fn fill_batch_slots_populates_and_sets_callback_on_last_slot() {
        let mut payloads = vec![mk_env(1), mk_env(2)].into_iter();
        let mut slots = vec![mk_slot(), mk_slot()];
        let (tx, _rx) = flume::bounded(1);
        let mut cb = Some(ResponseCallback { tx });

        let res = fill_batch_slots(slots.iter_mut(), &mut payloads, 100, 99, 2, &mut cb);

        assert_eq!(res, Ok(()));
        assert!(cb.is_none(), "callback should move to last slot");
        assert_eq!(slots[0].seq, 100);
        assert_eq!(slots[1].seq, 101);
        assert!(slots[0].response_cb.is_none());
        assert!(slots[1].response_cb.is_some());
    }

    #[test]
    fn fill_batch_slots_errors_when_slots_exceed_payloads() {
        let mut payloads = vec![mk_env(1)].into_iter();
        let mut slots = vec![mk_slot(), mk_slot()];
        let (tx, _rx) = flume::bounded(1);
        let mut cb = Some(ResponseCallback { tx });

        let res = fill_batch_slots(slots.iter_mut(), &mut payloads, 10, 5, 2, &mut cb);
        assert_eq!(res, Err(BatchFillError::IteratorLongerThanPayloads));
        assert!(cb.is_some(), "callback should remain untouched on error");
    }

    #[test]
    fn fill_batch_slots_errors_when_payloads_exceed_slots() {
        let mut payloads = vec![mk_env(1), mk_env(2), mk_env(3)].into_iter();
        let mut slots = vec![mk_slot(), mk_slot()];
        let (tx, _rx) = flume::bounded(1);
        let mut cb = Some(ResponseCallback { tx });

        let res = fill_batch_slots(slots.iter_mut(), &mut payloads, 10, 5, 3, &mut cb);
        assert_eq!(res, Err(BatchFillError::IteratorShorterThanPayloads));
        assert!(cb.is_some(), "callback should remain untouched on error");
    }
}