//! Mock implementations of runtime primitives for testing.

use crate::{
    signal::Signal,
    telemetry::metrics::{Metric, Registered},
    Blob, BufMut, BufferPool, BufferPooler, Clock, Error, Handle, IoBufs, IoBufsMut, Metrics, Name,
    Spawner, Storage, Supervisor,
};
use bytes::{Bytes, BytesMut};
use commonware_utils::{
    channel::{fallible::OneshotExt, oneshot},
    sync::Mutex,
};
use governor::clock::{Clock as GovernorClock, ReasonablyRealtime};
use rand::{TryCryptoRng, TryRng};
use std::{future::Future, mem, sync::Arc};

/// Default buffer size (64 KB). Controls both how much data the stream
/// pulls per recv and the backpressure threshold for send.
const DEFAULT_BUFFER_SIZE: usize = 64 * 1024;

/// A mock channel struct that is used internally by Sink and Stream.
pub struct Channel {
    /// Stores the bytes sent by the sink that are not yet read by the stream.
    buffer: BytesMut,

    /// If the stream is waiting to read bytes, the waiter stores the number of
    /// bytes that the stream is waiting for, as well as the oneshot sender that
    /// the sink uses to send the bytes to the stream directly.
    waiter: Option<(usize, oneshot::Sender<Bytes>)>,

    /// Target buffer size, used to bound both the stream's local buffer
    /// and the shared buffer (backpressure threshold).
    buffer_size: usize,

    /// If the sink is blocked waiting for the buffer to drain, this holds
    /// the oneshot sender that the stream uses to wake the sink.
    drain_waiter: Option<oneshot::Sender<()>>,

    /// Tracks whether the sink is still alive and able to send messages.
    sink_alive: bool,

    /// Tracks whether the stream is still alive and able to receive messages.
    stream_alive: bool,
}

impl Channel {
    /// Returns an async-safe Sink/Stream pair with default buffer size.
    pub fn init() -> (Sink, Stream) {
        Self::init_with_buffer_size(DEFAULT_BUFFER_SIZE)
    }

    /// Returns an async-safe Sink/Stream pair with the specified buffer size.
    pub fn init_with_buffer_size(buffer_size: usize) -> (Sink, Stream) {
        let channel = Arc::new(Mutex::new(Self {
            buffer: BytesMut::new(),
            waiter: None,
            buffer_size,
            drain_waiter: None,
            sink_alive: true,
            stream_alive: true,
        }));
        (
            Sink {
                channel: channel.clone(),
                state: SinkState::Open,
            },
            Stream {
                channel,
                buffer: BytesMut::new(),
                poisoned: false,
            },
        )
    }

    /// Restores bytes that were detached from the front of the shared buffer.
    fn restore_front(&mut self, data: Bytes) {
        if data.is_empty() {
            return;
        }

        let mut restored = BytesMut::with_capacity(data.len() + self.buffer.len());
        restored.extend_from_slice(&data);
        restored.extend_from_slice(&self.buffer);
        self.buffer = restored;
    }

    /// Marks the sink as closed and wakes any waiter.
    fn close_sink(&mut self) {
        self.sink_alive = false;

        // If there is a waiter, resolve it by dropping the oneshot sender.
        self.waiter.take();
    }
}

struct RecvWaiterGuard {
    channel: Arc<Mutex<Channel>>,
    active: bool,
}

impl RecvWaiterGuard {
    const fn new(channel: Arc<Mutex<Channel>>) -> Self {
        Self {
            channel,
            active: true,
        }
    }

    const fn disarm(&mut self) {
        self.active = false;
    }
}

impl Drop for RecvWaiterGuard {
    fn drop(&mut self) {
        if !self.active {
            return;
        }

        self.channel.lock().waiter.take();
    }
}

/// A mock sink that implements the Sink trait.
pub struct Sink {
    channel: Arc<Mutex<Channel>>,
    state: SinkState,
}

/// Lifecycle state for the mock sink half.
enum SinkState {
    /// Sends may be attempted.
    Open,
    /// A send is currently in progress.
    Sending,
    /// The sink has been closed.
    Closed,
}

impl Sink {
    fn close(&mut self) {
        if matches!(self.state, SinkState::Closed) {
            return;
        }
        self.channel.lock().close_sink();
        self.state = SinkState::Closed;
    }
}

impl crate::Sink for Sink {
    async fn send(&mut self, bufs: impl Into<IoBufs> + Send) -> Result<(), Error> {
        match self.state {
            SinkState::Open => {}
            SinkState::Sending => {
                self.close();
                return Err(Error::Closed);
            }
            SinkState::Closed => return Err(Error::Closed),
        }

        let drain_recv = {
            let mut channel = self.channel.lock();

            // If the receiver is dead, we cannot send any more messages.
            if !channel.stream_alive {
                channel.close_sink();
                self.state = SinkState::Closed;
                return Err(Error::SendFailed);
            }

            channel.buffer.put(bufs.into());

            // If there is a waiter and the buffer is large enough,
            // resolve the waiter (while clearing the waiter field).
            if channel
                .waiter
                .as_ref()
                .is_some_and(|(requested, _)| *requested <= channel.buffer.len())
            {
                // Send up to buffer_size bytes (but at least requested amount)
                let (requested, os_send) = channel.waiter.take().unwrap();
                let send_amount = channel.buffer.len().min(requested.max(channel.buffer_size));
                let data = channel.buffer.split_to(send_amount).freeze();

                // A canceled recv should behave like a buffered transport:
                // preserve the bytes and allow a subsequent recv to consume them.
                if let Err(data) = os_send.send(data) {
                    channel.restore_front(data);
                    if !channel.stream_alive {
                        channel.close_sink();
                        self.state = SinkState::Closed;
                        return Err(Error::SendFailed);
                    }
                }
            }

            // If the buffer exceeds the write limit, block until the
            // receiver drains enough data.
            if channel.buffer.len() > channel.buffer_size {
                assert!(channel.drain_waiter.is_none());
                let (os_send, os_recv) = oneshot::channel();
                channel.drain_waiter = Some(os_send);
                os_recv
            } else {
                return Ok(());
            }
        };

        // Mark the sink as sending before awaiting so cancellation can be
        // detected by the next send.
        self.state = SinkState::Sending;

        // Wait for the receiver to drain the buffer.
        match drain_recv.await {
            Ok(()) => {
                self.state = SinkState::Open;
                Ok(())
            }
            Err(_) => {
                self.close();
                Err(Error::SendFailed)
            }
        }
    }
}

impl Drop for Sink {
    fn drop(&mut self) {
        self.close();
    }
}

/// A mock stream that implements the Stream trait.
pub struct Stream {
    channel: Arc<Mutex<Channel>>,
    /// Local buffer for data that has been received but not yet consumed.
    buffer: BytesMut,
    poisoned: bool,
}

impl crate::Stream for Stream {
    async fn recv(&mut self, len: usize) -> Result<IoBufs, Error> {
        if self.poisoned {
            return Err(Error::Closed);
        }

        let os_recv = {
            let mut channel = self.channel.lock();

            // Pull data from channel buffer into local buffer.
            let target = len.max(channel.buffer_size);
            let pull_amount = channel
                .buffer
                .len()
                .min(target.saturating_sub(self.buffer.len()));
            if pull_amount > 0 {
                let data = channel.buffer.split_to(pull_amount);
                self.buffer.extend_from_slice(&data);

                // Wake a blocked sender if the buffer drained below the limit.
                if channel.buffer.len() <= channel.buffer_size {
                    if let Some(sender) = channel.drain_waiter.take() {
                        sender.send_lossy(());
                    }
                }
            }

            // If we have enough, return immediately.
            if self.buffer.len() >= len {
                return Ok(IoBufs::from(self.buffer.split_to(len).freeze()));
            }

            // If the sink is dead, we cannot receive any more messages.
            if !channel.sink_alive {
                self.poisoned = true;
                return Err(Error::RecvFailed);
            }

            // Set up waiter for remaining amount.
            let remaining = len - self.buffer.len();
            assert!(channel.waiter.is_none());
            let (os_send, os_recv) = oneshot::channel();
            channel.waiter = Some((remaining, os_send));
            os_recv
        };

        let mut waiter_guard = RecvWaiterGuard::new(self.channel.clone());

        // Pre-poison so that cancellation  leaves the stream permanently closed.
        self.poisoned = true;

        // Wait for the waiter to be resolved.
        let data = match os_recv.await {
            Ok(data) => {
                waiter_guard.disarm();
                self.poisoned = false;
                data
            }
            Err(_) => {
                waiter_guard.disarm();
                return Err(Error::RecvFailed);
            }
        };
        self.buffer.extend_from_slice(&data);

        assert!(self.buffer.len() >= len);
        Ok(IoBufs::from(self.buffer.split_to(len).freeze()))
    }

    fn peek(&self, max_len: usize) -> &[u8] {
        let len = max_len.min(self.buffer.len());
        &self.buffer[..len]
    }
}

impl Drop for Stream {
    fn drop(&mut self) {
        let mut channel = self.channel.lock();
        channel.stream_alive = false;

        // Wake a blocked sender so it can observe the closed stream.
        channel.drain_waiter.take();
    }
}

/// A sync deferred by a [DelayedSyncBlob], held open until explicitly completed.
pub struct DeferredSync {
    /// Completes the sync with the provided result (success runs the inner blob's sync).
    pub release: oneshot::Sender<Result<(), Error>>,

    /// Resolves once the deferred sync's handle begins waiting on `release`.
    pub blocked: oneshot::Receiver<()>,
}

/// Coordinates durability operations for a [DelayedSyncContext] or [DelayedSyncBlob].
///
/// Every started sync parks in a deferred queue (in start order) until a test
/// releases it. [Self::arm] additionally installs a one-shot gate that blocks
/// the next durability operation and counts operations from that point on
/// ([Self::calls]). The gate is pushed onto the deferred queue when [Self::arm]
/// is called, before any operation reaches it.
#[derive(Clone, Default)]
pub struct PendingSyncs {
    syncs: Arc<Mutex<Vec<DeferredSync>>>,
    gate: Arc<Mutex<SyncGateState>>,
}

/// Forwards [Supervisor], [Clock], [GovernorClock], [ReasonablyRealtime],
/// [Metrics], [BufferPooler], [TryRng], and [TryCryptoRng] to the wrapped
/// context for test context wrappers with one extra field (named by the
/// second argument).
macro_rules! forward_context {
    ($wrapper:ident, $field:ident) => {
        impl<E: Supervisor> Supervisor for $wrapper<E> {
            fn name(&self) -> Name {
                self.inner.name()
            }

            fn child(&self, label: &'static str) -> Self {
                Self {
                    inner: self.inner.child(label),
                    $field: self.$field.clone(),
                }
            }

            fn with_attribute(self, key: &'static str, value: impl std::fmt::Display) -> Self {
                Self {
                    inner: self.inner.with_attribute(key, value),
                    $field: self.$field,
                }
            }
        }

        impl<E: Clock> Clock for $wrapper<E> {
            fn current(&self) -> std::time::SystemTime {
                self.inner.current()
            }

            fn sleep(
                &self,
                duration: std::time::Duration,
            ) -> impl Future<Output = ()> + Send + 'static {
                self.inner.sleep(duration)
            }

            fn sleep_until(
                &self,
                deadline: std::time::SystemTime,
            ) -> impl Future<Output = ()> + Send + 'static {
                self.inner.sleep_until(deadline)
            }
        }

        impl<E: Clock> GovernorClock for $wrapper<E> {
            type Instant = std::time::SystemTime;

            fn now(&self) -> Self::Instant {
                self.current()
            }
        }

        impl<E: Clock> ReasonablyRealtime for $wrapper<E> {}

        impl<E: Metrics> Metrics for $wrapper<E> {
            fn register<N: Into<String>, H: Into<String>, M: Metric>(
                &self,
                name: N,
                help: H,
                metric: M,
            ) -> Registered<M> {
                self.inner.register(name, help, metric)
            }

            fn encode(&self) -> String {
                self.inner.encode()
            }
        }

        impl<E: BufferPooler> BufferPooler for $wrapper<E> {
            fn network_buffer_pool(&self) -> &BufferPool {
                self.inner.network_buffer_pool()
            }

            fn storage_buffer_pool(&self) -> &BufferPool {
                self.inner.storage_buffer_pool()
            }
        }

        impl<E: TryRng> TryRng for $wrapper<E> {
            type Error = E::Error;

            fn try_next_u32(&mut self) -> Result<u32, Self::Error> {
                self.inner.try_next_u32()
            }

            fn try_next_u64(&mut self) -> Result<u64, Self::Error> {
                self.inner.try_next_u64()
            }

            fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Self::Error> {
                self.inner.try_fill_bytes(dest)
            }
        }

        impl<E: TryCryptoRng> TryCryptoRng for $wrapper<E> {}
    };
}

/// Context wrapper whose blobs defer [Blob::start_sync] and can gate blocking syncs in tests.
#[derive(Clone)]
pub struct DelayedSyncContext<E> {
    pub inner: E,
    pub pending: PendingSyncs,
}

forward_context!(DelayedSyncContext, pending);

impl<E: Spawner> Spawner for DelayedSyncContext<E> {
    fn shared(mut self, blocking: bool) -> Self {
        self.inner = self.inner.shared(blocking);
        self
    }

    fn dedicated(mut self) -> Self {
        self.inner = self.inner.dedicated();
        self
    }

    fn spawn<F, Fut, T>(self, f: F) -> Handle<T>
    where
        F: FnOnce(Self) -> Fut + Send + 'static,
        Fut: Future<Output = T> + Send + 'static,
        T: Send + 'static,
    {
        let pending = self.pending;
        self.inner.spawn(move |inner| f(Self { inner, pending }))
    }

    async fn stop(self, value: i32, timeout: Option<std::time::Duration>) -> Result<(), Error> {
        self.inner.stop(value, timeout).await
    }

    fn stopped(&self) -> Signal {
        self.inner.stopped()
    }
}

impl<E: Storage> Storage for DelayedSyncContext<E> {
    type Blob = DelayedSyncBlob<E::Blob>;

    async fn open_versioned(
        &self,
        partition: &str,
        name: &[u8],
        versions: std::ops::RangeInclusive<u16>,
    ) -> Result<(Self::Blob, u64, u16), Error> {
        let (inner, len, version) = self.inner.open_versioned(partition, name, versions).await?;
        Ok((
            DelayedSyncBlob {
                inner,
                pending: self.pending.clone(),
            },
            len,
            version,
        ))
    }

    async fn remove(&self, partition: &str, name: Option<&[u8]>) -> Result<(), Error> {
        self.inner.remove(partition, name).await
    }

    async fn scan(&self, partition: &str) -> Result<Vec<Vec<u8>>, Error> {
        self.inner.scan(partition).await
    }
}

/// Blob wrapper that parks each started sync and supports one-shot blocking sync tracking.
#[derive(Clone)]
pub struct DelayedSyncBlob<B> {
    inner: B,
    pending: PendingSyncs,
}

impl<B> DelayedSyncBlob<B> {
    /// Wrap `inner`, returning the blob and the list its deferred syncs are pushed onto.
    pub fn new(inner: B) -> (Self, PendingSyncs) {
        let pending = PendingSyncs::default();
        (
            Self {
                inner,
                pending: pending.clone(),
            },
            pending,
        )
    }
}

impl<B: Blob> Blob for DelayedSyncBlob<B> {
    async fn read_at_buf(
        &self,
        offset: u64,
        len: usize,
        bufs: impl Into<IoBufsMut> + Send,
    ) -> Result<IoBufsMut, Error> {
        self.inner.read_at_buf(offset, len, bufs).await
    }

    async fn read_at(&self, offset: u64, len: usize) -> Result<IoBufsMut, Error> {
        self.inner.read_at(offset, len).await
    }

    async fn write_at(&self, offset: u64, bufs: impl Into<IoBufs> + Send) -> Result<(), Error> {
        self.inner.write_at(offset, bufs).await
    }

    async fn write_at_sync(
        &self,
        offset: u64,
        bufs: impl Into<IoBufs> + Send,
    ) -> Result<(), Error> {
        if !self.pending.tracking() {
            return self.inner.write_at_sync(offset, bufs).await;
        }
        self.inner.write_at(offset, bufs).await?;
        self.pending.wait().await?;
        self.inner.sync().await
    }

    async fn resize(&self, len: u64) -> Result<(), Error> {
        self.inner.resize(len).await
    }

    async fn sync(&self) -> Result<(), Error> {
        self.pending.wait().await?;
        self.inner.sync().await
    }

    async fn start_sync(&self) -> Handle<()> {
        let inner = self.inner.clone();
        let waiter = self
            .pending
            .observe()
            .unwrap_or_else(|| self.pending.defer());
        Handle::from_future(async move {
            waiter.wait().await?;
            inner.sync().await
        })
    }
}

/// Take the oldest pending sync, panicking if none was started.
pub fn next_pending_sync(pending: &PendingSyncs) -> DeferredSync {
    let mut pending = pending.lock();
    assert!(!pending.is_empty(), "no pending sync was started");
    pending.remove(0)
}

/// Complete the oldest `count` pending syncs successfully.
pub fn release_next_pending_syncs(pending: &PendingSyncs, count: usize) {
    let syncs = {
        let mut pending = pending.lock();
        assert!(
            pending.len() >= count,
            "not enough pending syncs: have {}, need {count}",
            pending.len()
        );
        pending.drain(..count).collect::<Vec<_>>()
    };
    for sync in syncs {
        let _ = sync.release.send(Ok(()));
    }
}

/// Complete all pending syncs successfully.
pub fn release_pending_syncs(pending: &PendingSyncs) {
    for sync in mem::take(&mut *pending.lock()) {
        let _ = sync.release.send(Ok(()));
    }
}

/// Fail all pending syncs with an injected I/O error.
pub fn fail_pending_syncs(pending: &PendingSyncs) {
    for sync in mem::take(&mut *pending.lock()) {
        let err = std::io::Error::other("injected sync failure");
        let _ = sync.release.send(Err(Error::Io(err.into())));
    }
}

struct SyncWaiter {
    entered: oneshot::Sender<()>,
    release: oneshot::Receiver<Result<(), Error>>,
}

impl SyncWaiter {
    async fn wait(self) -> Result<(), Error> {
        self.entered.send_lossy(());
        self.release.await.map_err(|_| Error::Closed)??;
        Ok(())
    }
}

#[derive(Default)]
struct SyncGateState {
    tracking: bool,
    calls: usize,
    waiter: Option<SyncWaiter>,
}

impl PendingSyncs {
    /// Locks the deferred sync queue.
    pub fn lock(&self) -> commonware_utils::sync::MutexGuard<'_, Vec<DeferredSync>> {
        self.syncs.lock()
    }

    /// Begins counting durability operations and blocks the next one behind a
    /// one-shot gate (pushed onto the deferred queue so tests can release it).
    ///
    /// Once the gate is consumed, started syncs park in the deferred queue as
    /// usual while [Self::calls] keeps counting.
    pub fn arm(&self) {
        let mut state = self.gate.lock();
        assert!(!state.tracking, "sync gate already armed");
        assert!(state.waiter.is_none(), "sync gate already has a waiter");
        state.tracking = true;
        state.calls = 0;
        state.waiter = Some(self.defer());
    }

    /// Returns the number of durability operations observed since [Self::arm].
    pub fn calls(&self) -> usize {
        self.gate.lock().calls
    }

    fn tracking(&self) -> bool {
        self.gate.lock().tracking
    }

    fn defer(&self) -> SyncWaiter {
        let (release, release_rx) = oneshot::channel();
        let (entered, blocked) = oneshot::channel();
        self.syncs.lock().push(DeferredSync { release, blocked });
        SyncWaiter {
            entered,
            release: release_rx,
        }
    }

    /// Records a durability operation if the gate is armed, returning the
    /// one-shot gate waiter if it has not been consumed yet.
    fn observe(&self) -> Option<SyncWaiter> {
        let mut state = self.gate.lock();
        if !state.tracking {
            return None;
        }
        state.calls += 1;
        state.waiter.take()
    }

    async fn wait(&self) -> Result<(), Error> {
        match self.observe() {
            Some(waiter) => waiter.wait().await,
            None => Ok(()),
        }
    }
}

/// Context wrapper whose blobs fail `sync` and `start_sync` for a single partition.
#[derive(Clone)]
pub struct SyncFaultContext<E> {
    pub inner: E,
    pub fail_partition: String,
}

forward_context!(SyncFaultContext, fail_partition);

impl<E: Storage> Storage for SyncFaultContext<E> {
    type Blob = SyncFaultBlob<E::Blob>;

    async fn open_versioned(
        &self,
        partition: &str,
        name: &[u8],
        versions: std::ops::RangeInclusive<u16>,
    ) -> Result<(Self::Blob, u64, u16), Error> {
        let (inner, len, version) = self.inner.open_versioned(partition, name, versions).await?;
        Ok((
            SyncFaultBlob {
                inner,
                faulty: partition == self.fail_partition,
            },
            len,
            version,
        ))
    }

    async fn remove(&self, partition: &str, name: Option<&[u8]>) -> Result<(), Error> {
        self.inner.remove(partition, name).await
    }

    async fn scan(&self, partition: &str) -> Result<Vec<Vec<u8>>, Error> {
        self.inner.scan(partition).await
    }
}

/// Blob wrapper that fails `sync` and `start_sync` when marked faulty.
#[derive(Clone)]
pub struct SyncFaultBlob<B> {
    inner: B,
    faulty: bool,
}

impl<B: Blob> Blob for SyncFaultBlob<B> {
    async fn read_at_buf(
        &self,
        offset: u64,
        len: usize,
        bufs: impl Into<IoBufsMut> + Send,
    ) -> Result<IoBufsMut, Error> {
        self.inner.read_at_buf(offset, len, bufs).await
    }

    async fn read_at(&self, offset: u64, len: usize) -> Result<IoBufsMut, Error> {
        self.inner.read_at(offset, len).await
    }

    async fn write_at(&self, offset: u64, bufs: impl Into<IoBufs> + Send) -> Result<(), Error> {
        self.inner.write_at(offset, bufs).await
    }

    async fn write_at_sync(
        &self,
        offset: u64,
        bufs: impl Into<IoBufs> + Send,
    ) -> Result<(), Error> {
        self.inner.write_at_sync(offset, bufs).await
    }

    async fn resize(&self, len: u64) -> Result<(), Error> {
        self.inner.resize(len).await
    }

    async fn sync(&self) -> Result<(), Error> {
        if self.faulty {
            let err = std::io::Error::other("injected partition sync fault");
            return Err(Error::Io(err.into()));
        }
        self.inner.sync().await
    }

    async fn start_sync(&self) -> Handle<()> {
        if self.faulty {
            return Handle::ready(self.sync().await);
        }
        self.inner.start_sync().await
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{deterministic, Clock, Runner, Sink, Spawner, Stream};
    use commonware_macros::select;
    use std::{thread::sleep, time::Duration};

    #[test]
    fn test_send_recv() {
        let (mut sink, mut stream) = Channel::init();
        let data = b"hello world";

        let executor = deterministic::Runner::default();
        executor.start(|_| async move {
            sink.send(data.as_slice()).await.unwrap();
            let received = stream.recv(data.len()).await.unwrap();
            assert_eq!(received.coalesce(), data);
        });
    }

    #[test]
    fn test_send_recv_partial_multiple() {
        let (mut sink, mut stream) = Channel::init();
        let data = b"hello";
        let data2 = b" world";

        let executor = deterministic::Runner::default();
        executor.start(|_| async move {
            sink.send(data.as_slice()).await.unwrap();
            sink.send(data2.as_slice()).await.unwrap();
            let received = stream.recv(5).await.unwrap();
            assert_eq!(received.coalesce(), b"hello");
            let received = stream.recv(5).await.unwrap();
            assert_eq!(received.coalesce(), b" worl");
            let received = stream.recv(1).await.unwrap();
            assert_eq!(received.coalesce(), b"d");
        });
    }

    #[test]
    fn test_send_recv_async() {
        let (mut sink, mut stream) = Channel::init();
        let data = b"hello world";

        let executor = deterministic::Runner::default();
        executor.start(|_| async move {
            let (received, _) = futures::try_join!(stream.recv(data.len()), async {
                sleep(Duration::from_millis(50));
                sink.send(data.as_slice()).await
            })
            .unwrap();
            assert_eq!(received.coalesce(), data);
        });
    }

    #[test]
    fn test_recv_error_sink_dropped_while_waiting() {
        let (sink, mut stream) = Channel::init();

        let executor = deterministic::Runner::default();
        executor.start(|context| async move {
            futures::join!(
                async {
                    let result = stream.recv(5).await;
                    assert!(matches!(result, Err(Error::RecvFailed)));
                    let result = stream.recv(5).await;
                    assert!(matches!(result, Err(Error::Closed)));
                },
                async {
                    // Wait for the stream to start waiting
                    context.sleep(Duration::from_millis(50)).await;
                    drop(sink);
                }
            );
        });
    }

    #[test]
    fn test_recv_error_sink_dropped_before_recv() {
        let (sink, mut stream) = Channel::init();
        drop(sink); // Drop sink immediately

        let executor = deterministic::Runner::default();
        executor.start(|_| async move {
            let result = stream.recv(5).await;
            assert!(matches!(result, Err(Error::RecvFailed)));
            let result = stream.recv(5).await;
            assert!(matches!(result, Err(Error::Closed)));
        });
    }

    #[test]
    fn test_send_error_stream_dropped() {
        let (mut sink, mut stream) = Channel::init();

        let executor = deterministic::Runner::default();
        executor.start(|context| async move {
            // Send some bytes
            assert!(sink.send(b"7 bytes".as_slice()).await.is_ok());

            // Spawn a task to initiate recv's where the first one will succeed and then will drop.
            let handle = context.child("recv").spawn(|_| async move {
                let _ = stream.recv(5).await;
                let _ = stream.recv(5).await;
            });

            // Give the async task a moment to start
            context.sleep(Duration::from_millis(50)).await;

            // Drop the stream by aborting the handle
            handle.abort();
            assert!(matches!(handle.await, Err(Error::Closed)));

            // Try to send a message. The stream is dropped, so this should fail.
            let result = sink.send(b"hello world".as_slice()).await;
            assert!(matches!(result, Err(Error::SendFailed)));
            let result = sink.send(b"hello world".as_slice()).await;
            assert!(matches!(result, Err(Error::Closed)));
        });
    }

    #[test]
    fn test_send_error_stream_dropped_before_send() {
        let (mut sink, stream) = Channel::init();
        drop(stream); // Drop stream immediately

        let executor = deterministic::Runner::default();
        executor.start(|_| async move {
            let result = sink.send(b"hello world".as_slice()).await;
            assert!(matches!(result, Err(Error::SendFailed)));
            let result = sink.send(b"hello world".as_slice()).await;
            assert!(matches!(result, Err(Error::Closed)));
        });
    }

    #[test]
    fn test_recv_timeout() {
        let (_sink, mut stream) = Channel::init();

        // If there is no data to read, test that the recv function just blocks.
        // The timeout should return first.
        let executor = deterministic::Runner::default();
        executor.start(|context| async move {
            select! {
                v = stream.recv(5) => {
                    panic!("unexpected value: {v:?}");
                },
                _ = context.sleep(Duration::from_millis(100)) => "timeout",
            };
        });
    }

    #[test]
    fn test_peek_empty() {
        let (_sink, stream) = Channel::init();

        // Peek on a fresh stream should return empty slice
        assert!(stream.peek(10).is_empty());
    }

    #[test]
    fn test_peek_after_partial_recv() {
        let (mut sink, mut stream) = Channel::init();

        let executor = deterministic::Runner::default();
        executor.start(|_| async move {
            // Send more data than we'll consume
            sink.send(b"hello world".as_slice()).await.unwrap();

            // Recv only part of it
            let received = stream.recv(5).await.unwrap();
            assert_eq!(received.coalesce(), b"hello");

            // Peek should show the remaining data
            assert_eq!(stream.peek(100), b" world");

            // Peek with smaller max_len
            assert_eq!(stream.peek(3), b" wo");

            // Peek doesn't consume - can peek again
            assert_eq!(stream.peek(100), b" world");

            // Recv consumes the peeked data
            let received = stream.recv(6).await.unwrap();
            assert_eq!(received.coalesce(), b" world");

            // Peek is now empty
            assert!(stream.peek(100).is_empty());
        });
    }

    #[test]
    fn test_peek_after_recv_wakeup() {
        let (mut sink, mut stream) = Channel::init_with_buffer_size(64);

        let executor = deterministic::Runner::default();
        executor.start(|context| async move {
            // Spawn recv that will block waiting
            let (tx, rx) = oneshot::channel();
            let recv_handle = context.child("recv").spawn(|_| async move {
                let data = stream.recv(3).await.unwrap();
                tx.send(stream).ok();
                data
            });

            // Let recv set up waiter
            context.sleep(Duration::from_millis(10)).await;

            // Send more than requested
            sink.send(b"ABCDEFGHIJ".as_slice()).await.unwrap();

            // Recv gets its 3 bytes
            let received = recv_handle.await.unwrap();
            assert_eq!(received.coalesce(), b"ABC");

            // Get stream back and verify peek sees remaining data
            let stream = rx.await.unwrap();
            assert_eq!(stream.peek(100), b"DEFGHIJ");
        });
    }

    #[test]
    fn test_peek_multiple_sends() {
        let (mut sink, mut stream) = Channel::init();

        let executor = deterministic::Runner::default();
        executor.start(|_| async move {
            // Send multiple chunks
            sink.send(b"aaa".as_slice()).await.unwrap();
            sink.send(b"bbb".as_slice()).await.unwrap();
            sink.send(b"ccc".as_slice()).await.unwrap();

            // Recv less than total
            let received = stream.recv(4).await.unwrap();
            assert_eq!(received.coalesce(), b"aaab");

            // Peek should show remaining
            assert_eq!(stream.peek(100), b"bbccc");
        });
    }

    #[test]
    fn test_buffer_size_limit() {
        // Use a small buffer capacity for testing
        let (mut sink, mut stream) = Channel::init_with_buffer_size(10);

        let executor = deterministic::Runner::default();
        executor.start(|context| async move {
            // Send more than buffer capacity concurrently with recv
            // so the sender can drain via backpressure.
            let send_handle = context.child("sender").spawn(|_| async move {
                sink.send(b"0123456789ABCDEF".as_slice()).await.unwrap();
                sink
            });

            // Recv a small amount - should only pull up to capacity (10 bytes)
            let received = stream.recv(2).await.unwrap();
            assert_eq!(received.coalesce(), b"01");

            // Peek should show remaining buffered data (8 bytes, not 14)
            assert_eq!(stream.peek(100), b"23456789");

            // The rest should still be in the channel buffer
            // Recv more to pull the remaining data
            let received = stream.recv(8).await.unwrap();
            assert_eq!(received.coalesce(), b"23456789");

            // Now peek should show next chunk from channel (up to capacity)
            let received = stream.recv(2).await.unwrap();
            assert_eq!(received.coalesce(), b"AB");

            assert_eq!(stream.peek(100), b"CDEF");

            // Ensure the sender completes
            send_handle.await.unwrap();
        });
    }

    #[test]
    fn test_recv_before_send() {
        // Use a small buffer capacity for testing
        let (mut sink, mut stream) = Channel::init_with_buffer_size(10);

        let executor = deterministic::Runner::default();
        executor.start(|context| async move {
            // Start recv before send (will wait)
            let recv_handle = context
                .child("recv")
                .spawn(|_| async move { stream.recv(3).await.unwrap() });

            // Give recv time to set up waiter
            context.sleep(Duration::from_millis(10)).await;

            // Send more than capacity
            sink.send(b"ABCDEFGHIJKLMNOP".as_slice()).await.unwrap();

            // Recv should get its 3 bytes
            let received = recv_handle.await.unwrap();
            assert_eq!(received.coalesce(), b"ABC");
        });
    }
}