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    1025

    use std::collections::{HashMap, HashSet, VecDeque};
    use std::fmt;
    use std::ops::{Deref, DerefMut};
    use std::sync::{Arc, Mutex, Weak};

    #[cfg(not(feature = "runtime"))]
    use std::time::{Duration, Instant};

    use futures_channel::oneshot;
    #[cfg(feature = "runtime")]
    use tokio::time::{Duration, Instant, Interval};

    use super::client::Ver;
    use crate::common::{task, exec::Exec, Future, Pin, Poll, Unpin};

    // FIXME: allow() required due to `impl Trait` leaking types to this lint
    #[allow(missing_debug_implementations)]
    pub(super) struct Pool<T> {
        // If the pool is disabled, this is None.
        inner: Option<Arc<Mutex<PoolInner<T>>>>,
    }

    // Before using a pooled connection, make sure the sender is not dead.
    //
    // This is a trait to allow the `client::pool::tests` to work for `i32`.
    //
    // See https://github.com/hyperium/hyper/issues/1429
    pub(super) trait Poolable: Unpin + Send + Sized + 'static {
        fn is_open(&self) -> bool;
        /// Reserve this connection.
        ///
        /// Allows for HTTP/2 to return a shared reservation.
        fn reserve(self) -> Reservation<Self>;
        fn can_share(&self) -> bool;
    }

    /// When checking out a pooled connection, it might be that the connection
    /// only supports a single reservation, or it might be usable for many.
    ///
    /// Specifically, HTTP/1 requires a unique reservation, but HTTP/2 can be
    /// used for multiple requests.
    // FIXME: allow() required due to `impl Trait` leaking types to this lint
    #[allow(missing_debug_implementations)]
    pub(super) enum Reservation<T> {
        /// This connection could be used multiple times, the first one will be
        /// reinserted into the `idle` pool, and the second will be given to
        /// the `Checkout`.
        #[cfg(feature = "http2")]
        Shared(T, T),
        /// This connection requires unique access. It will be returned after
        /// use is complete.
        Unique(T),
    }

    /// Simple type alias in case the key type needs to be adjusted.
    pub(super) type Key = (http::uri::Scheme, http::uri::Authority); //Arc<String>;

    struct PoolInner<T> {
        // A flag that a connection is being established, and the connection
        // should be shared. This prevents making multiple HTTP/2 connections
        // to the same host.
        connecting: HashSet<Key>,
        // These are internal Conns sitting in the event loop in the KeepAlive
        // state, waiting to receive a new Request to send on the socket.
        idle: HashMap<Key, Vec<Idle<T>>>,
        max_idle_per_host: usize,
        // These are outstanding Checkouts that are waiting for a socket to be
        // able to send a Request one. This is used when "racing" for a new
        // connection.
        //
        // The Client starts 2 tasks, 1 to connect a new socket, and 1 to wait
        // for the Pool to receive an idle Conn. When a Conn becomes idle,
        // this list is checked for any parked Checkouts, and tries to notify
        // them that the Conn could be used instead of waiting for a brand new
        // connection.
        waiters: HashMap<Key, VecDeque<oneshot::Sender<T>>>,
        // A oneshot channel is used to allow the interval to be notified when
        // the Pool completely drops. That way, the interval can cancel immediately.
        #[cfg(feature = "runtime")]
        idle_interval_ref: Option<oneshot::Sender<crate::common::Never>>,
        #[cfg(feature = "runtime")]
        exec: Exec,
        timeout: Option<Duration>,
    }

    // This is because `Weak::new()` *allocates* space for `T`, even if it
    // doesn't need it!
    struct WeakOpt<T>(Option<Weak<T>>);

    #[derive(Clone, Copy, Debug)]
    pub(super) struct Config {
        pub(super) idle_timeout: Option<Duration>,
        pub(super) max_idle_per_host: usize,
    }

    impl Config {
        pub(super) fn is_enabled(&self) -> bool {
            self.max_idle_per_host > 0
        }
    }

    impl<T> Pool<T> {
        pub(super) fn new(config: Config, __exec: &Exec) -> Pool<T> {
            let inner = if config.is_enabled() {
                Some(Arc::new(Mutex::new(PoolInner {
                    connecting: HashSet::new(),
                    idle: HashMap::new(),
                    #[cfg(feature = "runtime")]
                    idle_interval_ref: None,
                    max_idle_per_host: config.max_idle_per_host,
                    waiters: HashMap::new(),
                    #[cfg(feature = "runtime")]
                    exec: __exec.clone(),
                    timeout: config.idle_timeout,
                })))
            } else {
                None
            };

            Pool { inner }
        }

        fn is_enabled(&self) -> bool {
            self.inner.is_some()
        }

        #[cfg(test)]
        pub(super) fn no_timer(&self) {
            // Prevent an actual interval from being created for this pool...
            #[cfg(feature = "runtime")]
            {
                let mut inner = self.inner.as_ref().unwrap().lock().unwrap();
                assert!(inner.idle_interval_ref.is_none(), "timer already spawned");
                let (tx, _) = oneshot::channel();
                inner.idle_interval_ref = Some(tx);
            }
        }
    }

    impl<T: Poolable> Pool<T> {
        /// Returns a `Checkout` which is a future that resolves if an idle
        /// connection becomes available.
        pub(super) fn checkout(&self, key: Key) -> Checkout<T> {
            Checkout {
                key,
                pool: self.clone(),
                waiter: None,
            }
        }

        /// Ensure that there is only ever 1 connecting task for HTTP/2
        /// connections. This does nothing for HTTP/1.
        pub(super) fn connecting(&self, key: &Key, ver: Ver) -> Option<Connecting<T>> {
            if ver == Ver::Http2 {
                if let Some(ref enabled) = self.inner {
                    let mut inner = enabled.lock().unwrap();
                    return if inner.connecting.insert(key.clone()) {
                        let connecting = Connecting {
                            key: key.clone(),
                            pool: WeakOpt::downgrade(enabled),
                        };
                        Some(connecting)
                    } else {
                        trace!("HTTP/2 connecting already in progress for {:?}", key);
                        None
                    };
                }
            }

            // else
            Some(Connecting {
                key: key.clone(),
                // in HTTP/1's case, there is never a lock, so we don't
                // need to do anything in Drop.
                pool: WeakOpt::none(),
            })
        }

        #[cfg(test)]
        fn locked(&self) -> std::sync::MutexGuard<'_, PoolInner<T>> {
            self.inner.as_ref().expect("enabled").lock().expect("lock")
        }

        /* Used in client/tests.rs...
        #[cfg(feature = "runtime")]
        #[cfg(test)]
        pub(super) fn h1_key(&self, s: &str) -> Key {
            Arc::new(s.to_string())
        }

        #[cfg(feature = "runtime")]
        #[cfg(test)]
        pub(super) fn idle_count(&self, key: &Key) -> usize {
            self
                .locked()
                .idle
                .get(key)
                .map(|list| list.len())
                .unwrap_or(0)
        }
        */

        pub(super) fn pooled(
            &self,
            #[cfg_attr(not(feature = "http2"), allow(unused_mut))] mut connecting: Connecting<T>,
            value: T,
        ) -> Pooled<T> {
            let (value, pool_ref) = if let Some(ref enabled) = self.inner {
                match value.reserve() {
                    #[cfg(feature = "http2")]
                    Reservation::Shared(to_insert, to_return) => {
                        let mut inner = enabled.lock().unwrap();
                        inner.put(connecting.key.clone(), to_insert, enabled);
                        // Do this here instead of Drop for Connecting because we
                        // already have a lock, no need to lock the mutex twice.
                        inner.connected(&connecting.key);
                        // prevent the Drop of Connecting from repeating inner.connected()
                        connecting.pool = WeakOpt::none();

                        // Shared reservations don't need a reference to the pool,
                        // since the pool always keeps a copy.
                        (to_return, WeakOpt::none())
                    }
                    Reservation::Unique(value) => {
                        // Unique reservations must take a reference to the pool
                        // since they hope to reinsert once the reservation is
                        // completed
                        (value, WeakOpt::downgrade(enabled))
                    }
                }
            } else {
                // If pool is not enabled, skip all the things...

                // The Connecting should have had no pool ref
                debug_assert!(connecting.pool.upgrade().is_none());

                (value, WeakOpt::none())
            };
            Pooled {
                key: connecting.key.clone(),
                is_reused: false,
                pool: pool_ref,
                value: Some(value),
            }
        }

        fn reuse(&self, key: &Key, value: T) -> Pooled<T> {
            debug!("reuse idle connection for {:?}", key);
            // TODO: unhack this
            // In Pool::pooled(), which is used for inserting brand new connections,
            // there's some code that adjusts the pool reference taken depending
            // on if the Reservation can be shared or is unique. By the time
            // reuse() is called, the reservation has already been made, and
            // we just have the final value, without knowledge of if this is
            // unique or shared. So, the hack is to just assume Ver::Http2 means
            // shared... :(
            let mut pool_ref = WeakOpt::none();
            if !value.can_share() {
                if let Some(ref enabled) = self.inner {
                    pool_ref = WeakOpt::downgrade(enabled);
                }
            }

            Pooled {
                is_reused: true,
                key: key.clone(),
                pool: pool_ref,
                value: Some(value),
            }
        }
    }

    /// Pop off this list, looking for a usable connection that hasn't expired.
    struct IdlePopper<'a, T> {
        key: &'a Key,
        list: &'a mut Vec<Idle<T>>,
    }

    impl<'a, T: Poolable + 'a> IdlePopper<'a, T> {
        fn pop(self, expiration: &Expiration) -> Option<Idle<T>> {
            while let Some(entry) = self.list.pop() {
                // If the connection has been closed, or is older than our idle
                // timeout, simply drop it and keep looking...
                if !entry.value.is_open() {
                    trace!("removing closed connection for {:?}", self.key);
                    continue;
                }
                // TODO: Actually, since the `idle` list is pushed to the end always,
                // that would imply that if *this* entry is expired, then anything
                // "earlier" in the list would *have* to be expired also... Right?
                //
                // In that case, we could just break out of the loop and drop the
                // whole list...
                if expiration.expires(entry.idle_at) {
                    trace!("removing expired connection for {:?}", self.key);
                    continue;
                }

                let value = match entry.value.reserve() {
                    #[cfg(feature = "http2")]
                    Reservation::Shared(to_reinsert, to_checkout) => {
                        self.list.push(Idle {
                            idle_at: Instant::now(),
                            value: to_reinsert,
                        });
                        to_checkout
                    }
                    Reservation::Unique(unique) => unique,
                };

                return Some(Idle {
                    idle_at: entry.idle_at,
                    value,
                });
            }

            None
        }
    }

    impl<T: Poolable> PoolInner<T> {
        fn put(&mut self, key: Key, value: T, __pool_ref: &Arc<Mutex<PoolInner<T>>>) {
            if value.can_share() && self.idle.contains_key(&key) {
                trace!("put; existing idle HTTP/2 connection for {:?}", key);
                return;
            }
            trace!("put; add idle connection for {:?}", key);
            let mut remove_waiters = false;
            let mut value = Some(value);
            if let Some(waiters) = self.waiters.get_mut(&key) {
                while let Some(tx) = waiters.pop_front() {
                    if !tx.is_canceled() {
                        let reserved = value.take().expect("value already sent");
                        let reserved = match reserved.reserve() {
                            #[cfg(feature = "http2")]
                            Reservation::Shared(to_keep, to_send) => {
                                value = Some(to_keep);
                                to_send
                            }
                            Reservation::Unique(uniq) => uniq,
                        };
                        match tx.send(reserved) {
                            Ok(()) => {
                                if value.is_none() {
                                    break;
                                } else {
                                    continue;
                                }
                            }
                            Err(e) => {
                                value = Some(e);
                            }
                        }
                    }

                    trace!("put; removing canceled waiter for {:?}", key);
                }
                remove_waiters = waiters.is_empty();
            }
            if remove_waiters {
                self.waiters.remove(&key);
            }

            match value {
                Some(value) => {
                    // borrow-check scope...
                    {
                        let idle_list = self.idle.entry(key.clone()).or_insert_with(Vec::new);
                        if self.max_idle_per_host <= idle_list.len() {
                            trace!("max idle per host for {:?}, dropping connection", key);
                            return;
                        }

                        debug!("pooling idle connection for {:?}", key);
                        idle_list.push(Idle {
                            value,
                            idle_at: Instant::now(),
                        });
                    }

                    #[cfg(feature = "runtime")]
                    {
                        self.spawn_idle_interval(__pool_ref);
                    }
                }
                None => trace!("put; found waiter for {:?}", key),
            }
        }

        /// A `Connecting` task is complete. Not necessarily successfully,
        /// but the lock is going away, so clean up.
        fn connected(&mut self, key: &Key) {
            let existed = self.connecting.remove(key);
            debug_assert!(existed, "Connecting dropped, key not in pool.connecting");
            // cancel any waiters. if there are any, it's because
            // this Connecting task didn't complete successfully.
            // those waiters would never receive a connection.
            self.waiters.remove(key);
        }

        #[cfg(feature = "runtime")]
        fn spawn_idle_interval(&mut self, pool_ref: &Arc<Mutex<PoolInner<T>>>) {
            let (dur, rx) = {
                if self.idle_interval_ref.is_some() {
                    return;
                }

                if let Some(dur) = self.timeout {
                    let (tx, rx) = oneshot::channel();
                    self.idle_interval_ref = Some(tx);
                    (dur, rx)
                } else {
                    return;
                }
            };

            let interval = IdleTask {
                interval: tokio::time::interval(dur),
                pool: WeakOpt::downgrade(pool_ref),
                pool_drop_notifier: rx,
            };

            self.exec.execute(interval);
        }
    }

    impl<T> PoolInner<T> {
        /// Any `FutureResponse`s that were created will have made a `Checkout`,
        /// and possibly inserted into the pool that it is waiting for an idle
        /// connection. If a user ever dropped that future, we need to clean out
        /// those parked senders.
        fn clean_waiters(&mut self, key: &Key) {
            let mut remove_waiters = false;
            if let Some(waiters) = self.waiters.get_mut(key) {
                waiters.retain(|tx| !tx.is_canceled());
                remove_waiters = waiters.is_empty();
            }
            if remove_waiters {
                self.waiters.remove(key);
            }
        }
    }

    #[cfg(feature = "runtime")]
    impl<T: Poolable> PoolInner<T> {
        /// This should *only* be called by the IdleTask
        fn clear_expired(&mut self) {
            let dur = self.timeout.expect("interval assumes timeout");

            let now = Instant::now();
            //self.last_idle_check_at = now;

            self.idle.retain(|key, values| {
                values.retain(|entry| {
                    if !entry.value.is_open() {
                        trace!("idle interval evicting closed for {:?}", key);
                        return false;
                    }
                    if now - entry.idle_at > dur {
                        trace!("idle interval evicting expired for {:?}", key);
                        return false;
                    }

                    // Otherwise, keep this value...
                    true
                });

                // returning false evicts this key/val
                !values.is_empty()
            });
        }
    }

    impl<T> Clone for Pool<T> {
        fn clone(&self) -> Pool<T> {
            Pool {
                inner: self.inner.clone(),
            }
        }
    }

    /// A wrapped poolable value that tries to reinsert to the Pool on Drop.
    // Note: The bounds `T: Poolable` is needed for the Drop impl.
    pub(super) struct Pooled<T: Poolable> {
        value: Option<T>,
        is_reused: bool,
        key: Key,
        pool: WeakOpt<Mutex<PoolInner<T>>>,
    }

    impl<T: Poolable> Pooled<T> {
        pub(super) fn is_reused(&self) -> bool {
            self.is_reused
        }

        pub(super) fn is_pool_enabled(&self) -> bool {
            self.pool.0.is_some()
        }

        fn as_ref(&self) -> &T {
            self.value.as_ref().expect("not dropped")
        }

        fn as_mut(&mut self) -> &mut T {
            self.value.as_mut().expect("not dropped")
        }
    }

    impl<T: Poolable> Deref for Pooled<T> {
        type Target = T;
        fn deref(&self) -> &T {
            self.as_ref()
        }
    }

    impl<T: Poolable> DerefMut for Pooled<T> {
        fn deref_mut(&mut self) -> &mut T {
            self.as_mut()
        }
    }

    impl<T: Poolable> Drop for Pooled<T> {
        fn drop(&mut self) {
            if let Some(value) = self.value.take() {
                if !value.is_open() {
                    // If we *already* know the connection is done here,
                    // it shouldn't be re-inserted back into the pool.
                    return;
                }

                if let Some(pool) = self.pool.upgrade() {
                    if let Ok(mut inner) = pool.lock() {
                        inner.put(self.key.clone(), value, &pool);
                    }
                } else if !value.can_share() {
                    trace!("pool dropped, dropping pooled ({:?})", self.key);
                }
                // Ver::Http2 is already in the Pool (or dead), so we wouldn't
                // have an actual reference to the Pool.
            }
        }
    }

    impl<T: Poolable> fmt::Debug for Pooled<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Pooled").field("key", &self.key).finish()
        }
    }

    struct Idle<T> {
        idle_at: Instant,
        value: T,
    }

    // FIXME: allow() required due to `impl Trait` leaking types to this lint
    #[allow(missing_debug_implementations)]
    pub(super) struct Checkout<T> {
        key: Key,
        pool: Pool<T>,
        waiter: Option<oneshot::Receiver<T>>,
    }

    impl<T: Poolable> Checkout<T> {
        fn poll_waiter(
            &mut self,
            cx: &mut task::Context<'_>,
        ) -> Poll<Option<crate::Result<Pooled<T>>>> {
            static CANCELED: &str = "pool checkout failed";
            if let Some(mut rx) = self.waiter.take() {
                match Pin::new(&mut rx).poll(cx) {
                    Poll::Ready(Ok(value)) => {
                        if value.is_open() {
                            Poll::Ready(Some(Ok(self.pool.reuse(&self.key, value))))
                        } else {
                            Poll::Ready(Some(Err(crate::Error::new_canceled().with(CANCELED))))
                        }
                    }
                    Poll::Pending => {
                        self.waiter = Some(rx);
                        Poll::Pending
                    }
                    Poll::Ready(Err(_canceled)) => {
                        Poll::Ready(Some(Err(crate::Error::new_canceled().with(CANCELED))))
                    }
                }
            } else {
                Poll::Ready(None)
            }
        }

        fn checkout(&mut self, cx: &mut task::Context<'_>) -> Option<Pooled<T>> {
            let entry = {
                let mut inner = self.pool.inner.as_ref()?.lock().unwrap();
                let expiration = Expiration::new(inner.timeout);
                let maybe_entry = inner.idle.get_mut(&self.key).and_then(|list| {
                    trace!("take? {:?}: expiration = {:?}", self.key, expiration.0);
                    // A block to end the mutable borrow on list,
                    // so the map below can check is_empty()
                    {
                        let popper = IdlePopper {
                            key: &self.key,
                            list,
                        };
                        popper.pop(&expiration)
                    }
                    .map(|e| (e, list.is_empty()))
                });

                let (entry, empty) = if let Some((e, empty)) = maybe_entry {
                    (Some(e), empty)
                } else {
                    // No entry found means nuke the list for sure.
                    (None, true)
                };
                if empty {
                    //TODO: This could be done with the HashMap::entry API instead.
                    inner.idle.remove(&self.key);
                }

                if entry.is_none() && self.waiter.is_none() {
                    let (tx, mut rx) = oneshot::channel();
                    trace!("checkout waiting for idle connection: {:?}", self.key);
                    inner
                        .waiters
                        .entry(self.key.clone())
                        .or_insert_with(VecDeque::new)
                        .push_back(tx);

                    // register the waker with this oneshot
                    assert!(Pin::new(&mut rx).poll(cx).is_pending());
                    self.waiter = Some(rx);
                }

                entry
            };

            entry.map(|e| self.pool.reuse(&self.key, e.value))
        }
    }

    impl<T: Poolable> Future for Checkout<T> {
        type Output = crate::Result<Pooled<T>>;

        fn poll(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
            if let Some(pooled) = ready!(self.poll_waiter(cx)?) {
                return Poll::Ready(Ok(pooled));
            }

            if let Some(pooled) = self.checkout(cx) {
                Poll::Ready(Ok(pooled))
            } else if !self.pool.is_enabled() {
                Poll::Ready(Err(crate::Error::new_canceled().with("pool is disabled")))
            } else {
                // There's a new waiter, already registered in self.checkout()
                debug_assert!(self.waiter.is_some());
                Poll::Pending
            }
        }
    }

    impl<T> Drop for Checkout<T> {
        fn drop(&mut self) {
            if self.waiter.take().is_some() {
                trace!("checkout dropped for {:?}", self.key);
                if let Some(Ok(mut inner)) = self.pool.inner.as_ref().map(|i| i.lock()) {
                    inner.clean_waiters(&self.key);
                }
            }
        }
    }

    // FIXME: allow() required due to `impl Trait` leaking types to this lint
    #[allow(missing_debug_implementations)]
    pub(super) struct Connecting<T: Poolable> {
        key: Key,
        pool: WeakOpt<Mutex<PoolInner<T>>>,
    }

    impl<T: Poolable> Connecting<T> {
        pub(super) fn alpn_h2(self, pool: &Pool<T>) -> Option<Self> {
            debug_assert!(
                self.pool.0.is_none(),
                "Connecting::alpn_h2 but already Http2"
            );

            pool.connecting(&self.key, Ver::Http2)
        }
    }

    impl<T: Poolable> Drop for Connecting<T> {
        fn drop(&mut self) {
            if let Some(pool) = self.pool.upgrade() {
                // No need to panic on drop, that could abort!
                if let Ok(mut inner) = pool.lock() {
                    inner.connected(&self.key);
                }
            }
        }
    }

    struct Expiration(Option<Duration>);

    impl Expiration {
        fn new(dur: Option<Duration>) -> Expiration {
            Expiration(dur)
        }

        fn expires(&self, instant: Instant) -> bool {
            match self.0 {
                Some(timeout) => instant.elapsed() > timeout,
                None => false,
            }
        }
    }

    #[cfg(feature = "runtime")]
    #[pin_project::pin_project]
    struct IdleTask<T> {
        #[pin]
        interval: Interval,
        pool: WeakOpt<Mutex<PoolInner<T>>>,
        // This allows the IdleTask to be notified as soon as the entire
        // Pool is fully dropped, and shutdown. This channel is never sent on,
        // but Err(Canceled) will be received when the Pool is dropped.
        #[pin]
        pool_drop_notifier: oneshot::Receiver<crate::common::Never>,
    }

    #[cfg(feature = "runtime")]
    impl<T: Poolable + 'static> Future for IdleTask<T> {
        type Output = ();

        fn poll(self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
            let mut this = self.project();
            loop {
                match this.pool_drop_notifier.as_mut().poll(cx) {
                    Poll::Ready(Ok(n)) => match n {},
                    Poll::Pending => (),
                    Poll::Ready(Err(_canceled)) => {
                        trace!("pool closed, canceling idle interval");
                        return Poll::Ready(());
                    }
                }

                ready!(this.interval.as_mut().poll_tick(cx));

                if let Some(inner) = this.pool.upgrade() {
                    if let Ok(mut inner) = inner.lock() {
                        trace!("idle interval checking for expired");
                        inner.clear_expired();
                        continue;
                    }
                }
                return Poll::Ready(());
            }
        }
    }

    impl<T> WeakOpt<T> {
        fn none() -> Self {
            WeakOpt(None)
        }

        fn downgrade(arc: &Arc<T>) -> Self {
            WeakOpt(Some(Arc::downgrade(arc)))
        }

        fn upgrade(&self) -> Option<Arc<T>> {
            self.0.as_ref().and_then(Weak::upgrade)
        }
    }

    #[cfg(test)]
    mod tests {
        use std::task::Poll;
        use std::time::Duration;

        use super::{Connecting, Key, Pool, Poolable, Reservation, WeakOpt};
        use crate::common::{task, exec::Exec, Future, Pin};

        /// Test unique reservations.
        #[derive(Debug, PartialEq, Eq)]
        struct Uniq<T>(T);

        impl<T: Send + 'static + Unpin> Poolable for Uniq<T> {
            fn is_open(&self) -> bool {
                true
            }

            fn reserve(self) -> Reservation<Self> {
                Reservation::Unique(self)
            }

            fn can_share(&self) -> bool {
                false
            }
        }

        fn c<T: Poolable>(key: Key) -> Connecting<T> {
            Connecting {
                key,
                pool: WeakOpt::none(),
            }
        }

        fn host_key(s: &str) -> Key {
            (http::uri::Scheme::HTTP, s.parse().expect("host key"))
        }

        fn pool_no_timer<T>() -> Pool<T> {
            pool_max_idle_no_timer(::std::usize::MAX)
        }

        fn pool_max_idle_no_timer<T>(max_idle: usize) -> Pool<T> {
            let pool = Pool::new(
                super::Config {
                    idle_timeout: Some(Duration::from_millis(100)),
                    max_idle_per_host: max_idle,
                },
                &Exec::Default,
            );
            pool.no_timer();
            pool
        }

        #[tokio::test]
        async fn test_pool_checkout_smoke() {
            let pool = pool_no_timer();
            let key = host_key("foo");
            let pooled = pool.pooled(c(key.clone()), Uniq(41));

            drop(pooled);

            match pool.checkout(key).await {
                Ok(pooled) => assert_eq!(*pooled, Uniq(41)),
                Err(_) => panic!("not ready"),
            };
        }

        /// Helper to check if the future is ready after polling once.
        struct PollOnce<'a, F>(&'a mut F);

        impl<F, T, U> Future for PollOnce<'_, F>
        where
            F: Future<Output = Result<T, U>> + Unpin,
        {
            type Output = Option<()>;

            fn poll(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
                match Pin::new(&mut self.0).poll(cx) {
                    Poll::Ready(Ok(_)) => Poll::Ready(Some(())),
                    Poll::Ready(Err(_)) => Poll::Ready(Some(())),
                    Poll::Pending => Poll::Ready(None),
                }
            }
        }

        #[tokio::test]
        async fn test_pool_checkout_returns_none_if_expired() {
            let pool = pool_no_timer();
            let key = host_key("foo");
            let pooled = pool.pooled(c(key.clone()), Uniq(41));

            drop(pooled);
            tokio::time::sleep(pool.locked().timeout.unwrap()).await;
            let mut checkout = pool.checkout(key);
            let poll_once = PollOnce(&mut checkout);
            let is_not_ready = poll_once.await.is_none();
            assert!(is_not_ready);
        }

        #[cfg(feature = "runtime")]
        #[tokio::test]
        async fn test_pool_checkout_removes_expired() {
            let pool = pool_no_timer();
            let key = host_key("foo");

            pool.pooled(c(key.clone()), Uniq(41));
            pool.pooled(c(key.clone()), Uniq(5));
            pool.pooled(c(key.clone()), Uniq(99));

            assert_eq!(
                pool.locked().idle.get(&key).map(|entries| entries.len()),
                Some(3)
            );
            tokio::time::sleep(pool.locked().timeout.unwrap()).await;

            let mut checkout = pool.checkout(key.clone());
            let poll_once = PollOnce(&mut checkout);
            // checkout.await should clean out the expired
            poll_once.await;
            assert!(pool.locked().idle.get(&key).is_none());
        }

        #[test]
        fn test_pool_max_idle_per_host() {
            let pool = pool_max_idle_no_timer(2);
            let key = host_key("foo");

            pool.pooled(c(key.clone()), Uniq(41));
            pool.pooled(c(key.clone()), Uniq(5));
            pool.pooled(c(key.clone()), Uniq(99));

            // pooled and dropped 3, max_idle should only allow 2
            assert_eq!(
                pool.locked().idle.get(&key).map(|entries| entries.len()),
                Some(2)
            );
        }

        #[cfg(feature = "runtime")]
        #[tokio::test]
        async fn test_pool_timer_removes_expired() {
            let _ = pretty_env_logger::try_init();
            tokio::time::pause();

            let pool = Pool::new(
                super::Config {
                    idle_timeout: Some(Duration::from_millis(10)),
                    max_idle_per_host: std::usize::MAX,
                },
                &Exec::Default,
            );

            let key = host_key("foo");

            pool.pooled(c(key.clone()), Uniq(41));
            pool.pooled(c(key.clone()), Uniq(5));
            pool.pooled(c(key.clone()), Uniq(99));

            assert_eq!(
                pool.locked().idle.get(&key).map(|entries| entries.len()),
                Some(3)
            );

            // Let the timer tick passed the expiration...
            tokio::time::advance(Duration::from_millis(30)).await;
            // Yield so the Interval can reap...
            tokio::task::yield_now().await;

            assert!(pool.locked().idle.get(&key).is_none());
        }

        #[tokio::test]
        async fn test_pool_checkout_task_unparked() {
            use futures_util::future::join;
            use futures_util::FutureExt;

            let pool = pool_no_timer();
            let key = host_key("foo");
            let pooled = pool.pooled(c(key.clone()), Uniq(41));

            let checkout = join(pool.checkout(key), async {
                // the checkout future will park first,
                // and then this lazy future will be polled, which will insert
                // the pooled back into the pool
                //
                // this test makes sure that doing so will unpark the checkout
                drop(pooled);
            })
            .map(|(entry, _)| entry);

            assert_eq!(*checkout.await.unwrap(), Uniq(41));
        }

        #[tokio::test]
        async fn test_pool_checkout_drop_cleans_up_waiters() {
            let pool = pool_no_timer::<Uniq<i32>>();
            let key = host_key("foo");

            let mut checkout1 = pool.checkout(key.clone());
            let mut checkout2 = pool.checkout(key.clone());

            let poll_once1 = PollOnce(&mut checkout1);
            let poll_once2 = PollOnce(&mut checkout2);

            // first poll needed to get into Pool's parked
            poll_once1.await;
            assert_eq!(pool.locked().waiters.get(&key).unwrap().len(), 1);
            poll_once2.await;
            assert_eq!(pool.locked().waiters.get(&key).unwrap().len(), 2);

            // on drop, clean up Pool
            drop(checkout1);
            assert_eq!(pool.locked().waiters.get(&key).unwrap().len(), 1);

            drop(checkout2);
            assert!(pool.locked().waiters.get(&key).is_none());
        }

        #[derive(Debug)]
        struct CanClose {
            val: i32,
            closed: bool,
        }

        impl Poolable for CanClose {
            fn is_open(&self) -> bool {
                !self.closed
            }

            fn reserve(self) -> Reservation<Self> {
                Reservation::Unique(self)
            }

            fn can_share(&self) -> bool {
                false
            }
        }

        #[test]
        fn pooled_drop_if_closed_doesnt_reinsert() {
            let pool = pool_no_timer();
            let key = host_key("foo");
            pool.pooled(
                c(key.clone()),
                CanClose {
                    val: 57,
                    closed: true,
                },
            );

            assert!(!pool.locked().idle.contains_key(&key));
        }
    }