1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
#![allow(dead_code)]

use std::collections::{HashMap, HashSet, VecDeque};
use std::convert::Infallible;
use std::error::Error as StdError;
use std::fmt::{self, Debug};
use std::future::Future;
use std::hash::Hash;
use std::ops::{Deref, DerefMut};
use std::pin::Pin;
use std::sync::{Arc, Mutex, Weak};
use std::task::{self, Poll};

use std::time::{Duration, Instant};

use futures_channel::oneshot;
use futures_util::ready;
use tracing::{debug, trace};

use hyper::rt::Sleep;
use hyper::rt::Timer as _;

use crate::common::{exec, exec::Exec, timer::Timer};

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

// 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 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;
}

pub trait Key: Eq + Hash + Clone + Debug + Unpin + Send + 'static {}

impl<T> Key for T where T: Eq + Hash + Clone + Debug + Unpin + Send + 'static {}

/// A marker to identify what version a pooled connection is.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
#[allow(dead_code)]
pub enum Ver {
    Auto,
    Http2,
}

/// 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 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 type Key = (http::uri::Scheme, http::uri::Authority); //Arc<String>;

struct PoolInner<T, K: Eq + Hash> {
    // 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<K>,
    // 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<K, 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<K, 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.
    idle_interval_ref: Option<oneshot::Sender<Infallible>>,
    exec: Exec,
    timer: Option<Timer>,
    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 struct Config {
    pub idle_timeout: Option<Duration>,
    pub max_idle_per_host: usize,
}

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

impl<T, K: Key> Pool<T, K> {
    pub fn new<E, M>(config: Config, executor: E, timer: Option<M>) -> Pool<T, K>
    where
        E: hyper::rt::Executor<exec::BoxSendFuture> + Send + Sync + Clone + 'static,
        M: hyper::rt::Timer + Send + Sync + Clone + 'static,
    {
        let exec = Exec::new(executor);
        let timer = timer.map(|t| Timer::new(t));
        let inner = if config.is_enabled() {
            Some(Arc::new(Mutex::new(PoolInner {
                connecting: HashSet::new(),
                idle: HashMap::new(),
                idle_interval_ref: None,
                max_idle_per_host: config.max_idle_per_host,
                waiters: HashMap::new(),
                exec,
                timer,
                timeout: config.idle_timeout,
            })))
        } else {
            None
        };

        Pool { inner }
    }

    pub(crate) 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...
        {
            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, K: Key> Pool<T, K> {
    /// Returns a `Checkout` which is a future that resolves if an idle
    /// connection becomes available.
    pub fn checkout(&self, key: K) -> Checkout<T, K> {
        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 fn connecting(&self, key: &K, ver: Ver) -> Option<Connecting<T, K>> {
        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, K>> {
        self.inner.as_ref().expect("enabled").lock().expect("lock")
    }

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

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

    pub fn pooled(
        &self,
        #[cfg_attr(not(feature = "http2"), allow(unused_mut))] mut connecting: Connecting<T, K>,
        value: T,
    ) -> Pooled<T, K> {
        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: &K, value: T) -> Pooled<T, K> {
        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, K> {
    key: &'a K,
    list: &'a mut Vec<Idle<T>>,
}

impl<'a, T: Poolable + 'a, K: Debug> IdlePopper<'a, T, K> {
    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, K: Key> PoolInner<T, K> {
    fn put(&mut self, key: K, value: T, __pool_ref: &Arc<Mutex<PoolInner<T, K>>>) {
        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_default();
                    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(),
                    });
                }

                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: &K) {
        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);
    }

    fn spawn_idle_interval(&mut self, pool_ref: &Arc<Mutex<PoolInner<T, K>>>) {
        if self.idle_interval_ref.is_some() {
            return;
        }
        let dur = if let Some(dur) = self.timeout {
            dur
        } else {
            return;
        };
        let timer = if let Some(timer) = self.timer.clone() {
            timer
        } else {
            return;
        };
        let (tx, rx) = oneshot::channel();
        self.idle_interval_ref = Some(tx);

        let interval = IdleTask {
            timer: timer.clone(),
            duration: dur,
            deadline: Instant::now(),
            fut: timer.sleep_until(Instant::now()), // ready at first tick
            pool: WeakOpt::downgrade(pool_ref),
            pool_drop_notifier: rx,
        };

        self.exec.execute(interval);
    }
}

impl<T, K: Eq + Hash> PoolInner<T, K> {
    /// 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: &K) {
        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);
        }
    }
}

impl<T: Poolable, K: Key> PoolInner<T, K> {
    /// 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;
                }

                // Avoid `Instant::sub` to avoid issues like rust-lang/rust#86470.
                if now.saturating_duration_since(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, K: Key> Clone for Pool<T, K> {
    fn clone(&self) -> Pool<T, K> {
        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 struct Pooled<T: Poolable, K: Key> {
    value: Option<T>,
    is_reused: bool,
    key: K,
    pool: WeakOpt<Mutex<PoolInner<T, K>>>,
}

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

    pub 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, K: Key> Deref for Pooled<T, K> {
    type Target = T;
    fn deref(&self) -> &T {
        self.as_ref()
    }
}

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

impl<T: Poolable, K: Key> Drop for Pooled<T, K> {
    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, K: Key> fmt::Debug for Pooled<T, K> {
    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 struct Checkout<T, K: Key> {
    key: K,
    pool: Pool<T, K>,
    waiter: Option<oneshot::Receiver<T>>,
}

#[derive(Debug)]
#[non_exhaustive]
pub enum Error {
    PoolDisabled,
    CheckoutNoLongerWanted,
    CheckedOutClosedValue,
}

impl Error {
    pub(super) fn is_canceled(&self) -> bool {
        matches!(self, Error::CheckedOutClosedValue)
    }
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(match self {
            Error::PoolDisabled => "pool is disabled",
            Error::CheckedOutClosedValue => "checked out connection was closed",
            Error::CheckoutNoLongerWanted => "request was canceled",
        })
    }
}

impl StdError for Error {}

impl<T: Poolable, K: Key> Checkout<T, K> {
    fn poll_waiter(
        &mut self,
        cx: &mut task::Context<'_>,
    ) -> Poll<Option<Result<Pooled<T, K>, Error>>> {
        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(Error::CheckedOutClosedValue)))
                    }
                }
                Poll::Pending => {
                    self.waiter = Some(rx);
                    Poll::Pending
                }
                Poll::Ready(Err(_canceled)) => {
                    Poll::Ready(Some(Err(Error::CheckoutNoLongerWanted)))
                }
            }
        } else {
            Poll::Ready(None)
        }
    }

    fn checkout(&mut self, cx: &mut task::Context<'_>) -> Option<Pooled<T, K>> {
        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, K: Key> Future for Checkout<T, K> {
    type Output = Result<Pooled<T, K>, Error>;

    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(Error::PoolDisabled))
        } else {
            // There's a new waiter, already registered in self.checkout()
            debug_assert!(self.waiter.is_some());
            Poll::Pending
        }
    }
}

impl<T, K: Key> Drop for Checkout<T, K> {
    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 struct Connecting<T: Poolable, K: Key> {
    key: K,
    pool: WeakOpt<Mutex<PoolInner<T, K>>>,
}

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

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

impl<T: Poolable, K: Key> Drop for Connecting<T, K> {
    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 {
            // Avoid `Instant::elapsed` to avoid issues like rust-lang/rust#86470.
            Some(timeout) => Instant::now().saturating_duration_since(instant) > timeout,
            None => false,
        }
    }
}

pin_project_lite::pin_project! {
    struct IdleTask<T, K: Key> {
        timer: Timer,
        duration: Duration,
        deadline: Instant,
        fut: Pin<Box<dyn Sleep>>,
        pool: WeakOpt<Mutex<PoolInner<T, K>>>,
        // 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<Infallible>,
    }
}

impl<T: Poolable + 'static, K: Key> Future for IdleTask<T, K> {
    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!(Pin::new(&mut this.fut).poll(cx));
            // Set this task to run after the next deadline
            // If the poll missed the deadline by a lot, set the deadline
            // from the current time instead
            *this.deadline += *this.duration;
            if *this.deadline < Instant::now() - Duration::from_millis(5) {
                *this.deadline = Instant::now() + *this.duration;
            }
            *this.fut = this.timer.sleep_until(*this.deadline);

            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(all(test, not(miri)))]
mod tests {
    use std::fmt::Debug;
    use std::future::Future;
    use std::hash::Hash;
    use std::pin::Pin;
    use std::task::{self, Poll};
    use std::time::Duration;

    use super::{Connecting, Key, Pool, Poolable, Reservation, WeakOpt};
    use crate::rt::{TokioExecutor, TokioTimer};

    use crate::common::timer;

    #[derive(Clone, Debug, PartialEq, Eq, Hash)]
    struct KeyImpl(http::uri::Scheme, http::uri::Authority);

    type KeyTuple = (http::uri::Scheme, http::uri::Authority);

    /// 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, K: Key>(key: K) -> Connecting<T, K> {
        Connecting {
            key,
            pool: WeakOpt::none(),
        }
    }

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

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

    fn pool_max_idle_no_timer<T, K: Key>(max_idle: usize) -> Pool<T, K> {
        let pool = Pool::new(
            super::Config {
                idle_timeout: Some(Duration::from_millis(100)),
                max_idle_per_host: max_idle,
            },
            TokioExecutor::new(),
            Option::<timer::Timer>::None,
        );
        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);
    }

    #[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)
        );
    }

    #[tokio::test]
    async fn test_pool_timer_removes_expired() {
        let pool = Pool::new(
            super::Config {
                idle_timeout: Some(Duration::from_millis(10)),
                max_idle_per_host: std::usize::MAX,
            },
            TokioExecutor::new(),
            Some(TokioTimer::new()),
        );

        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::sleep(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>, KeyImpl>();
        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 {
        #[allow(unused)]
        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));
    }
}