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
#![doc(html_root_url = "https://docs.rs/tokio-reactor/0.1.12")]
#![deny(missing_docs, missing_debug_implementations)]

//! Event loop that drives Tokio I/O resources.
//!
//! > **Note:** This crate is **deprecated in tokio 0.2.x** and has been moved
//! > and refactored into various places in the [`tokio::runtime`] and
//! > [`tokio::io`] modules of the [`tokio`] crate. The Reactor has also been
//! > renamed the "I/O Driver".
//!
//! [`tokio::runtime`]: https://docs.rs/tokio/latest/tokio/runtime/index.html
//! [`tokio::io`]: https://docs.rs/tokio/latest/tokio/io/index.html
//! [`tokio`]: https://docs.rs/tokio/latest/tokio/index.html
//! [`io-driver` feature]: https://docs.rs/tokio/0.2.9/tokio/index.html#feature-flags
//!
//! The reactor is the engine that drives asynchronous I/O resources (like TCP and
//! UDP sockets). It is backed by [`mio`] and acts as a bridge between [`mio`] and
//! [`futures`].
//!
//! The crate provides:
//!
//! * [`Reactor`] is the main type of this crate. It performs the event loop logic.
//!
//! * [`Handle`] provides a reference to a reactor instance.
//!
//! * [`Registration`] and [`PollEvented`] allow third parties to implement I/O
//!   resources that are driven by the reactor.
//!
//! Application authors will not use this crate directly. Instead, they will use the
//! `tokio` crate. Library authors should only depend on `tokio-reactor` if they
//! are building a custom I/O resource.
//!
//! For more details, see [reactor module] documentation in the Tokio crate.
//!
//! [`mio`]: http://github.com/carllerche/mio
//! [`futures`]: http://github.com/rust-lang-nursery/futures-rs
//! [`Reactor`]: struct.Reactor.html
//! [`Handle`]: struct.Handle.html
//! [`Registration`]: struct.Registration.html
//! [`PollEvented`]: struct.PollEvented.html
//! [reactor module]: https://docs.rs/tokio/0.1/tokio/reactor/index.html

extern crate crossbeam_utils;
#[macro_use]
extern crate futures;
#[macro_use]
extern crate lazy_static;
#[macro_use]
extern crate log;
extern crate mio;
extern crate num_cpus;
extern crate parking_lot;
extern crate slab;
extern crate tokio_executor;
extern crate tokio_io;
extern crate tokio_sync;

pub(crate) mod background;
mod poll_evented;
mod registration;
mod sharded_rwlock;

// ===== Public re-exports =====

pub use self::background::{Background, Shutdown};
pub use self::poll_evented::PollEvented;
pub use self::registration::Registration;

// ===== Private imports =====

use sharded_rwlock::RwLock;

use futures::task::Task;
use tokio_executor::park::{Park, Unpark};
use tokio_executor::Enter;
use tokio_sync::task::AtomicTask;

use std::cell::RefCell;
use std::error::Error;
use std::io;
use std::mem;
#[cfg(all(unix, not(target_os = "fuchsia")))]
use std::os::unix::io::{AsRawFd, RawFd};
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::{Relaxed, SeqCst};
use std::sync::{Arc, Weak};
use std::time::{Duration, Instant};
use std::{fmt, usize};

use log::Level;
use mio::event::Evented;
use slab::Slab;

/// The core reactor, or event loop.
///
/// The event loop is the main source of blocking in an application which drives
/// all other I/O events and notifications happening. Each event loop can have
/// multiple handles pointing to it, each of which can then be used to create
/// various I/O objects to interact with the event loop in interesting ways.
pub struct Reactor {
    /// Reuse the `mio::Events` value across calls to poll.
    events: mio::Events,

    /// State shared between the reactor and the handles.
    inner: Arc<Inner>,

    _wakeup_registration: mio::Registration,
}

/// A reference to a reactor.
///
/// A `Handle` is used for associating I/O objects with an event loop
/// explicitly. Typically though you won't end up using a `Handle` that often
/// and will instead use the default reactor for the execution context.
///
/// By default, most components bind lazily to reactors.
/// To get this behavior when manually passing a `Handle`, use `default()`.
#[derive(Clone)]
pub struct Handle {
    inner: Option<HandlePriv>,
}

/// Like `Handle`, but never `None`.
#[derive(Clone)]
struct HandlePriv {
    inner: Weak<Inner>,
}

/// Return value from the `turn` method on `Reactor`.
///
/// Currently this value doesn't actually provide any functionality, but it may
/// in the future give insight into what happened during `turn`.
#[derive(Debug)]
pub struct Turn {
    _priv: (),
}

/// Error returned from `Handle::set_fallback`.
#[derive(Clone, Debug)]
pub struct SetFallbackError(());

#[deprecated(since = "0.1.2", note = "use SetFallbackError instead")]
#[doc(hidden)]
pub type SetDefaultError = SetFallbackError;

/// Ensure that the default reactor is removed from the thread-local context
/// when leaving the scope. This handles cases that involve panicking.
#[derive(Debug)]
pub struct DefaultGuard {
    _p: (),
}

#[test]
fn test_handle_size() {
    use std::mem;
    assert_eq!(mem::size_of::<Handle>(), mem::size_of::<HandlePriv>());
}

struct Inner {
    /// The underlying system event queue.
    io: mio::Poll,

    /// ABA guard counter
    next_aba_guard: AtomicUsize,

    /// Dispatch slabs for I/O and futures events
    io_dispatch: RwLock<Slab<ScheduledIo>>,

    /// Used to wake up the reactor from a call to `turn`
    wakeup: mio::SetReadiness,
}

struct ScheduledIo {
    aba_guard: usize,
    readiness: AtomicUsize,
    reader: AtomicTask,
    writer: AtomicTask,
}

#[derive(Debug, Eq, PartialEq, Clone, Copy)]
pub(crate) enum Direction {
    Read,
    Write,
}

/// The global fallback reactor.
static HANDLE_FALLBACK: AtomicUsize = AtomicUsize::new(0);

thread_local! {
    /// Tracks the reactor for the current execution context.
    static CURRENT_REACTOR: RefCell<Option<HandlePriv>> = RefCell::new(None)
}

const TOKEN_SHIFT: usize = 22;

// Kind of arbitrary, but this reserves some token space for later usage.
const MAX_SOURCES: usize = (1 << TOKEN_SHIFT) - 1;
const TOKEN_WAKEUP: mio::Token = mio::Token(MAX_SOURCES);

fn _assert_kinds() {
    fn _assert<T: Send + Sync>() {}

    _assert::<Handle>();
}

// ===== impl Reactor =====

/// Set the default reactor for the duration of the closure
///
/// # Panics
///
/// This function panics if there already is a default reactor set.
pub fn with_default<F, R>(handle: &Handle, enter: &mut Enter, f: F) -> R
where
    F: FnOnce(&mut Enter) -> R,
{
    // This ensures the value for the current reactor gets reset even if there
    // is a panic.
    let _guard = set_default(handle);
    f(enter)
}

/// Sets `handle` as the default reactor, returning a guard that unsets it when
/// dropped.
///
/// # Panics
///
/// This function panics if there already is a default reactor set.
pub fn set_default(handle: &Handle) -> DefaultGuard {
    CURRENT_REACTOR.with(|current| {
        let mut current = current.borrow_mut();

        assert!(
            current.is_none(),
            "default Tokio reactor already set \
             for execution context"
        );

        let handle = match handle.as_priv() {
            Some(handle) => handle,
            None => {
                panic!("`handle` does not reference a reactor");
            }
        };

        *current = Some(handle.clone());
    });
    DefaultGuard { _p: () }
}

impl Reactor {
    /// Creates a new event loop, returning any error that happened during the
    /// creation.
    pub fn new() -> io::Result<Reactor> {
        let io = mio::Poll::new()?;
        let wakeup_pair = mio::Registration::new2();

        io.register(
            &wakeup_pair.0,
            TOKEN_WAKEUP,
            mio::Ready::readable(),
            mio::PollOpt::level(),
        )?;

        Ok(Reactor {
            events: mio::Events::with_capacity(1024),
            _wakeup_registration: wakeup_pair.0,
            inner: Arc::new(Inner {
                io: io,
                next_aba_guard: AtomicUsize::new(0),
                io_dispatch: RwLock::new(Slab::with_capacity(1)),
                wakeup: wakeup_pair.1,
            }),
        })
    }

    /// Returns a handle to this event loop which can be sent across threads
    /// and can be used as a proxy to the event loop itself.
    ///
    /// Handles are cloneable and clones always refer to the same event loop.
    /// This handle is typically passed into functions that create I/O objects
    /// to bind them to this event loop.
    pub fn handle(&self) -> Handle {
        Handle {
            inner: Some(HandlePriv {
                inner: Arc::downgrade(&self.inner),
            }),
        }
    }

    /// Configures the fallback handle to be returned from `Handle::default`.
    ///
    /// The `Handle::default()` function will by default lazily spin up a global
    /// thread and run a reactor on this global thread. This behavior is not
    /// always desirable in all applications, however, and sometimes a different
    /// fallback reactor is desired.
    ///
    /// This function will attempt to globally alter the return value of
    /// `Handle::default()` to return the `handle` specified rather than a
    /// lazily initialized global thread. If successful then all future calls to
    /// `Handle::default()` which would otherwise fall back to the global thread
    /// will instead return a clone of the handle specified.
    ///
    /// # Errors
    ///
    /// This function may not always succeed in configuring the fallback handle.
    /// If this function was previously called (or perhaps concurrently called
    /// on many threads) only the *first* invocation of this function will
    /// succeed. All other invocations will return an error.
    ///
    /// Additionally if the global reactor thread has already been initialized
    /// then this function will also return an error. (aka if `Handle::default`
    /// has been called previously in this program).
    pub fn set_fallback(&self) -> Result<(), SetFallbackError> {
        set_fallback(self.handle().into_priv().unwrap())
    }

    /// Performs one iteration of the event loop, blocking on waiting for events
    /// for at most `max_wait` (forever if `None`).
    ///
    /// This method is the primary method of running this reactor and processing
    /// I/O events that occur. This method executes one iteration of an event
    /// loop, blocking at most once waiting for events to happen.
    ///
    /// If a `max_wait` is specified then the method should block no longer than
    /// the duration specified, but this shouldn't be used as a super-precise
    /// timer but rather a "ballpark approximation"
    ///
    /// # Return value
    ///
    /// This function returns an instance of `Turn`
    ///
    /// `Turn` as of today has no extra information with it and can be safely
    /// discarded.  In the future `Turn` may contain information about what
    /// happened while this reactor blocked.
    ///
    /// # Errors
    ///
    /// This function may also return any I/O error which occurs when polling
    /// for readiness of I/O objects with the OS. This is quite unlikely to
    /// arise and typically mean that things have gone horribly wrong at that
    /// point. Currently this is primarily only known to happen for internal
    /// bugs to `tokio` itself.
    pub fn turn(&mut self, max_wait: Option<Duration>) -> io::Result<Turn> {
        self.poll(max_wait)?;
        Ok(Turn { _priv: () })
    }

    /// Returns true if the reactor is currently idle.
    ///
    /// Idle is defined as all tasks that have been spawned have completed,
    /// either successfully or with an error.
    pub fn is_idle(&self) -> bool {
        self.inner.io_dispatch.read().is_empty()
    }

    /// Run this reactor on a background thread.
    ///
    /// This function takes ownership, spawns a new thread, and moves the
    /// reactor to this new thread. It then runs the reactor, driving all
    /// associated I/O resources, until the `Background` handle is dropped or
    /// explicitly shutdown.
    pub fn background(self) -> io::Result<Background> {
        Background::new(self)
    }

    fn poll(&mut self, max_wait: Option<Duration>) -> io::Result<()> {
        // Block waiting for an event to happen, peeling out how many events
        // happened.
        match self.inner.io.poll(&mut self.events, max_wait) {
            Ok(_) => {}
            Err(e) => return Err(e),
        }

        let start = if log_enabled!(Level::Debug) {
            Some(Instant::now())
        } else {
            None
        };

        // Process all the events that came in, dispatching appropriately
        let mut events = 0;
        for event in self.events.iter() {
            events += 1;
            let token = event.token();
            trace!("event {:?} {:?}", event.readiness(), event.token());

            if token == TOKEN_WAKEUP {
                self.inner
                    .wakeup
                    .set_readiness(mio::Ready::empty())
                    .unwrap();
            } else {
                self.dispatch(token, event.readiness());
            }
        }

        if let Some(start) = start {
            let dur = start.elapsed();
            trace!(
                "loop process - {} events, {}.{:03}s",
                events,
                dur.as_secs(),
                dur.subsec_nanos() / 1_000_000
            );
        }

        Ok(())
    }

    fn dispatch(&self, token: mio::Token, ready: mio::Ready) {
        let aba_guard = token.0 & !MAX_SOURCES;
        let token = token.0 & MAX_SOURCES;

        let mut rd = None;
        let mut wr = None;

        // Create a scope to ensure that notifying the tasks stays out of the
        // lock's critical section.
        {
            let io_dispatch = self.inner.io_dispatch.read();

            let io = match io_dispatch.get(token) {
                Some(io) => io,
                None => return,
            };

            if aba_guard != io.aba_guard {
                return;
            }

            io.readiness.fetch_or(ready.as_usize(), Relaxed);

            if ready.is_writable() || platform::is_hup(&ready) {
                wr = io.writer.take_task();
            }

            if !(ready & (!mio::Ready::writable())).is_empty() {
                rd = io.reader.take_task();
            }
        }

        if let Some(task) = rd {
            task.notify();
        }

        if let Some(task) = wr {
            task.notify();
        }
    }
}

#[cfg(all(unix, not(target_os = "fuchsia")))]
impl AsRawFd for Reactor {
    fn as_raw_fd(&self) -> RawFd {
        self.inner.io.as_raw_fd()
    }
}

impl Park for Reactor {
    type Unpark = Handle;
    type Error = io::Error;

    fn unpark(&self) -> Self::Unpark {
        self.handle()
    }

    fn park(&mut self) -> io::Result<()> {
        self.turn(None)?;
        Ok(())
    }

    fn park_timeout(&mut self, duration: Duration) -> io::Result<()> {
        self.turn(Some(duration))?;
        Ok(())
    }
}

impl fmt::Debug for Reactor {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "Reactor")
    }
}

// ===== impl Handle =====

impl Handle {
    #[doc(hidden)]
    #[deprecated(note = "semantics were sometimes surprising, use Handle::default()")]
    pub fn current() -> Handle {
        // TODO: Should this panic on error?
        HandlePriv::try_current()
            .map(|handle| Handle {
                inner: Some(handle),
            })
            .unwrap_or(Handle {
                inner: Some(HandlePriv { inner: Weak::new() }),
            })
    }

    fn as_priv(&self) -> Option<&HandlePriv> {
        self.inner.as_ref()
    }

    fn into_priv(self) -> Option<HandlePriv> {
        self.inner
    }

    fn wakeup(&self) {
        if let Some(handle) = self.as_priv() {
            handle.wakeup();
        }
    }
}

impl Unpark for Handle {
    fn unpark(&self) {
        if let Some(ref h) = self.inner {
            h.wakeup();
        }
    }
}

impl Default for Handle {
    /// Returns a "default" handle, i.e., a handle that lazily binds to a reactor.
    fn default() -> Handle {
        Handle { inner: None }
    }
}

impl fmt::Debug for Handle {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "Handle")
    }
}

fn set_fallback(handle: HandlePriv) -> Result<(), SetFallbackError> {
    unsafe {
        let val = handle.into_usize();
        match HANDLE_FALLBACK.compare_exchange(0, val, SeqCst, SeqCst) {
            Ok(_) => Ok(()),
            Err(_) => {
                drop(HandlePriv::from_usize(val));
                Err(SetFallbackError(()))
            }
        }
    }
}

// ===== impl HandlePriv =====

impl HandlePriv {
    /// Try to get a handle to the current reactor.
    ///
    /// Returns `Err` if no handle is found.
    pub(crate) fn try_current() -> io::Result<HandlePriv> {
        CURRENT_REACTOR.with(|current| match *current.borrow() {
            Some(ref handle) => Ok(handle.clone()),
            None => HandlePriv::fallback(),
        })
    }

    /// Returns a handle to the fallback reactor.
    fn fallback() -> io::Result<HandlePriv> {
        let mut fallback = HANDLE_FALLBACK.load(SeqCst);

        // If the fallback hasn't been previously initialized then let's spin
        // up a helper thread and try to initialize with that. If we can't
        // actually create a helper thread then we'll just return a "defunct"
        // handle which will return errors when I/O objects are attempted to be
        // associated.
        if fallback == 0 {
            let reactor = match Reactor::new() {
                Ok(reactor) => reactor,
                Err(_) => {
                    return Err(io::Error::new(
                        io::ErrorKind::Other,
                        "failed to create reactor",
                    ));
                }
            };

            // If we successfully set ourselves as the actual fallback then we
            // want to `forget` the helper thread to ensure that it persists
            // globally. If we fail to set ourselves as the fallback that means
            // that someone was racing with this call to `Handle::default`.
            // They ended up winning so we'll destroy our helper thread (which
            // shuts down the thread) and reload the fallback.
            if set_fallback(reactor.handle().into_priv().unwrap()).is_ok() {
                let ret = reactor.handle().into_priv().unwrap();

                match reactor.background() {
                    Ok(bg) => bg.forget(),
                    // The global handle is fubar, but y'all probably got bigger
                    // problems if a thread can't spawn.
                    Err(_) => {}
                }

                return Ok(ret);
            }

            fallback = HANDLE_FALLBACK.load(SeqCst);
        }

        // At this point our fallback handle global was configured so we use
        // its value to reify a handle, clone it, and then forget our reified
        // handle as we don't actually have an owning reference to it.
        assert!(fallback != 0);

        let ret = unsafe {
            let handle = HandlePriv::from_usize(fallback);
            let ret = handle.clone();

            // This prevents `handle` from being dropped and having the ref
            // count decremented.
            drop(handle.into_usize());

            ret
        };

        Ok(ret)
    }

    /// Forces a reactor blocked in a call to `turn` to wakeup, or otherwise
    /// makes the next call to `turn` return immediately.
    ///
    /// This method is intended to be used in situations where a notification
    /// needs to otherwise be sent to the main reactor. If the reactor is
    /// currently blocked inside of `turn` then it will wake up and soon return
    /// after this method has been called. If the reactor is not currently
    /// blocked in `turn`, then the next call to `turn` will not block and
    /// return immediately.
    fn wakeup(&self) {
        if let Some(inner) = self.inner() {
            inner.wakeup.set_readiness(mio::Ready::readable()).unwrap();
        }
    }

    fn into_usize(self) -> usize {
        unsafe { mem::transmute::<Weak<Inner>, usize>(self.inner) }
    }

    unsafe fn from_usize(val: usize) -> HandlePriv {
        let inner = mem::transmute::<usize, Weak<Inner>>(val);
        HandlePriv { inner }
    }

    fn inner(&self) -> Option<Arc<Inner>> {
        self.inner.upgrade()
    }
}

impl fmt::Debug for HandlePriv {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "HandlePriv")
    }
}

// ===== impl Inner =====

impl Inner {
    /// Register an I/O resource with the reactor.
    ///
    /// The registration token is returned.
    fn add_source(&self, source: &dyn Evented) -> io::Result<usize> {
        // Get an ABA guard value
        let aba_guard = self.next_aba_guard.fetch_add(1 << TOKEN_SHIFT, Relaxed);

        let key = {
            // Block to contain the write lock
            let mut io_dispatch = self.io_dispatch.write();

            if io_dispatch.len() == MAX_SOURCES {
                return Err(io::Error::new(
                    io::ErrorKind::Other,
                    "reactor at max \
                     registered I/O resources",
                ));
            }

            io_dispatch.insert(ScheduledIo {
                aba_guard,
                readiness: AtomicUsize::new(0),
                reader: AtomicTask::new(),
                writer: AtomicTask::new(),
            })
        };

        let token = aba_guard | key;
        debug!("adding I/O source: {}", token);

        self.io.register(
            source,
            mio::Token(token),
            mio::Ready::all(),
            mio::PollOpt::edge(),
        )?;

        Ok(key)
    }

    /// Deregisters an I/O resource from the reactor.
    fn deregister_source(&self, source: &dyn Evented) -> io::Result<()> {
        self.io.deregister(source)
    }

    fn drop_source(&self, token: usize) {
        debug!("dropping I/O source: {}", token);
        self.io_dispatch.write().remove(token);
    }

    /// Registers interest in the I/O resource associated with `token`.
    fn register(&self, token: usize, dir: Direction, t: Task) {
        debug!("scheduling {:?} for: {}", dir, token);
        let io_dispatch = self.io_dispatch.read();
        let sched = io_dispatch.get(token).unwrap();

        let (task, ready) = match dir {
            Direction::Read => (&sched.reader, !mio::Ready::writable()),
            Direction::Write => (&sched.writer, mio::Ready::writable()),
        };

        task.register_task(t);

        if sched.readiness.load(SeqCst) & ready.as_usize() != 0 {
            task.notify();
        }
    }
}

impl Drop for Inner {
    fn drop(&mut self) {
        // When a reactor is dropped it needs to wake up all blocked tasks as
        // they'll never receive a notification, and all connected I/O objects
        // will start returning errors pretty quickly.
        let io = self.io_dispatch.read();
        for (_, io) in io.iter() {
            io.writer.notify();
            io.reader.notify();
        }
    }
}

impl Direction {
    fn mask(&self) -> mio::Ready {
        match *self {
            Direction::Read => {
                // Everything except writable is signaled through read.
                mio::Ready::all() - mio::Ready::writable()
            }
            Direction::Write => mio::Ready::writable() | platform::hup(),
        }
    }
}

impl Drop for DefaultGuard {
    fn drop(&mut self) {
        let _ = CURRENT_REACTOR.try_with(|current| {
            let mut current = current.borrow_mut();
            *current = None;
        });
    }
}

#[cfg(unix)]
mod platform {
    use mio::unix::UnixReady;
    use mio::Ready;

    pub fn hup() -> Ready {
        UnixReady::hup().into()
    }

    pub fn is_hup(ready: &Ready) -> bool {
        UnixReady::from(*ready).is_hup()
    }
}

#[cfg(windows)]
mod platform {
    use mio::Ready;

    pub fn hup() -> Ready {
        Ready::empty()
    }

    pub fn is_hup(_: &Ready) -> bool {
        false
    }
}

// ===== impl SetFallbackError =====

impl fmt::Display for SetFallbackError {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        write!(fmt, "{}", self.description())
    }
}

impl Error for SetFallbackError {
    fn description(&self) -> &str {
        "attempted to set fallback reactor while already configured"
    }
}