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
// Copyright 2018 Parity Technologies (UK) Ltd.
//
// Permission is hereby granted, free of charge, to any person obtaining a
// copy of this software and associated documentation files (the "Software"),
// to deal in the Software without restriction, including without limitation
// the rights to use, copy, modify, merge, publish, distribute, sublicense,
// and/or sell copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.

//! Muxing is the process of splitting a connection into multiple substreams.
//!
//! The main item of this module is the `StreamMuxer` trait. An implementation of `StreamMuxer`
//! has ownership of a connection, lets you open and close substreams, and read/write data
//! on open substreams.
//!
//! > **Note**: You normally don't need to use the methods of the `StreamMuxer` directly, as this
//! >           is managed by the library's internals.
//!
//! Each substream of a connection is an isolated stream of data. All the substreams are muxed
//! together so that the data read from or written to each substream doesn't influence the other
//! substreams.
//!
//! In the context of libp2p, each substream can use a different protocol. Contrary to opening a
//! connection, opening a substream is almost free in terms of resources. This means that you
//! shouldn't hesitate to rapidly open and close substreams, and to design protocols that don't
//! require maintaining long-lived channels of communication.
//!
//! > **Example**: The Kademlia protocol opens a new substream for each request it wants to
//! >              perform. Multiple requests can be performed simultaneously by opening multiple
//! >              substreams, without having to worry about associating responses with the
//! >              right request.
//!
//! # Implementing a muxing protocol
//!
//! In order to implement a muxing protocol, create an object that implements the `UpgradeInfo`,
//! `InboundUpgrade` and `OutboundUpgrade` traits. See the `upgrade` module for more information.
//! The `Output` associated type of the `InboundUpgrade` and `OutboundUpgrade` traits should be
//! identical, and should be an object that implements the `StreamMuxer` trait.
//!
//! The upgrade process will take ownership of the connection, which makes it possible for the
//! implementation of `StreamMuxer` to control everything that happens on the wire.

use fnv::FnvHashMap;
use futures::{future, prelude::*, task::Context, task::Poll};
use multiaddr::Multiaddr;
use parking_lot::Mutex;
use std::{io, ops::Deref, fmt, pin::Pin, sync::atomic::{AtomicUsize, Ordering}};

pub use self::singleton::SingletonMuxer;

mod singleton;

/// Implemented on objects that can open and manage substreams.
///
/// The state of a muxer, as exposed by this API, is the following:
///
/// - A connection to the remote. The `poll_event`, `flush_all` and `close` methods operate
///   on this.
/// - A list of substreams that are open. The `poll_outbound`, `read_substream`, `write_substream`,
///   `flush_substream`, `shutdown_substream` and `destroy_substream` methods allow controlling
///   these entries.
/// - A list of outbound substreams being opened. The `open_outbound`, `poll_outbound` and
///   `destroy_outbound` methods allow controlling these entries.
///
pub trait StreamMuxer {
    /// Type of the object that represents the raw substream where data can be read and written.
    type Substream;

    /// Future that will be resolved when the outgoing substream is open.
    type OutboundSubstream;

    /// Error type of the muxer
    type Error: Into<io::Error>;

    /// Polls for a connection-wide event.
    ///
    /// This function behaves the same as a `Stream`.
    ///
    /// If `Pending` is returned, then the current task will be notified once the muxer
    /// is ready to be polled, similar to the API of `Stream::poll()`.
    /// Only the latest task that was used to call this method may be notified.
    ///
    /// An error can be generated if the connection has been closed.
    fn poll_event(&self, cx: &mut Context<'_>) -> Poll<Result<StreamMuxerEvent<Self::Substream>, Self::Error>>;

    /// Opens a new outgoing substream, and produces the equivalent to a future that will be
    /// resolved when it becomes available.
    ///
    /// The API of `OutboundSubstream` is totally opaque, and the object can only be interfaced
    /// through the methods on the `StreamMuxer` trait.
    fn open_outbound(&self) -> Self::OutboundSubstream;

    /// Polls the outbound substream.
    ///
    /// If `Pending` is returned, then the current task will be notified once the substream
    /// is ready to be polled, similar to the API of `Future::poll()`.
    /// However, for each individual outbound substream, only the latest task that was used to
    /// call this method may be notified.
    ///
    /// May panic or produce an undefined result if an earlier polling of the same substream
    /// returned `Ready` or `Err`.
    fn poll_outbound(&self, cx: &mut Context<'_>, s: &mut Self::OutboundSubstream)
        -> Poll<Result<Self::Substream, Self::Error>>;

    /// Destroys an outbound substream future. Use this after the outbound substream has finished,
    /// or if you want to interrupt it.
    fn destroy_outbound(&self, s: Self::OutboundSubstream);

    /// Reads data from a substream. The behaviour is the same as `futures::AsyncRead::poll_read`.
    ///
    /// If `Pending` is returned, then the current task will be notified once the substream
    /// is ready to be read. However, for each individual substream, only the latest task that
    /// was used to call this method may be notified.
    ///
    /// If `Async::Ready(0)` is returned, the substream has been closed by the remote and should
    /// no longer be read afterwards.
    ///
    /// An error can be generated if the connection has been closed, or if a protocol misbehaviour
    /// happened.
    fn read_substream(&self, cx: &mut Context<'_>, s: &mut Self::Substream, buf: &mut [u8])
        -> Poll<Result<usize, Self::Error>>;

    /// Write data to a substream. The behaviour is the same as `futures::AsyncWrite::poll_write`.
    ///
    /// If `Pending` is returned, then the current task will be notified once the substream
    /// is ready to be read. For each individual substream, only the latest task that was used to
    /// call this method may be notified.
    ///
    /// Calling `write_substream` does not guarantee that data will arrive to the remote. To
    /// ensure that, you should call `flush_substream`.
    ///
    /// It is incorrect to call this method on a substream if you called `shutdown_substream` on
    /// this substream earlier.
    fn write_substream(&self, cx: &mut Context<'_>, s: &mut Self::Substream, buf: &[u8])
        -> Poll<Result<usize, Self::Error>>;

    /// Flushes a substream. The behaviour is the same as `futures::AsyncWrite::poll_flush`.
    ///
    /// After this method has been called, data written earlier on the substream is guaranteed to
    /// be received by the remote.
    ///
    /// If `Pending` is returned, then the current task will be notified once the substream
    /// is ready to be read. For each individual substream, only the latest task that was used to
    /// call this method may be notified.
    ///
    /// > **Note**: This method may be implemented as a call to `flush_all`.
    fn flush_substream(&self, cx: &mut Context<'_>, s: &mut Self::Substream)
        -> Poll<Result<(), Self::Error>>;

    /// Attempts to shut down the writing side of a substream. The behaviour is similar to
    /// `AsyncWrite::poll_close`.
    ///
    /// Contrary to `AsyncWrite::poll_close`, shutting down a substream does not imply
    /// `flush_substream`. If you want to make sure that the remote is immediately informed about
    /// the shutdown, use `flush_substream` or `flush_all`.
    ///
    /// After this method has been called, you should no longer attempt to write to this substream.
    ///
    /// An error can be generated if the connection has been closed, or if a protocol misbehaviour
    /// happened.
    fn shutdown_substream(&self, cx: &mut Context<'_>, s: &mut Self::Substream)
        -> Poll<Result<(), Self::Error>>;

    /// Destroys a substream.
    fn destroy_substream(&self, s: Self::Substream);

    /// Returns `true` if the remote has shown any sign of activity after the muxer has been open.
    ///
    /// For optimisation purposes, the connection handshake of libp2p can be very optimistic and is
    /// allowed to assume that the handshake has succeeded when it didn't in fact succeed. This
    /// method can be called in order to determine whether the remote has accepted our handshake or
    /// has potentially not received it yet.
    #[deprecated(note = "This method is unused and will be removed in the future")]
    fn is_remote_acknowledged(&self) -> bool {
        true
    }

    /// Closes this `StreamMuxer`.
    ///
    /// After this has returned `Poll::Ready(Ok(()))`, the muxer has become useless. All
    /// subsequent reads must return either `EOF` or an error. All subsequent writes, shutdowns,
    /// or polls must generate an error or be ignored.
    ///
    /// Calling this method implies `flush_all`.
    ///
    /// > **Note**: You are encouraged to call this method and wait for it to return `Ready`, so
    /// >           that the remote is properly informed of the shutdown. However, apart from
    /// >           properly informing the remote, there is no difference between this and
    /// >           immediately dropping the muxer.
    fn close(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>;

    /// Flush this `StreamMuxer`.
    ///
    /// This drains any write buffers of substreams and delivers any pending shutdown notifications
    /// due to `shutdown_substream` or `close`. One may thus shutdown groups of substreams
    /// followed by a final `flush_all` instead of having to do `flush_substream` for each.
    fn flush_all(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>;
}

/// Event about a connection, reported by an implementation of [`StreamMuxer`].
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum StreamMuxerEvent<T> {
    /// Remote has opened a new substream. Contains the substream in question.
    InboundSubstream(T),

    /// Address to the remote has changed. The previous one is now obsolete.
    ///
    /// > **Note**: This can for example happen when using the QUIC protocol, where the two nodes
    /// >           can change their IP address while retaining the same QUIC connection.
    AddressChange(Multiaddr),
}

impl<T> StreamMuxerEvent<T> {
    /// If `self` is a [`StreamMuxerEvent::InboundSubstream`], returns the content. Otherwise
    /// returns `None`.
    pub fn into_inbound_substream(self) -> Option<T> {
        if let StreamMuxerEvent::InboundSubstream(s) = self {
            Some(s)
        } else {
            None
        }
    }
}

/// Polls for an event from the muxer and, if an inbound substream, wraps this substream in an
/// object that implements `Read`/`Write`/`AsyncRead`/`AsyncWrite`.
pub fn event_from_ref_and_wrap<P>(
    muxer: P,
) -> impl Future<Output = Result<StreamMuxerEvent<SubstreamRef<P>>, <P::Target as StreamMuxer>::Error>>
where
    P: Deref + Clone,
    P::Target: StreamMuxer,
{
    let muxer2 = muxer.clone();
    future::poll_fn(move |cx| muxer.poll_event(cx))
        .map_ok(|event| {
            match event {
                StreamMuxerEvent::InboundSubstream(substream) =>
                    StreamMuxerEvent::InboundSubstream(substream_from_ref(muxer2, substream)),
                StreamMuxerEvent::AddressChange(addr) => StreamMuxerEvent::AddressChange(addr),
            }
        })
}

/// Same as `outbound_from_ref`, but wraps the output in an object that
/// implements `Read`/`Write`/`AsyncRead`/`AsyncWrite`.
pub fn outbound_from_ref_and_wrap<P>(muxer: P) -> OutboundSubstreamRefWrapFuture<P>
where
    P: Deref + Clone,
    P::Target: StreamMuxer,
{
    let inner = outbound_from_ref(muxer);
    OutboundSubstreamRefWrapFuture { inner }
}

/// Future returned by `outbound_from_ref_and_wrap`.
pub struct OutboundSubstreamRefWrapFuture<P>
where
    P: Deref + Clone,
    P::Target: StreamMuxer,
{
    inner: OutboundSubstreamRefFuture<P>,
}

impl<P> Future for OutboundSubstreamRefWrapFuture<P>
where
    P: Deref + Clone,
    P::Target: StreamMuxer,
{
    type Output = Result<SubstreamRef<P>, <P::Target as StreamMuxer>::Error>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        match Future::poll(Pin::new(&mut self.inner), cx) {
            Poll::Ready(Ok(substream)) => {
                let out = substream_from_ref(self.inner.muxer.clone(), substream);
                Poll::Ready(Ok(out))
            }
            Poll::Pending => Poll::Pending,
            Poll::Ready(Err(err)) => Poll::Ready(Err(err)),
        }
    }
}

/// Builds a new future for an outbound substream, where the muxer is a reference.
pub fn outbound_from_ref<P>(muxer: P) -> OutboundSubstreamRefFuture<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    let outbound = muxer.open_outbound();
    OutboundSubstreamRefFuture {
        muxer,
        outbound: Some(outbound),
    }
}

/// Future returned by `outbound_from_ref`.
pub struct OutboundSubstreamRefFuture<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    muxer: P,
    outbound: Option<<P::Target as StreamMuxer>::OutboundSubstream>,
}

impl<P> Unpin for OutboundSubstreamRefFuture<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
}

impl<P> Future for OutboundSubstreamRefFuture<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    type Output = Result<<P::Target as StreamMuxer>::Substream, <P::Target as StreamMuxer>::Error>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        // We use a `this` because the compiler isn't smart enough to allow mutably borrowing
        // multiple different fields from the `Pin` at the same time.
        let this = &mut *self;
        this.muxer.poll_outbound(cx, this.outbound.as_mut().expect("outbound was empty"))
    }
}

impl<P> Drop for OutboundSubstreamRefFuture<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    fn drop(&mut self) {
        self.muxer
            .destroy_outbound(self.outbound.take().expect("outbound was empty"))
    }
}

/// Builds an implementation of `Read`/`Write`/`AsyncRead`/`AsyncWrite` from an `Arc` to the
/// muxer and a substream.
pub fn substream_from_ref<P>(
    muxer: P,
    substream: <P::Target as StreamMuxer>::Substream,
) -> SubstreamRef<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    SubstreamRef {
        muxer,
        substream: Some(substream),
        shutdown_state: ShutdownState::Shutdown,
    }
}

/// Stream returned by `substream_from_ref`.
pub struct SubstreamRef<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    muxer: P,
    substream: Option<<P::Target as StreamMuxer>::Substream>,
    shutdown_state: ShutdownState,
}

enum ShutdownState {
    Shutdown,
    Flush,
    Done,
}

impl<P> fmt::Debug for SubstreamRef<P>
where
    P: Deref,
    P::Target: StreamMuxer,
    <P::Target as StreamMuxer>::Substream: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
        write!(f, "Substream({:?})", self.substream)
    }
}

impl<P> Unpin for SubstreamRef<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
}

impl<P> AsyncRead for SubstreamRef<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    fn poll_read(mut self: Pin<&mut Self>, cx: &mut Context<'_>, buf: &mut [u8]) -> Poll<Result<usize, io::Error>> {
        // We use a `this` because the compiler isn't smart enough to allow mutably borrowing
        // multiple different fields from the `Pin` at the same time.
        let this = &mut *self;

        let s = this.substream.as_mut().expect("substream was empty");
        this.muxer.read_substream(cx, s, buf).map_err(|e| e.into())
    }
}

impl<P> AsyncWrite for SubstreamRef<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    fn poll_write(mut self: Pin<&mut Self>, cx: &mut Context<'_>, buf: &[u8]) -> Poll<Result<usize, io::Error>> {
        // We use a `this` because the compiler isn't smart enough to allow mutably borrowing
        // multiple different fields from the `Pin` at the same time.
        let this = &mut *self;

        let s = this.substream.as_mut().expect("substream was empty");
        this.muxer.write_substream(cx, s, buf).map_err(|e| e.into())
    }

    fn poll_close(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
        // We use a `this` because the compiler isn't smart enough to allow mutably borrowing
        // multiple different fields from the `Pin` at the same time.
        let this = &mut *self;

        let s = this.substream.as_mut().expect("substream was empty");
        loop {
            match this.shutdown_state {
                ShutdownState::Shutdown => {
                    match this.muxer.shutdown_substream(cx, s) {
                        Poll::Ready(Ok(())) => this.shutdown_state = ShutdownState::Flush,
                        Poll::Ready(Err(err)) => return Poll::Ready(Err(err.into())),
                        Poll::Pending => return Poll::Pending,
                    }
                }
                ShutdownState::Flush => {
                    match this.muxer.flush_substream(cx, s) {
                        Poll::Ready(Ok(())) => this.shutdown_state = ShutdownState::Done,
                        Poll::Ready(Err(err)) => return Poll::Ready(Err(err.into())),
                        Poll::Pending => return Poll::Pending,
                    }
                }
                ShutdownState::Done => {
                    return Poll::Ready(Ok(()));
                }
            }
        }
    }

    fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
        // We use a `this` because the compiler isn't smart enough to allow mutably borrowing
        // multiple different fields from the `Pin` at the same time.
        let this = &mut *self;

        let s = this.substream.as_mut().expect("substream was empty");
        this.muxer.flush_substream(cx, s).map_err(|e| e.into())
    }
}

impl<P> Drop for SubstreamRef<P>
where
    P: Deref,
    P::Target: StreamMuxer,
{
    fn drop(&mut self) {
        self.muxer.destroy_substream(self.substream.take().expect("substream was empty"))
    }
}

/// Abstract `StreamMuxer`.
pub struct StreamMuxerBox {
    inner: Box<dyn StreamMuxer<Substream = usize, OutboundSubstream = usize, Error = io::Error> + Send + Sync>,
}

impl StreamMuxerBox {
    /// Turns a stream muxer into a `StreamMuxerBox`.
    pub fn new<T>(muxer: T) -> StreamMuxerBox
    where
        T: StreamMuxer + Send + Sync + 'static,
        T::OutboundSubstream: Send,
        T::Substream: Send,
    {
        let wrap = Wrap {
            inner: muxer,
            substreams: Mutex::new(Default::default()),
            next_substream: AtomicUsize::new(0),
            outbound: Mutex::new(Default::default()),
            next_outbound: AtomicUsize::new(0),
        };

        StreamMuxerBox {
            inner: Box::new(wrap),
        }
    }
}

impl StreamMuxer for StreamMuxerBox {
    type Substream = usize; // TODO: use a newtype
    type OutboundSubstream = usize; // TODO: use a newtype
    type Error = io::Error;

    #[inline]
    fn poll_event(&self, cx: &mut Context<'_>) -> Poll<Result<StreamMuxerEvent<Self::Substream>, Self::Error>> {
        self.inner.poll_event(cx)
    }

    #[inline]
    fn open_outbound(&self) -> Self::OutboundSubstream {
        self.inner.open_outbound()
    }

    #[inline]
    fn poll_outbound(&self, cx: &mut Context<'_>, s: &mut Self::OutboundSubstream) -> Poll<Result<Self::Substream, Self::Error>> {
        self.inner.poll_outbound(cx, s)
    }

    #[inline]
    fn destroy_outbound(&self, substream: Self::OutboundSubstream) {
        self.inner.destroy_outbound(substream)
    }

    #[inline]
    fn read_substream(&self, cx: &mut Context<'_>, s: &mut Self::Substream, buf: &mut [u8]) -> Poll<Result<usize, Self::Error>> {
        self.inner.read_substream(cx, s, buf)
    }

    #[inline]
    fn write_substream(&self, cx: &mut Context<'_>, s: &mut Self::Substream, buf: &[u8]) -> Poll<Result<usize, Self::Error>> {
        self.inner.write_substream(cx, s, buf)
    }

    #[inline]
    fn flush_substream(&self, cx: &mut Context<'_>, s: &mut Self::Substream) -> Poll<Result<(), Self::Error>> {
        self.inner.flush_substream(cx, s)
    }

    #[inline]
    fn shutdown_substream(&self, cx: &mut Context<'_>, s: &mut Self::Substream) -> Poll<Result<(), Self::Error>> {
        self.inner.shutdown_substream(cx, s)
    }

    #[inline]
    fn destroy_substream(&self, s: Self::Substream) {
        self.inner.destroy_substream(s)
    }

    #[inline]
    fn close(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        self.inner.close(cx)
    }

    #[inline]
    fn flush_all(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        self.inner.flush_all(cx)
    }
}

struct Wrap<T> where T: StreamMuxer {
    inner: T,
    substreams: Mutex<FnvHashMap<usize, T::Substream>>,
    next_substream: AtomicUsize,
    outbound: Mutex<FnvHashMap<usize, T::OutboundSubstream>>,
    next_outbound: AtomicUsize,
}

impl<T> StreamMuxer for Wrap<T>
where
    T: StreamMuxer,
{
    type Substream = usize; // TODO: use a newtype
    type OutboundSubstream = usize; // TODO: use a newtype
    type Error = io::Error;

    #[inline]
    fn poll_event(&self, cx: &mut Context<'_>) -> Poll<Result<StreamMuxerEvent<Self::Substream>, Self::Error>> {
        let substream = match self.inner.poll_event(cx) {
            Poll::Pending => return Poll::Pending,
            Poll::Ready(Ok(StreamMuxerEvent::AddressChange(a))) =>
                return Poll::Ready(Ok(StreamMuxerEvent::AddressChange(a))),
            Poll::Ready(Ok(StreamMuxerEvent::InboundSubstream(s))) => s,
            Poll::Ready(Err(err)) => return Poll::Ready(Err(err.into())),
        };

        let id = self.next_substream.fetch_add(1, Ordering::Relaxed);
        self.substreams.lock().insert(id, substream);
        Poll::Ready(Ok(StreamMuxerEvent::InboundSubstream(id)))
    }

    #[inline]
    fn open_outbound(&self) -> Self::OutboundSubstream {
        let outbound = self.inner.open_outbound();
        let id = self.next_outbound.fetch_add(1, Ordering::Relaxed);
        self.outbound.lock().insert(id, outbound);
        id
    }

    #[inline]
    fn poll_outbound(
        &self,
        cx: &mut Context<'_>,
        substream: &mut Self::OutboundSubstream,
    ) -> Poll<Result<Self::Substream, Self::Error>> {
        let mut list = self.outbound.lock();
        let substream = match self.inner.poll_outbound(cx, list.get_mut(substream).unwrap()) {
            Poll::Pending => return Poll::Pending,
            Poll::Ready(Ok(s)) => s,
            Poll::Ready(Err(err)) => return Poll::Ready(Err(err.into())),
        };
        let id = self.next_substream.fetch_add(1, Ordering::Relaxed);
        self.substreams.lock().insert(id, substream);
        Poll::Ready(Ok(id))
    }

    #[inline]
    fn destroy_outbound(&self, substream: Self::OutboundSubstream) {
        let mut list = self.outbound.lock();
        self.inner.destroy_outbound(list.remove(&substream).unwrap())
    }

    #[inline]
    fn read_substream(&self, cx: &mut Context<'_>, s: &mut Self::Substream, buf: &mut [u8]) -> Poll<Result<usize, Self::Error>> {
        let mut list = self.substreams.lock();
        self.inner.read_substream(cx, list.get_mut(s).unwrap(), buf).map_err(|e| e.into())
    }

    #[inline]
    fn write_substream(&self, cx: &mut Context<'_>, s: &mut Self::Substream, buf: &[u8]) -> Poll<Result<usize, Self::Error>> {
        let mut list = self.substreams.lock();
        self.inner.write_substream(cx, list.get_mut(s).unwrap(), buf).map_err(|e| e.into())
    }

    #[inline]
    fn flush_substream(&self, cx: &mut Context<'_>, s: &mut Self::Substream) -> Poll<Result<(), Self::Error>> {
        let mut list = self.substreams.lock();
        self.inner.flush_substream(cx, list.get_mut(s).unwrap()).map_err(|e| e.into())
    }

    #[inline]
    fn shutdown_substream(&self, cx: &mut Context<'_>, s: &mut Self::Substream) -> Poll<Result<(), Self::Error>> {
        let mut list = self.substreams.lock();
        self.inner.shutdown_substream(cx, list.get_mut(s).unwrap()).map_err(|e| e.into())
    }

    #[inline]
    fn destroy_substream(&self, substream: Self::Substream) {
        let mut list = self.substreams.lock();
        self.inner.destroy_substream(list.remove(&substream).unwrap())
    }

    #[inline]
    fn close(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        self.inner.close(cx).map_err(|e| e.into())
    }

    #[inline]
    fn flush_all(&self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        self.inner.flush_all(cx).map_err(|e| e.into())
    }
}