      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

    use crate::codec::UserError;
    use crate::frame::Reason;
    use crate::proto::{self, WindowSize};

    use bytes::{Buf, Bytes};
    use http::HeaderMap;

    use crate::PollExt;
    use std::fmt;
    #[cfg(feature = "stream")]
    use std::pin::Pin;
    use std::task::{Context, Poll};

    /// Sends the body stream and trailers to the remote peer.
    ///
    /// # Overview
    ///
    /// A `SendStream` is provided by [`SendRequest`] and [`SendResponse`] once the
    /// HTTP/2.0 message header has been sent sent. It is used to stream the message
    /// body and send the message trailers. See method level documentation for more
    /// details.
    ///
    /// The `SendStream` instance is also used to manage outbound flow control.
    ///
    /// If a `SendStream` is dropped without explicitly closing the send stream, a
    /// `RST_STREAM` frame will be sent. This essentially cancels the request /
    /// response exchange.
    ///
    /// The ways to explicitly close the send stream are:
    ///
    /// * Set `end_of_stream` to true when calling [`send_request`],
    ///   [`send_response`], or [`send_data`].
    /// * Send trailers with [`send_trailers`].
    /// * Explicitly reset the stream with [`send_reset`].
    ///
    /// # Flow control
    ///
    /// In HTTP/2.0, data cannot be sent to the remote peer unless there is
    /// available window capacity on both the stream and the connection. When a data
    /// frame is sent, both the stream window and the connection window are
    /// decremented. When the stream level window reaches zero, no further data can
    /// be sent on that stream. When the connection level window reaches zero, no
    /// further data can be sent on any stream for that connection.
    ///
    /// When the remote peer is ready to receive more data, it sends `WINDOW_UPDATE`
    /// frames. These frames increment the windows. See the [specification] for more
    /// details on the principles of HTTP/2.0 flow control.
    ///
    /// The implications for sending data are that the caller **should** ensure that
    /// both the stream and the connection has available window capacity before
    /// loading the data to send into memory. The `SendStream` instance provides the
    /// necessary APIs to perform this logic. This, however, is not an obligation.
    /// If the caller attempts to send data on a stream when there is no available
    /// window capacity, the library will buffer the data until capacity becomes
    /// available, at which point the buffer will be flushed to the connection.
    ///
    /// **NOTE**: There is no bound on the amount of data that the library will
    /// buffer. If you are sending large amounts of data, you really should hook
    /// into the flow control lifecycle. Otherwise, you risk using up significant
    /// amounts of memory.
    ///
    /// To hook into the flow control lifecycle, the caller signals to the library
    /// that it intends to send data by calling [`reserve_capacity`], specifying the
    /// amount of data, in octets, that the caller intends to send. After this,
    /// `poll_capacity` is used to be notified when the requested capacity is
    /// assigned to the stream. Once [`poll_capacity`] returns `Ready` with the number
    /// of octets available to the stream, the caller is able to actually send the
    /// data using [`send_data`].
    ///
    /// Because there is also a connection level window that applies to **all**
    /// streams on a connection, when capacity is assigned to a stream (indicated by
    /// `poll_capacity` returning `Ready`), this capacity is reserved on the
    /// connection and will **not** be assigned to any other stream. If data is
    /// never written to the stream, that capacity is effectively lost to other
    /// streams and this introduces the risk of deadlocking a connection.
    ///
    /// To avoid throttling data on a connection, the caller should not reserve
    /// capacity until ready to send data and once any capacity is assigned to the
    /// stream, the caller should immediately send data consuming this capacity.
    /// There is no guarantee as to when the full capacity requested will become
    /// available. For example, if the caller requests 64 KB of data and 512 bytes
    /// become available, the caller should immediately send 512 bytes of data.
    ///
    /// See [`reserve_capacity`] documentation for more details.
    ///
    /// [`SendRequest`]: client/struct.SendRequest.html
    /// [`SendResponse`]: server/struct.SendResponse.html
    /// [specification]: http://httpwg.org/specs/rfc7540.html#FlowControl
    /// [`reserve_capacity`]: #method.reserve_capacity
    /// [`poll_capacity`]: #method.poll_capacity
    /// [`send_data`]: #method.send_data
    /// [`send_request`]: client/struct.SendRequest.html#method.send_request
    /// [`send_response`]: server/struct.SendResponse.html#method.send_response
    /// [`send_data`]: #method.send_data
    /// [`send_trailers`]: #method.send_trailers
    /// [`send_reset`]: #method.send_reset
    #[derive(Debug)]
    pub struct SendStream<B: Buf> {
        inner: proto::StreamRef<B>,
    }

    /// A stream identifier, as described in [Section 5.1.1] of RFC 7540.
    ///
    /// Streams are identified with an unsigned 31-bit integer. Streams
    /// initiated by a client MUST use odd-numbered stream identifiers; those
    /// initiated by the server MUST use even-numbered stream identifiers.  A
    /// stream identifier of zero (0x0) is used for connection control
    /// messages; the stream identifier of zero cannot be used to establish a
    /// new stream.
    ///
    /// [Section 5.1.1]: https://tools.ietf.org/html/rfc7540#section-5.1.1
    #[derive(Debug, Clone, Eq, PartialEq, Hash)]
    pub struct StreamId(u32);

    /// Receives the body stream and trailers from the remote peer.
    ///
    /// A `RecvStream` is provided by [`client::ResponseFuture`] and
    /// [`server::Connection`] with the received HTTP/2.0 message head (the response
    /// and request head respectively).
    ///
    /// A `RecvStream` instance is used to receive the streaming message body and
    /// any trailers from the remote peer. It is also used to manage inbound flow
    /// control.
    ///
    /// See method level documentation for more details on receiving data. See
    /// [`FlowControl`] for more details on inbound flow control.
    ///
    /// Note that this type implements [`Stream`], yielding the received data frames.
    /// When this implementation is used, the capacity is immediately released when
    /// the data is yielded. It is recommended to only use this API when the data
    /// will not be retained in memory for extended periods of time.
    ///
    /// [`client::ResponseFuture`]: client/struct.ResponseFuture.html
    /// [`server::Connection`]: server/struct.Connection.html
    /// [`FlowControl`]: struct.FlowControl.html
    /// [`Stream`]: https://docs.rs/futures/0.1/futures/stream/trait.Stream.html
    #[must_use = "streams do nothing unless polled"]
    pub struct RecvStream {
        inner: FlowControl,
    }

    /// A handle to release window capacity to a remote stream.
    ///
    /// This type allows the caller to manage inbound data [flow control]. The
    /// caller is expected to call [`release_capacity`] after dropping data frames.
    ///
    /// # Overview
    ///
    /// Each stream has a window size. This window size is the maximum amount of
    /// inbound data that can be in-flight. In-flight data is defined as data that
    /// has been received, but not yet released.
    ///
    /// When a stream is created, the window size is set to the connection's initial
    /// window size value. When a data frame is received, the window size is then
    /// decremented by size of the data frame before the data is provided to the
    /// caller. As the caller finishes using the data, [`release_capacity`] must be
    /// called. This will then increment the window size again, allowing the peer to
    /// send more data.
    ///
    /// There is also a connection level window as well as the stream level window.
    /// Received data counts against the connection level window as well and calls
    /// to [`release_capacity`] will also increment the connection level window.
    ///
    /// # Sending `WINDOW_UPDATE` frames
    ///
    /// `WINDOW_UPDATE` frames will not be sent out for **every** call to
    /// `release_capacity`, as this would end up slowing down the protocol. Instead,
    /// `h2` waits until the window size is increased to a certain threshold and
    /// then sends out a single `WINDOW_UPDATE` frame representing all the calls to
    /// `release_capacity` since the last `WINDOW_UPDATE` frame.
    ///
    /// This essentially batches window updating.
    ///
    /// # Scenarios
    ///
    /// Following is a basic scenario with an HTTP/2.0 connection containing a
    /// single active stream.
    ///
    /// * A new stream is activated. The receive window is initialized to 1024 (the
    ///   value of the initial window size for this connection).
    /// * A `DATA` frame is received containing a payload of 400 bytes.
    /// * The receive window size is reduced to 424 bytes.
    /// * [`release_capacity`] is called with 200.
    /// * The receive window size is now 624 bytes. The peer may send no more than
    ///   this.
    /// * A `DATA` frame is received with a payload of 624 bytes.
    /// * The window size is now 0 bytes. The peer may not send any more data.
    /// * [`release_capacity`] is called with 1024.
    /// * The receive window size is now 1024 bytes. The peer may now send more
    /// data.
    ///
    /// [flow control]: ../index.html#flow-control
    /// [`release_capacity`]: struct.FlowControl.html#method.release_capacity
    #[derive(Clone, Debug)]
    pub struct FlowControl {
        inner: proto::OpaqueStreamRef,
    }

    /// A handle to send and receive PING frames with the peer.
    // NOT Clone on purpose
    pub struct PingPong {
        inner: proto::UserPings,
    }

    /// Sent via [`PingPong`][] to send a PING frame to a peer.
    ///
    /// [`PingPong`]: struct.PingPong.html
    pub struct Ping {
        _p: (),
    }

    /// Received via [`PingPong`][] when a peer acknowledges a [`Ping`][].
    ///
    /// [`PingPong`]: struct.PingPong.html
    /// [`Ping`]: struct.Ping.html
    pub struct Pong {
        _p: (),
    }

    // ===== impl SendStream =====

    impl<B: Buf> SendStream<B> {
        pub(crate) fn new(inner: proto::StreamRef<B>) -> Self {
            SendStream { inner }
        }

        /// Requests capacity to send data.
        ///
        /// This function is used to express intent to send data. This requests
        /// connection level capacity. Once the capacity is available, it is
        /// assigned to the stream and not reused by other streams.
        ///
        /// This function may be called repeatedly. The `capacity` argument is the
        /// **total** amount of requested capacity. Sequential calls to
        /// `reserve_capacity` are *not* additive. Given the following:
        ///
        /// ```rust
        /// # use h2::*;
        /// # fn doc(mut send_stream: SendStream<&'static [u8]>) {
        /// send_stream.reserve_capacity(100);
        /// send_stream.reserve_capacity(200);
        /// # }
        /// ```
        ///
        /// After the second call to `reserve_capacity`, the *total* requested
        /// capacity will be 200.
        ///
        /// `reserve_capacity` is also used to cancel previous capacity requests.
        /// Given the following:
        ///
        /// ```rust
        /// # use h2::*;
        /// # fn doc(mut send_stream: SendStream<&'static [u8]>) {
        /// send_stream.reserve_capacity(100);
        /// send_stream.reserve_capacity(0);
        /// # }
        /// ```
        ///
        /// After the second call to `reserve_capacity`, the *total* requested
        /// capacity will be 0, i.e. there is no requested capacity for the stream.
        ///
        /// If `reserve_capacity` is called with a lower value than the amount of
        /// capacity **currently** assigned to the stream, this capacity will be
        /// returned to the connection to be re-assigned to other streams.
        ///
        /// Also, the amount of capacity that is reserved gets decremented as data
        /// is sent. For example:
        ///
        /// ```rust
        /// # use h2::*;
        /// # async fn doc(mut send_stream: SendStream<&'static [u8]>) {
        /// send_stream.reserve_capacity(100);
        ///
        /// send_stream.send_data(b"hello", false).unwrap();
        /// // At this point, the total amount of requested capacity is 95 bytes.
        ///
        /// // Calling `reserve_capacity` with `100` again essentially requests an
        /// // additional 5 bytes.
        /// send_stream.reserve_capacity(100);
        /// # }
        /// ```
        ///
        /// See [Flow control](struct.SendStream.html#flow-control) for an overview
        /// of how send flow control works.
        pub fn reserve_capacity(&mut self, capacity: usize) {
            // TODO: Check for overflow
            self.inner.reserve_capacity(capacity as WindowSize)
        }

        /// Returns the stream's current send capacity.
        ///
        /// This allows the caller to check the current amount of available capacity
        /// before sending data.
        pub fn capacity(&self) -> usize {
            self.inner.capacity() as usize
        }

        /// Requests to be notified when the stream's capacity increases.
        ///
        /// Before calling this, capacity should be requested with
        /// `reserve_capacity`. Once capacity is requested, the connection will
        /// assign capacity to the stream **as it becomes available**. There is no
        /// guarantee as to when and in what increments capacity gets assigned to
        /// the stream.
        ///
        /// To get notified when the available capacity increases, the caller calls
        /// `poll_capacity`, which returns `Ready(Some(n))` when `n` has been
        /// increased by the connection. Note that `n` here represents the **total**
        /// amount of assigned capacity at that point in time. It is also possible
        /// that `n` is lower than the previous call if, since then, the caller has
        /// sent data.
        pub fn poll_capacity(&mut self, cx: &mut Context) -> Poll<Option<Result<usize, crate::Error>>> {
            self.inner
                .poll_capacity(cx)
                .map_ok_(|w| w as usize)
                .map_err_(Into::into)
        }

        /// Sends a single data frame to the remote peer.
        ///
        /// This function may be called repeatedly as long as `end_of_stream` is set
        /// to `false`. Setting `end_of_stream` to `true` sets the end stream flag
        /// on the data frame. Any further calls to `send_data` or `send_trailers`
        /// will return an [`Error`].
        ///
        /// `send_data` can be called without reserving capacity. In this case, the
        /// data is buffered and the capacity is implicitly requested. Once the
        /// capacity becomes available, the data is flushed to the connection.
        /// However, this buffering is unbounded. As such, sending large amounts of
        /// data without reserving capacity before hand could result in large
        /// amounts of data being buffered in memory.
        ///
        /// [`Error`]: struct.Error.html
        pub fn send_data(&mut self, data: B, end_of_stream: bool) -> Result<(), crate::Error> {
            self.inner
                .send_data(data, end_of_stream)
                .map_err(Into::into)
        }

        /// Sends trailers to the remote peer.
        ///
        /// Sending trailers implicitly closes the send stream. Once the send stream
        /// is closed, no more data can be sent.
        pub fn send_trailers(&mut self, trailers: HeaderMap) -> Result<(), crate::Error> {
            self.inner.send_trailers(trailers).map_err(Into::into)
        }

        /// Resets the stream.
        ///
        /// This cancels the request / response exchange. If the response has not
        /// yet been received, the associated `ResponseFuture` will return an
        /// [`Error`] to reflect the canceled exchange.
        ///
        /// [`Error`]: struct.Error.html
        pub fn send_reset(&mut self, reason: Reason) {
            self.inner.send_reset(reason)
        }

        /// Polls to be notified when the client resets this stream.
        ///
        /// If stream is still open, this returns `Poll::Pending`, and
        /// registers the task to be notified if a `RST_STREAM` is received.
        ///
        /// If a `RST_STREAM` frame is received for this stream, calling this
        /// method will yield the `Reason` for the reset.
        ///
        /// # Error
        ///
        /// If connection sees an error, this returns that error instead of a
        /// `Reason`.
        pub fn poll_reset(&mut self, cx: &mut Context) -> Poll<Result<Reason, crate::Error>> {
            self.inner.poll_reset(cx, proto::PollReset::Streaming)
        }

        /// Returns the stream ID of this `SendStream`.
        ///
        /// # Panics
        ///
        /// If the lock on the stream store has been poisoned.
        pub fn stream_id(&self) -> StreamId {
            StreamId::from_internal(self.inner.stream_id())
        }
    }

    // ===== impl StreamId =====

    impl StreamId {
        pub(crate) fn from_internal(id: crate::frame::StreamId) -> Self {
            StreamId(id.into())
        }
    }
    // ===== impl RecvStream =====

    impl RecvStream {
        pub(crate) fn new(inner: FlowControl) -> Self {
            RecvStream { inner }
        }

        /// Get the next data frame.
        pub async fn data(&mut self) -> Option<Result<Bytes, crate::Error>> {
            futures_util::future::poll_fn(move |cx| self.poll_data(cx)).await
        }

        /// Get optional trailers for this stream.
        pub async fn trailers(&mut self) -> Result<Option<HeaderMap>, crate::Error> {
            futures_util::future::poll_fn(move |cx| self.poll_trailers(cx)).await
        }

        #[doc(hidden)]
        pub fn poll_data(&mut self, cx: &mut Context<'_>) -> Poll<Option<Result<Bytes, crate::Error>>> {
            self.inner.inner.poll_data(cx).map_err_(Into::into)
        }

        #[doc(hidden)]
        pub fn poll_trailers(
            &mut self,
            cx: &mut Context,
        ) -> Poll<Result<Option<HeaderMap>, crate::Error>> {
            match ready!(self.inner.inner.poll_trailers(cx)) {
                Some(Ok(map)) => Poll::Ready(Ok(Some(map))),
                Some(Err(e)) => Poll::Ready(Err(e.into())),
                None => Poll::Ready(Ok(None)),
            }
        }

        /// Returns true if the receive half has reached the end of stream.
        ///
        /// A return value of `true` means that calls to `poll` and `poll_trailers`
        /// will both return `None`.
        pub fn is_end_stream(&self) -> bool {
            self.inner.inner.is_end_stream()
        }

        /// Get a mutable reference to this stream's `FlowControl`.
        ///
        /// It can be used immediately, or cloned to be used later.
        pub fn flow_control(&mut self) -> &mut FlowControl {
            &mut self.inner
        }

        /// Returns the stream ID of this stream.
        ///
        /// # Panics
        ///
        /// If the lock on the stream store has been poisoned.
        pub fn stream_id(&self) -> StreamId {
            self.inner.stream_id()
        }
    }

    #[cfg(feature = "stream")]
    impl futures_core::Stream for RecvStream {
        type Item = Result<Bytes, crate::Error>;

        fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
            self.poll_data(cx)
        }
    }

    impl fmt::Debug for RecvStream {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("RecvStream")
                .field("inner", &self.inner)
                .finish()
        }
    }

    impl Drop for RecvStream {
        fn drop(&mut self) {
            // Eagerly clear any received DATA frames now, since its no longer
            // possible to retrieve them. However, this will be called
            // again once *all* stream refs have been dropped, since
            // this won't send a RST_STREAM frame, in case the user wishes to
            // still *send* DATA.
            self.inner.inner.clear_recv_buffer();
        }
    }

    // ===== impl FlowControl =====

    impl FlowControl {
        pub(crate) fn new(inner: proto::OpaqueStreamRef) -> Self {
            FlowControl { inner }
        }

        /// Returns the stream ID of the stream whose capacity will
        /// be released by this `FlowControl`.
        pub fn stream_id(&self) -> StreamId {
            StreamId::from_internal(self.inner.stream_id())
        }

        /// Get the current available capacity of data this stream *could* receive.
        pub fn available_capacity(&self) -> isize {
            self.inner.available_recv_capacity()
        }

        /// Get the currently *used* capacity for this stream.
        ///
        /// This is the amount of bytes that can be released back to the remote.
        pub fn used_capacity(&self) -> usize {
            self.inner.used_recv_capacity() as usize
        }

        /// Release window capacity back to remote stream.
        ///
        /// This releases capacity back to the stream level and the connection level
        /// windows. Both window sizes will be increased by `sz`.
        ///
        /// See [struct level] documentation for more details.
        ///
        /// # Errors
        ///
        /// This function errors if increasing the receive window size by `sz` would
        /// result in a window size greater than the target window size. In other
        /// words, the caller cannot release more capacity than data has been
        /// received. If 1024 bytes of data have been received, at most 1024 bytes
        /// can be released.
        ///
        /// [struct level]: #
        pub fn release_capacity(&mut self, sz: usize) -> Result<(), crate::Error> {
            if sz > proto::MAX_WINDOW_SIZE as usize {
                return Err(UserError::ReleaseCapacityTooBig.into());
            }
            self.inner
                .release_capacity(sz as proto::WindowSize)
                .map_err(Into::into)
        }
    }

    // ===== impl PingPong =====

    impl PingPong {
        pub(crate) fn new(inner: proto::UserPings) -> Self {
            PingPong { inner }
        }

        /// Send a PING frame and wait for the peer to send the pong.
        pub async fn ping(&mut self, ping: Ping) -> Result<Pong, crate::Error> {
            self.send_ping(ping)?;
            futures_util::future::poll_fn(|cx| self.poll_pong(cx)).await
        }

        #[doc(hidden)]
        pub fn send_ping(&mut self, ping: Ping) -> Result<(), crate::Error> {
            // Passing a `Ping` here is just to be forwards-compatible with
            // eventually allowing choosing a ping payload. For now, we can
            // just drop it.
            drop(ping);

            self.inner.send_ping().map_err(|err| match err {
                Some(err) => err.into(),
                None => UserError::SendPingWhilePending.into(),
            })
        }

        #[doc(hidden)]
        pub fn poll_pong(&mut self, cx: &mut Context) -> Poll<Result<Pong, crate::Error>> {
            ready!(self.inner.poll_pong(cx))?;
            Poll::Ready(Ok(Pong { _p: () }))
        }
    }

    impl fmt::Debug for PingPong {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("PingPong").finish()
        }
    }

    // ===== impl Ping =====

    impl Ping {
        /// Creates a new opaque `Ping` to be sent via a [`PingPong`][].
        ///
        /// The payload is "opaque", such that it shouldn't be depended on.
        ///
        /// [`PingPong`]: struct.PingPong.html
        pub fn opaque() -> Ping {
            Ping { _p: () }
        }
    }

    impl fmt::Debug for Ping {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("Ping").finish()
        }
    }

    // ===== impl Pong =====

    impl fmt::Debug for Pong {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("Pong").finish()
        }
    }