      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

    /// Stream channels
    ///
    /// This is the flavor of channels which are optimized for one sender and one
    /// receiver. The sender will be upgraded to a shared channel if the channel is
    /// cloned.
    ///
    /// High level implementation details can be found in the comment of the parent
    /// module.
    pub use self::Failure::*;
    use self::Message::*;
    pub use self::UpgradeResult::*;

    use core::cmp;

    use crate::cell::UnsafeCell;
    use crate::ptr;
    use crate::thread;
    use crate::time::Instant;

    use crate::sync::atomic::{AtomicBool, AtomicIsize, AtomicUsize, Ordering};
    use crate::sync::mpsc::blocking::{self, SignalToken};
    use crate::sync::mpsc::spsc_queue as spsc;
    use crate::sync::mpsc::Receiver;

    const DISCONNECTED: isize = isize::MIN;
    #[cfg(test)]
    const MAX_STEALS: isize = 5;
    #[cfg(not(test))]
    const MAX_STEALS: isize = 1 << 20;

    pub struct Packet<T> {
        // internal queue for all messages
        queue: spsc::Queue<Message<T>, ProducerAddition, ConsumerAddition>,
    }

    struct ProducerAddition {
        cnt: AtomicIsize,     // How many items are on this channel
        to_wake: AtomicUsize, // SignalToken for the blocked thread to wake up

        port_dropped: AtomicBool, // flag if the channel has been destroyed.
    }

    struct ConsumerAddition {
        steals: UnsafeCell<isize>, // How many times has a port received without blocking?
    }

    pub enum Failure<T> {
        Empty,
        Disconnected,
        Upgraded(Receiver<T>),
    }

    pub enum UpgradeResult {
        UpSuccess,
        UpDisconnected,
        UpWoke(SignalToken),
    }

    // Any message could contain an "upgrade request" to a new shared port, so the
    // internal queue it's a queue of T, but rather Message<T>
    enum Message<T> {
        Data(T),
        GoUp(Receiver<T>),
    }

    impl<T> Packet<T> {
        pub fn new() -> Packet<T> {
            Packet {
                queue: unsafe {
                    spsc::Queue::with_additions(
                        128,
                        ProducerAddition {
                            cnt: AtomicIsize::new(0),
                            to_wake: AtomicUsize::new(0),

                            port_dropped: AtomicBool::new(false),
                        },
                        ConsumerAddition { steals: UnsafeCell::new(0) },
                    )
                },
            }
        }

        pub fn send(&self, t: T) -> Result<(), T> {
            // If the other port has deterministically gone away, then definitely
            // must return the data back up the stack. Otherwise, the data is
            // considered as being sent.
            if self.queue.producer_addition().port_dropped.load(Ordering::SeqCst) {
                return Err(t);
            }

            match self.do_send(Data(t)) {
                UpSuccess | UpDisconnected => {}
                UpWoke(token) => {
                    token.signal();
                }
            }
            Ok(())
        }

        pub fn upgrade(&self, up: Receiver<T>) -> UpgradeResult {
            // If the port has gone away, then there's no need to proceed any
            // further.
            if self.queue.producer_addition().port_dropped.load(Ordering::SeqCst) {
                return UpDisconnected;
            }

            self.do_send(GoUp(up))
        }

        fn do_send(&self, t: Message<T>) -> UpgradeResult {
            self.queue.push(t);
            match self.queue.producer_addition().cnt.fetch_add(1, Ordering::SeqCst) {
                // As described in the mod's doc comment, -1 == wakeup
                -1 => UpWoke(self.take_to_wake()),
                // As as described before, SPSC queues must be >= -2
                -2 => UpSuccess,

                // Be sure to preserve the disconnected state, and the return value
                // in this case is going to be whether our data was received or not.
                // This manifests itself on whether we have an empty queue or not.
                //
                // Primarily, are required to drain the queue here because the port
                // will never remove this data. We can only have at most one item to
                // drain (the port drains the rest).
                DISCONNECTED => {
                    self.queue.producer_addition().cnt.store(DISCONNECTED, Ordering::SeqCst);
                    let first = self.queue.pop();
                    let second = self.queue.pop();
                    assert!(second.is_none());

                    match first {
                        Some(..) => UpSuccess,  // we failed to send the data
                        None => UpDisconnected, // we successfully sent data
                    }
                }

                // Otherwise we just sent some data on a non-waiting queue, so just
                // make sure the world is sane and carry on!
                n => {
                    assert!(n >= 0);
                    UpSuccess
                }
            }
        }

        // Consumes ownership of the 'to_wake' field.
        fn take_to_wake(&self) -> SignalToken {
            let ptr = self.queue.producer_addition().to_wake.load(Ordering::SeqCst);
            self.queue.producer_addition().to_wake.store(0, Ordering::SeqCst);
            assert!(ptr != 0);
            unsafe { SignalToken::cast_from_usize(ptr) }
        }

        // Decrements the count on the channel for a sleeper, returning the sleeper
        // back if it shouldn't sleep. Note that this is the location where we take
        // steals into account.
        fn decrement(&self, token: SignalToken) -> Result<(), SignalToken> {
            assert_eq!(self.queue.producer_addition().to_wake.load(Ordering::SeqCst), 0);
            let ptr = unsafe { token.cast_to_usize() };
            self.queue.producer_addition().to_wake.store(ptr, Ordering::SeqCst);

            let steals = unsafe { ptr::replace(self.queue.consumer_addition().steals.get(), 0) };

            match self.queue.producer_addition().cnt.fetch_sub(1 + steals, Ordering::SeqCst) {
                DISCONNECTED => {
                    self.queue.producer_addition().cnt.store(DISCONNECTED, Ordering::SeqCst);
                }
                // If we factor in our steals and notice that the channel has no
                // data, we successfully sleep
                n => {
                    assert!(n >= 0);
                    if n - steals <= 0 {
                        return Ok(());
                    }
                }
            }

            self.queue.producer_addition().to_wake.store(0, Ordering::SeqCst);
            Err(unsafe { SignalToken::cast_from_usize(ptr) })
        }

        pub fn recv(&self, deadline: Option<Instant>) -> Result<T, Failure<T>> {
            // Optimistic preflight check (scheduling is expensive).
            match self.try_recv() {
                Err(Empty) => {}
                data => return data,
            }

            // Welp, our channel has no data. Deschedule the current thread and
            // initiate the blocking protocol.
            let (wait_token, signal_token) = blocking::tokens();
            if self.decrement(signal_token).is_ok() {
                if let Some(deadline) = deadline {
                    let timed_out = !wait_token.wait_max_until(deadline);
                    if timed_out {
                        self.abort_selection(/* was_upgrade = */ false).map_err(Upgraded)?;
                    }
                } else {
                    wait_token.wait();
                }
            }

            match self.try_recv() {
                // Messages which actually popped from the queue shouldn't count as
                // a steal, so offset the decrement here (we already have our
                // "steal" factored into the channel count above).
                data @ (Ok(..) | Err(Upgraded(..))) => unsafe {
                    *self.queue.consumer_addition().steals.get() -= 1;
                    data
                },

                data => data,
            }
        }

        pub fn try_recv(&self) -> Result<T, Failure<T>> {
            match self.queue.pop() {
                // If we stole some data, record to that effect (this will be
                // factored into cnt later on).
                //
                // Note that we don't allow steals to grow without bound in order to
                // prevent eventual overflow of either steals or cnt as an overflow
                // would have catastrophic results. Sometimes, steals > cnt, but
                // other times cnt > steals, so we don't know the relation between
                // steals and cnt. This code path is executed only rarely, so we do
                // a pretty slow operation, of swapping 0 into cnt, taking steals
                // down as much as possible (without going negative), and then
                // adding back in whatever we couldn't factor into steals.
                Some(data) => unsafe {
                    if *self.queue.consumer_addition().steals.get() > MAX_STEALS {
                        match self.queue.producer_addition().cnt.swap(0, Ordering::SeqCst) {
                            DISCONNECTED => {
                                self.queue
                                    .producer_addition()
                                    .cnt
                                    .store(DISCONNECTED, Ordering::SeqCst);
                            }
                            n => {
                                let m = cmp::min(n, *self.queue.consumer_addition().steals.get());
                                *self.queue.consumer_addition().steals.get() -= m;
                                self.bump(n - m);
                            }
                        }
                        assert!(*self.queue.consumer_addition().steals.get() >= 0);
                    }
                    *self.queue.consumer_addition().steals.get() += 1;
                    match data {
                        Data(t) => Ok(t),
                        GoUp(up) => Err(Upgraded(up)),
                    }
                },

                None => {
                    match self.queue.producer_addition().cnt.load(Ordering::SeqCst) {
                        n if n != DISCONNECTED => Err(Empty),

                        // This is a little bit of a tricky case. We failed to pop
                        // data above, and then we have viewed that the channel is
                        // disconnected. In this window more data could have been
                        // sent on the channel. It doesn't really make sense to
                        // return that the channel is disconnected when there's
                        // actually data on it, so be extra sure there's no data by
                        // popping one more time.
                        //
                        // We can ignore steals because the other end is
                        // disconnected and we'll never need to really factor in our
                        // steals again.
                        _ => match self.queue.pop() {
                            Some(Data(t)) => Ok(t),
                            Some(GoUp(up)) => Err(Upgraded(up)),
                            None => Err(Disconnected),
                        },
                    }
                }
            }
        }

        pub fn drop_chan(&self) {
            // Dropping a channel is pretty simple, we just flag it as disconnected
            // and then wakeup a blocker if there is one.
            match self.queue.producer_addition().cnt.swap(DISCONNECTED, Ordering::SeqCst) {
                -1 => {
                    self.take_to_wake().signal();
                }
                DISCONNECTED => {}
                n => {
                    assert!(n >= 0);
                }
            }
        }

        pub fn drop_port(&self) {
            // Dropping a port seems like a fairly trivial thing. In theory all we
            // need to do is flag that we're disconnected and then everything else
            // can take over (we don't have anyone to wake up).
            //
            // The catch for Ports is that we want to drop the entire contents of
            // the queue. There are multiple reasons for having this property, the
            // largest of which is that if another chan is waiting in this channel
            // (but not received yet), then waiting on that port will cause a
            // deadlock.
            //
            // So if we accept that we must now destroy the entire contents of the
            // queue, this code may make a bit more sense. The tricky part is that
            // we can't let any in-flight sends go un-dropped, we have to make sure
            // *everything* is dropped and nothing new will come onto the channel.

            // The first thing we do is set a flag saying that we're done for. All
            // sends are gated on this flag, so we're immediately guaranteed that
            // there are a bounded number of active sends that we'll have to deal
            // with.
            self.queue.producer_addition().port_dropped.store(true, Ordering::SeqCst);

            // Now that we're guaranteed to deal with a bounded number of senders,
            // we need to drain the queue. This draining process happens atomically
            // with respect to the "count" of the channel. If the count is nonzero
            // (with steals taken into account), then there must be data on the
            // channel. In this case we drain everything and then try again. We will
            // continue to fail while active senders send data while we're dropping
            // data, but eventually we're guaranteed to break out of this loop
            // (because there is a bounded number of senders).
            let mut steals = unsafe { *self.queue.consumer_addition().steals.get() };
            while {
                let cnt = self.queue.producer_addition().cnt.compare_and_swap(
                    steals,
                    DISCONNECTED,
                    Ordering::SeqCst,
                );
                cnt != DISCONNECTED && cnt != steals
            } {
                while self.queue.pop().is_some() {
                    steals += 1;
                }
            }

            // At this point in time, we have gated all future senders from sending,
            // and we have flagged the channel as being disconnected. The senders
            // still have some responsibility, however, because some sends may not
            // complete until after we flag the disconnection. There are more
            // details in the sending methods that see DISCONNECTED
        }

        ////////////////////////////////////////////////////////////////////////////
        // select implementation
        ////////////////////////////////////////////////////////////////////////////

        // increment the count on the channel (used for selection)
        fn bump(&self, amt: isize) -> isize {
            match self.queue.producer_addition().cnt.fetch_add(amt, Ordering::SeqCst) {
                DISCONNECTED => {
                    self.queue.producer_addition().cnt.store(DISCONNECTED, Ordering::SeqCst);
                    DISCONNECTED
                }
                n => n,
            }
        }

        // Removes a previous thread from being blocked in this port
        pub fn abort_selection(&self, was_upgrade: bool) -> Result<bool, Receiver<T>> {
            // If we're aborting selection after upgrading from a oneshot, then
            // we're guarantee that no one is waiting. The only way that we could
            // have seen the upgrade is if data was actually sent on the channel
            // half again. For us, this means that there is guaranteed to be data on
            // this channel. Furthermore, we're guaranteed that there was no
            // start_selection previously, so there's no need to modify `self.cnt`
            // at all.
            //
            // Hence, because of these invariants, we immediately return `Ok(true)`.
            // Note that the data may not actually be sent on the channel just yet.
            // The other end could have flagged the upgrade but not sent data to
            // this end. This is fine because we know it's a small bounded windows
            // of time until the data is actually sent.
            if was_upgrade {
                assert_eq!(unsafe { *self.queue.consumer_addition().steals.get() }, 0);
                assert_eq!(self.queue.producer_addition().to_wake.load(Ordering::SeqCst), 0);
                return Ok(true);
            }

            // We want to make sure that the count on the channel goes non-negative,
            // and in the stream case we can have at most one steal, so just assume
            // that we had one steal.
            let steals = 1;
            let prev = self.bump(steals + 1);

            // If we were previously disconnected, then we know for sure that there
            // is no thread in to_wake, so just keep going
            let has_data = if prev == DISCONNECTED {
                assert_eq!(self.queue.producer_addition().to_wake.load(Ordering::SeqCst), 0);
                true // there is data, that data is that we're disconnected
            } else {
                let cur = prev + steals + 1;
                assert!(cur >= 0);

                // If the previous count was negative, then we just made things go
                // positive, hence we passed the -1 boundary and we're responsible
                // for removing the to_wake() field and trashing it.
                //
                // If the previous count was positive then we're in a tougher
                // situation. A possible race is that a sender just incremented
                // through -1 (meaning it's going to try to wake a thread up), but it
                // hasn't yet read the to_wake. In order to prevent a future recv()
                // from waking up too early (this sender picking up the plastered
                // over to_wake), we spin loop here waiting for to_wake to be 0.
                // Note that this entire select() implementation needs an overhaul,
                // and this is *not* the worst part of it, so this is not done as a
                // final solution but rather out of necessity for now to get
                // something working.
                if prev < 0 {
                    drop(self.take_to_wake());
                } else {
                    while self.queue.producer_addition().to_wake.load(Ordering::SeqCst) != 0 {
                        thread::yield_now();
                    }
                }
                unsafe {
                    assert_eq!(*self.queue.consumer_addition().steals.get(), 0);
                    *self.queue.consumer_addition().steals.get() = steals;
                }

                // if we were previously positive, then there's surely data to
                // receive
                prev >= 0
            };

            // Now that we've determined that this queue "has data", we peek at the
            // queue to see if the data is an upgrade or not. If it's an upgrade,
            // then we need to destroy this port and abort selection on the
            // upgraded port.
            if has_data {
                match self.queue.peek() {
                    Some(&mut GoUp(..)) => match self.queue.pop() {
                        Some(GoUp(port)) => Err(port),
                        _ => unreachable!(),
                    },
                    _ => Ok(true),
                }
            } else {
                Ok(false)
            }
        }
    }

    impl<T> Drop for Packet<T> {
        fn drop(&mut self) {
            // Note that this load is not only an assert for correctness about
            // disconnection, but also a proper fence before the read of
            // `to_wake`, so this assert cannot be removed with also removing
            // the `to_wake` assert.
            assert_eq!(self.queue.producer_addition().cnt.load(Ordering::SeqCst), DISCONNECTED);
            assert_eq!(self.queue.producer_addition().to_wake.load(Ordering::SeqCst), 0);
        }
    }