shared.rs

/// Shared channels.
///
/// This is the flavor of channels which are not necessarily optimized for any
/// particular use case, but are the most general in how they are used. Shared
/// channels are cloneable allowing for multiple senders.
///
/// High level implementation details can be found in the comment of the parent
/// module. You'll also note that the implementation of the shared and stream
/// channels are quite similar, and this is no coincidence!
pub use self::Failure::*;
use self::StartResult::*;

use core::cmp;
use core::intrinsics::abort;

use crate::cell::UnsafeCell;
use crate::ptr;
use crate::sync::atomic::{AtomicBool, AtomicIsize, AtomicPtr, AtomicUsize, Ordering};
use crate::sync::mpsc::blocking::{self, SignalToken};
use crate::sync::mpsc::mpsc_queue as mpsc;
use crate::sync::{Mutex, MutexGuard};
use crate::thread;
use crate::time::Instant;

const DISCONNECTED: isize = isize::MIN;
const FUDGE: isize = 1024;
const MAX_REFCOUNT: usize = (isize::MAX) as usize;
#[cfg(test)]
const MAX_STEALS: isize = 5;
#[cfg(not(test))]
const MAX_STEALS: isize = 1 << 20;
const EMPTY: *mut u8 = ptr::null_mut(); // initial state: no data, no blocked receiver

pub struct Packet<T> {
    queue: mpsc::Queue<T>,
    cnt: AtomicIsize,          // How many items are on this channel
    steals: UnsafeCell<isize>, // How many times has a port received without blocking?
    to_wake: AtomicPtr<u8>,    // SignalToken for wake up

    // The number of channels which are currently using this packet.
    channels: AtomicUsize,

    // See the discussion in Port::drop and the channel send methods for what
    // these are used for
    port_dropped: AtomicBool,
    sender_drain: AtomicIsize,

    // this lock protects various portions of this implementation during
    // select()
    select_lock: Mutex<()>,
}

pub enum Failure {
    Empty,
    Disconnected,
}

#[derive(PartialEq, Eq)]
enum StartResult {
    Installed,
    Abort,
}

impl<T> Packet<T> {
    // Creation of a packet *must* be followed by a call to postinit_lock
    // and later by inherit_blocker
    pub fn new() -> Packet<T> {
        Packet {
            queue: mpsc::Queue::new(),
            cnt: AtomicIsize::new(0),
            steals: UnsafeCell::new(0),
            to_wake: AtomicPtr::new(EMPTY),
            channels: AtomicUsize::new(2),
            port_dropped: AtomicBool::new(false),
            sender_drain: AtomicIsize::new(0),
            select_lock: Mutex::new(()),
        }
    }

    // This function should be used after newly created Packet
    // was wrapped with an Arc
    // In other case mutex data will be duplicated while cloning
    // and that could cause problems on platforms where it is
    // represented by opaque data structure
    pub fn postinit_lock(&self) -> MutexGuard<'_, ()> {
        self.select_lock.lock().unwrap()
    }

    // This function is used at the creation of a shared packet to inherit a
    // previously blocked thread. This is done to prevent spurious wakeups of
    // threads in select().
    //
    // This can only be called at channel-creation time
    pub fn inherit_blocker(&self, token: Option<SignalToken>, guard: MutexGuard<'_, ()>) {
        if let Some(token) = token {
            assert_eq!(self.cnt.load(Ordering::SeqCst), 0);
            assert_eq!(self.to_wake.load(Ordering::SeqCst), EMPTY);
            self.to_wake.store(unsafe { token.to_raw() }, Ordering::SeqCst);
            self.cnt.store(-1, Ordering::SeqCst);

            // This store is a little sketchy. What's happening here is that
            // we're transferring a blocker from a oneshot or stream channel to
            // this shared channel. In doing so, we never spuriously wake them
            // up and rather only wake them up at the appropriate time. This
            // implementation of shared channels assumes that any blocking
            // recv() will undo the increment of steals performed in try_recv()
            // once the recv is complete.  This thread that we're inheriting,
            // however, is not in the middle of recv. Hence, the first time we
            // wake them up, they're going to wake up from their old port, move
            // on to the upgraded port, and then call the block recv() function.
            //
            // When calling this function, they'll find there's data immediately
            // available, counting it as a steal. This in fact wasn't a steal
            // because we appropriately blocked them waiting for data.
            //
            // To offset this bad increment, we initially set the steal count to
            // -1. You'll find some special code in abort_selection() as well to
            // ensure that this -1 steal count doesn't escape too far.
            unsafe {
                *self.steals.get() = -1;
            }
        }

        // When the shared packet is constructed, we grabbed this lock. The
        // purpose of this lock is to ensure that abort_selection() doesn't
        // interfere with this method. After we unlock this lock, we're
        // signifying that we're done modifying self.cnt and self.to_wake and
        // the port is ready for the world to continue using it.
        drop(guard);
    }

    pub fn send(&self, t: T) -> Result<(), T> {
        // See Port::drop for what's going on
        if self.port_dropped.load(Ordering::SeqCst) {
            return Err(t);
        }

        // Note that the multiple sender case is a little trickier
        // semantically than the single sender case. The logic for
        // incrementing is "add and if disconnected store disconnected".
        // This could end up leading some senders to believe that there
        // wasn't a disconnect if in fact there was a disconnect. This means
        // that while one thread is attempting to re-store the disconnected
        // states, other threads could walk through merrily incrementing
        // this very-negative disconnected count. To prevent senders from
        // spuriously attempting to send when the channels is actually
        // disconnected, the count has a ranged check here.
        //
        // This is also done for another reason. Remember that the return
        // value of this function is:
        //
        //  `true` == the data *may* be received, this essentially has no
        //            meaning
        //  `false` == the data will *never* be received, this has a lot of
        //             meaning
        //
        // In the SPSC case, we have a check of 'queue.is_empty()' to see
        // whether the data was actually received, but this same condition
        // means nothing in a multi-producer context. As a result, this
        // preflight check serves as the definitive "this will never be
        // received". Once we get beyond this check, we have permanently
        // entered the realm of "this may be received"
        if self.cnt.load(Ordering::SeqCst) < DISCONNECTED + FUDGE {
            return Err(t);
        }

        self.queue.push(t);
        match self.cnt.fetch_add(1, Ordering::SeqCst) {
            -1 => {
                self.take_to_wake().signal();
            }

            // In this case, we have possibly failed to send our data, and
            // we need to consider re-popping the data in order to fully
            // destroy it. We must arbitrate among the multiple senders,
            // however, because the queues that we're using are
            // single-consumer queues. In order to do this, all exiting
            // pushers will use an atomic count in order to count those
            // flowing through. Pushers who see 0 are required to drain as
            // much as possible, and then can only exit when they are the
            // only pusher (otherwise they must try again).
            n if n < DISCONNECTED + FUDGE => {
                // see the comment in 'try' for a shared channel for why this
                // window of "not disconnected" is ok.
                self.cnt.store(DISCONNECTED, Ordering::SeqCst);

                if self.sender_drain.fetch_add(1, Ordering::SeqCst) == 0 {
                    loop {
                        // drain the queue, for info on the thread yield see the
                        // discussion in try_recv
                        loop {
                            match self.queue.pop() {
                                mpsc::Data(..) => {}
                                mpsc::Empty => break,
                                mpsc::Inconsistent => thread::yield_now(),
                            }
                        }
                        // maybe we're done, if we're not the last ones
                        // here, then we need to go try again.
                        if self.sender_drain.fetch_sub(1, Ordering::SeqCst) == 1 {
                            break;
                        }
                    }

                    // At this point, there may still be data on the queue,
                    // but only if the count hasn't been incremented and
                    // some other sender hasn't finished pushing data just
                    // yet. That sender in question will drain its own data.
                }
            }

            // Can't make any assumptions about this case like in the SPSC case.
            _ => {}
        }

        Ok(())
    }

    pub fn recv(&self, deadline: Option<Instant>) -> Result<T, Failure> {
        // This code is essentially the exact same as that found in the stream
        // case (see stream.rs)
        match self.try_recv() {
            Err(Empty) => {}
            data => return data,
        }

        let (wait_token, signal_token) = blocking::tokens();
        if self.decrement(signal_token) == Installed {
            if let Some(deadline) = deadline {
                let timed_out = !wait_token.wait_max_until(deadline);
                if timed_out {
                    self.abort_selection(false);
                }
            } else {
                wait_token.wait();
            }
        }

        match self.try_recv() {
            data @ Ok(..) => unsafe {
                *self.steals.get() -= 1;
                data
            },
            data => data,
        }
    }

    // Essentially the exact same thing as the stream decrement function.
    // Returns true if blocking should proceed.
    fn decrement(&self, token: SignalToken) -> StartResult {
        unsafe {
            assert_eq!(
                self.to_wake.load(Ordering::SeqCst),
                EMPTY,
                "This is a known bug in the Rust standard library. See https://github.com/rust-lang/rust/issues/39364"
            );
            let ptr = token.to_raw();
            self.to_wake.store(ptr, Ordering::SeqCst);

            let steals = ptr::replace(self.steals.get(), 0);

            match self.cnt.fetch_sub(1 + steals, Ordering::SeqCst) {
                DISCONNECTED => {
                    self.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 Installed;
                    }
                }
            }

            self.to_wake.store(EMPTY, Ordering::SeqCst);
            drop(SignalToken::from_raw(ptr));
            Abort
        }
    }

    pub fn try_recv(&self) -> Result<T, Failure> {
        let ret = match self.queue.pop() {
            mpsc::Data(t) => Some(t),
            mpsc::Empty => None,

            // This is a bit of an interesting case. The channel is reported as
            // having data available, but our pop() has failed due to the queue
            // being in an inconsistent state.  This means that there is some
            // pusher somewhere which has yet to complete, but we are guaranteed
            // that a pop will eventually succeed. In this case, we spin in a
            // yield loop because the remote sender should finish their enqueue
            // operation "very quickly".
            //
            // Avoiding this yield loop would require a different queue
            // abstraction which provides the guarantee that after M pushes have
            // succeeded, at least M pops will succeed. The current queues
            // guarantee that if there are N active pushes, you can pop N times
            // once all N have finished.
            mpsc::Inconsistent => {
                let data;
                loop {
                    thread::yield_now();
                    match self.queue.pop() {
                        mpsc::Data(t) => {
                            data = t;
                            break;
                        }
                        mpsc::Empty => panic!("inconsistent => empty"),
                        mpsc::Inconsistent => {}
                    }
                }
                Some(data)
            }
        };
        match ret {
            // See the discussion in the stream implementation for why we
            // might decrement steals.
            Some(data) => unsafe {
                if *self.steals.get() > MAX_STEALS {
                    match self.cnt.swap(0, Ordering::SeqCst) {
                        DISCONNECTED => {
                            self.cnt.store(DISCONNECTED, Ordering::SeqCst);
                        }
                        n => {
                            let m = cmp::min(n, *self.steals.get());
                            *self.steals.get() -= m;
                            self.bump(n - m);
                        }
                    }
                    assert!(*self.steals.get() >= 0);
                }
                *self.steals.get() += 1;
                Ok(data)
            },

            // See the discussion in the stream implementation for why we try
            // again.
            None => {
                match self.cnt.load(Ordering::SeqCst) {
                    n if n != DISCONNECTED => Err(Empty),
                    _ => {
                        match self.queue.pop() {
                            mpsc::Data(t) => Ok(t),
                            mpsc::Empty => Err(Disconnected),
                            // with no senders, an inconsistency is impossible.
                            mpsc::Inconsistent => unreachable!(),
                        }
                    }
                }
            }
        }
    }

    // Prepares this shared packet for a channel clone, essentially just bumping
    // a refcount.
    pub fn clone_chan(&self) {
        let old_count = self.channels.fetch_add(1, Ordering::SeqCst);

        // See comments on Arc::clone() on why we do this (for `mem::forget`).
        if old_count > MAX_REFCOUNT {
            abort();
        }
    }

    // Decrement the reference count on a channel. This is called whenever a
    // Chan is dropped and may end up waking up a receiver. It's the receiver's
    // responsibility on the other end to figure out that we've disconnected.
    pub fn drop_chan(&self) {
        match self.channels.fetch_sub(1, Ordering::SeqCst) {
            1 => {}
            n if n > 1 => return,
            n => panic!("bad number of channels left {n}"),
        }

        match self.cnt.swap(DISCONNECTED, Ordering::SeqCst) {
            -1 => {
                self.take_to_wake().signal();
            }
            DISCONNECTED => {}
            n => {
                assert!(n >= 0);
            }
        }
    }

    // See the long discussion inside of stream.rs for why the queue is drained,
    // and why it is done in this fashion.
    pub fn drop_port(&self) {
        self.port_dropped.store(true, Ordering::SeqCst);
        let mut steals = unsafe { *self.steals.get() };
        while {
            match self.cnt.compare_exchange(
                steals,
                DISCONNECTED,
                Ordering::SeqCst,
                Ordering::SeqCst,
            ) {
                Ok(_) => false,
                Err(old) => old != DISCONNECTED,
            }
        } {
            // See the discussion in 'try_recv' for why we yield
            // control of this thread.
            loop {
                match self.queue.pop() {
                    mpsc::Data(..) => {
                        steals += 1;
                    }
                    mpsc::Empty | mpsc::Inconsistent => break,
                }
            }
        }
    }

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

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

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

    // Cancels a previous thread waiting on this port, returning whether there's
    // data on the port.
    //
    // This is similar to the stream implementation (hence fewer comments), but
    // uses a different value for the "steals" variable.
    pub fn abort_selection(&self, _was_upgrade: bool) -> bool {
        // Before we do anything else, we bounce on this lock. The reason for
        // doing this is to ensure that any upgrade-in-progress is gone and
        // done with. Without this bounce, we can race with inherit_blocker
        // about looking at and dealing with to_wake. Once we have acquired the
        // lock, we are guaranteed that inherit_blocker is done.
        {
            let _guard = self.select_lock.lock().unwrap();
        }

        // Like the stream implementation, we want to make sure that the count
        // on the channel goes non-negative. We don't know how negative the
        // stream currently is, so instead of using a steal value of 1, we load
        // the channel count and figure out what we should do to make it
        // positive.
        let steals = {
            let cnt = self.cnt.load(Ordering::SeqCst);
            if cnt < 0 && cnt != DISCONNECTED { -cnt } else { 0 }
        };
        let prev = self.bump(steals + 1);

        if prev == DISCONNECTED {
            assert_eq!(self.to_wake.load(Ordering::SeqCst), EMPTY);
            true
        } else {
            let cur = prev + steals + 1;
            assert!(cur >= 0);
            if prev < 0 {
                drop(self.take_to_wake());
            } else {
                while self.to_wake.load(Ordering::SeqCst) != EMPTY {
                    thread::yield_now();
                }
            }
            unsafe {
                // if the number of steals is -1, it was the pre-emptive -1 steal
                // count from when we inherited a blocker. This is fine because
                // we're just going to overwrite it with a real value.
                let old = self.steals.get();
                assert!(*old == 0 || *old == -1);
                *old = steals;
                prev >= 0
            }
        }
    }
}

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.cnt.load(Ordering::SeqCst), DISCONNECTED);
        assert_eq!(self.to_wake.load(Ordering::SeqCst), EMPTY);
        assert_eq!(self.channels.load(Ordering::SeqCst), 0);
    }
}