shared.rs.html -- source
Description: Source to the Rust file `src\libstd\sync\mpsc\shared.rs`.
Keywords: rust, rustlang, rust-lang

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    /// 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 core::isize;

    use crate::cell::UnsafeCell;
    use crate::ptr;
    use crate::sync::atomic::{AtomicUsize, AtomicIsize, AtomicBool, 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;

    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: AtomicUsize, // 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: AtomicUsize::new(0),
                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<'_, ()>) {
            token.map(|token| {
                assert_eq!(self.cnt.load(Ordering::SeqCst), 0);
                assert_eq!(self.to_wake.load(Ordering::SeqCst), 0);
                self.to_wake.store(unsafe { token.cast_to_usize() }, 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), 0);
                let ptr = token.cast_to_usize();
                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(0, Ordering::SeqCst);
                drop(SignalToken::cast_from_usize(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 {
                unsafe {
                    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 {
                let cnt = self.cnt.compare_and_swap(steals, DISCONNECTED, Ordering::SeqCst);
                cnt != DISCONNECTED && cnt != steals
            } {
                // 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(0, Ordering::SeqCst);
            assert!(ptr != 0);
            unsafe { SignalToken::cast_from_usize(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), 0);
                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) != 0 {
                        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), 0);
            assert_eq!(self.channels.load(Ordering::SeqCst), 0);
        }
    }