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    //! Selection over an array of receivers
    //!
    //! This module contains the implementation machinery necessary for selecting
    //! over a number of receivers. One large goal of this module is to provide an
    //! efficient interface to selecting over any receiver of any type.
    //!
    //! This is achieved through an architecture of a "receiver set" in which
    //! receivers are added to a set and then the entire set is waited on at once.
    //! The set can be waited on multiple times to prevent re-adding each receiver
    //! to the set.
    //!
    //! Usage of this module is currently encouraged to go through the use of the
    //! `select!` macro. This macro allows naturally binding of variables to the
    //! received values of receivers in a much more natural syntax then usage of the
    //! `Select` structure directly.
    //!
    //! # Examples
    //!
    //! ```rust
    //! #![feature(mpsc_select)]
    //!
    //! use std::sync::mpsc::channel;
    //!
    //! let (tx1, rx1) = channel();
    //! let (tx2, rx2) = channel();
    //!
    //! tx1.send(1).unwrap();
    //! tx2.send(2).unwrap();
    //!
    //! select! {
    //!     val = rx1.recv() => {
    //!         assert_eq!(val.unwrap(), 1);
    //!     },
    //!     val = rx2.recv() => {
    //!         assert_eq!(val.unwrap(), 2);
    //!     }
    //! }
    //! ```

    #![allow(dead_code)]
    #![unstable(feature = "mpsc_select",
                reason = "This implementation, while likely sufficient, is unsafe and \
                          likely to be error prone. At some point in the future this \
                          module will be removed.",
                issue = "27800")]
    #![rustc_deprecated(since = "1.32.0",
                        reason = "channel selection will be removed in a future release")]

    use core::cell::{Cell, UnsafeCell};
    use core::marker;
    use core::ptr;
    use core::usize;

    use crate::fmt;
    use crate::sync::mpsc::{Receiver, RecvError};
    use crate::sync::mpsc::blocking::{self, SignalToken};

    /// The "receiver set" of the select interface. This structure is used to manage
    /// a set of receivers which are being selected over.
    pub struct Select {
        inner: UnsafeCell<SelectInner>,
        next_id: Cell<usize>,
    }

    struct SelectInner {
        head: *mut Handle<'static, ()>,
        tail: *mut Handle<'static, ()>,
    }

    impl !marker::Send for Select {}

    /// A handle to a receiver which is currently a member of a `Select` set of
    /// receivers. This handle is used to keep the receiver in the set as well as
    /// interact with the underlying receiver.
    pub struct Handle<'rx, T:Send+'rx> {
        /// The ID of this handle, used to compare against the return value of
        /// `Select::wait()`.
        id: usize,
        selector: *mut SelectInner,
        next: *mut Handle<'static, ()>,
        prev: *mut Handle<'static, ()>,
        added: bool,
        packet: &'rx (dyn Packet+'rx),

        // due to our fun transmutes, we be sure to place this at the end. (nothing
        // previous relies on T)
        rx: &'rx Receiver<T>,
    }

    struct Packets { cur: *mut Handle<'static, ()> }

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

    #[doc(hidden)]
    pub trait Packet {
        fn can_recv(&self) -> bool;
        fn start_selection(&self, token: SignalToken) -> StartResult;
        fn abort_selection(&self) -> bool;
    }

    impl Select {
        /// Creates a new selection structure. This set is initially empty.
        ///
        /// Usage of this struct directly can sometimes be burdensome, and usage is much easier through
        /// the `select!` macro.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(mpsc_select)]
        ///
        /// use std::sync::mpsc::Select;
        ///
        /// let select = Select::new();
        /// ```
        pub fn new() -> Select {
            Select {
                inner: UnsafeCell::new(SelectInner {
                    head: ptr::null_mut(),
                    tail: ptr::null_mut(),
                }),
                next_id: Cell::new(1),
            }
        }

        /// Creates a new handle into this receiver set for a new receiver. Note
        /// that this does *not* add the receiver to the receiver set, for that you
        /// must call the `add` method on the handle itself.
        pub fn handle<'a, T: Send>(&'a self, rx: &'a Receiver<T>) -> Handle<'a, T> {
            let id = self.next_id.get();
            self.next_id.set(id + 1);
            Handle {
                id,
                selector: self.inner.get(),
                next: ptr::null_mut(),
                prev: ptr::null_mut(),
                added: false,
                rx,
                packet: rx,
            }
        }

        /// Waits for an event on this receiver set. The returned value is *not* an
        /// index, but rather an ID. This ID can be queried against any active
        /// `Handle` structures (each one has an `id` method). The handle with
        /// the matching `id` will have some sort of event available on it. The
        /// event could either be that data is available or the corresponding
        /// channel has been closed.
        pub fn wait(&self) -> usize {
            self.wait2(true)
        }

        /// Helper method for skipping the preflight checks during testing
        pub(super) fn wait2(&self, do_preflight_checks: bool) -> usize {
            // Note that this is currently an inefficient implementation. We in
            // theory have knowledge about all receivers in the set ahead of time,
            // so this method shouldn't really have to iterate over all of them yet
            // again. The idea with this "receiver set" interface is to get the
            // interface right this time around, and later this implementation can
            // be optimized.
            //
            // This implementation can be summarized by:
            //
            //      fn select(receivers) {
            //          if any receiver ready { return ready index }
            //          deschedule {
            //              block on all receivers
            //          }
            //          unblock on all receivers
            //          return ready index
            //      }
            //
            // Most notably, the iterations over all of the receivers shouldn't be
            // necessary.
            unsafe {
                // Stage 1: preflight checks. Look for any packets ready to receive
                if do_preflight_checks {
                    for handle in self.iter() {
                        if (*handle).packet.can_recv() {
                            return (*handle).id();
                        }
                    }
                }

                // Stage 2: begin the blocking process
                //
                // Create a number of signal tokens, and install each one
                // sequentially until one fails. If one fails, then abort the
                // selection on the already-installed tokens.
                let (wait_token, signal_token) = blocking::tokens();
                for (i, handle) in self.iter().enumerate() {
                    match (*handle).packet.start_selection(signal_token.clone()) {
                        StartResult::Installed => {}
                        StartResult::Abort => {
                            // Go back and abort the already-begun selections
                            for handle in self.iter().take(i) {
                                (*handle).packet.abort_selection();
                            }
                            return (*handle).id;
                        }
                    }
                }

                // Stage 3: no messages available, actually block
                wait_token.wait();

                // Stage 4: there *must* be message available; find it.
                //
                // Abort the selection process on each receiver. If the abort
                // process returns `true`, then that means that the receiver is
                // ready to receive some data. Note that this also means that the
                // receiver may have yet to have fully read the `to_wake` field and
                // woken us up (although the wakeup is guaranteed to fail).
                //
                // This situation happens in the window of where a sender invokes
                // increment(), sees -1, and then decides to wake up the thread. After
                // all this is done, the sending thread will set `selecting` to
                // `false`. Until this is done, we cannot return. If we were to
                // return, then a sender could wake up a receiver which has gone
                // back to sleep after this call to `select`.
                //
                // Note that it is a "fairly small window" in which an increment()
                // views that it should wake a thread up until the `selecting` bit
                // is set to false. For now, the implementation currently just spins
                // in a yield loop. This is very distasteful, but this
                // implementation is already nowhere near what it should ideally be.
                // A rewrite should focus on avoiding a yield loop, and for now this
                // implementation is tying us over to a more efficient "don't
                // iterate over everything every time" implementation.
                let mut ready_id = usize::MAX;
                for handle in self.iter() {
                    if (*handle).packet.abort_selection() {
                        ready_id = (*handle).id;
                    }
                }

                // We must have found a ready receiver
                assert!(ready_id != usize::MAX);
                return ready_id;
            }
        }

        fn iter(&self) -> Packets { Packets { cur: unsafe { &*self.inner.get() }.head } }
    }

    impl<'rx, T: Send> Handle<'rx, T> {
        /// Retrieves the ID of this handle.
        #[inline]
        pub fn id(&self) -> usize { self.id }

        /// Blocks to receive a value on the underlying receiver, returning `Some` on
        /// success or `None` if the channel disconnects. This function has the same
        /// semantics as `Receiver.recv`
        pub fn recv(&mut self) -> Result<T, RecvError> { self.rx.recv() }

        /// Adds this handle to the receiver set that the handle was created from. This
        /// method can be called multiple times, but it has no effect if `add` was
        /// called previously.
        ///
        /// This method is unsafe because it requires that the `Handle` is not moved
        /// while it is added to the `Select` set.
        pub unsafe fn add(&mut self) {
            if self.added { return }
            let selector = &mut *self.selector;
            let me = self as *mut Handle<'rx, T> as *mut Handle<'static, ()>;

            if selector.head.is_null() {
                selector.head = me;
                selector.tail = me;
            } else {
                (*me).prev = selector.tail;
                assert!((*me).next.is_null());
                (*selector.tail).next = me;
                selector.tail = me;
            }
            self.added = true;
        }

        /// Removes this handle from the `Select` set. This method is unsafe because
        /// it has no guarantee that the `Handle` was not moved since `add` was
        /// called.
        pub unsafe fn remove(&mut self) {
            if !self.added { return }

            let selector = &mut *self.selector;
            let me = self as *mut Handle<'rx, T> as *mut Handle<'static, ()>;

            if self.prev.is_null() {
                assert_eq!(selector.head, me);
                selector.head = self.next;
            } else {
                (*self.prev).next = self.next;
            }
            if self.next.is_null() {
                assert_eq!(selector.tail, me);
                selector.tail = self.prev;
            } else {
                (*self.next).prev = self.prev;
            }

            self.next = ptr::null_mut();
            self.prev = ptr::null_mut();

            self.added = false;
        }
    }

    impl Drop for Select {
        fn drop(&mut self) {
            unsafe {
                assert!((&*self.inner.get()).head.is_null());
                assert!((&*self.inner.get()).tail.is_null());
            }
        }
    }

    impl<T: Send> Drop for Handle<'_, T> {
        fn drop(&mut self) {
            unsafe { self.remove() }
        }
    }

    impl Iterator for Packets {
        type Item = *mut Handle<'static, ()>;

        fn next(&mut self) -> Option<*mut Handle<'static, ()>> {
            if self.cur.is_null() {
                None
            } else {
                let ret = Some(self.cur);
                unsafe { self.cur = (*self.cur).next; }
                ret
            }
        }
    }

    impl fmt::Debug for Select {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Select").finish()
        }
    }

    impl<T: Send> fmt::Debug for Handle<'_, T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Handle").finish()
        }
    }

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