      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

    //! 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()
        }
    }

# Help

## Keyboard Shortcuts

`?`
  Show this help dialog
`S`
  Focus the search field
`↑`
  Move up in search results
`↓`
  Move down in search results
`↹`
  Switch tab
`⏎`
  Go to active search result
`+`
  Expand all sections
`-`
  Collapse all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g., `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g., `vec -> usize` or `* -> vec`)

Search multiple things at once by splitting your query with comma (e.g., `str,u8` or `String,struct:Vec,test`)