DXR is a code search and navigation tool aimed at making sense of large projects. It supports full-text and regex searches as well as structural queries.

Mercurial (9b7cd94eaf0a)

VCS Links

Line Code
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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501
#![doc(html_root_url = "https://docs.rs/want/0.0.6")]
#![deny(warnings)]
#![deny(missing_docs)]
#![deny(missing_debug_implementations)]

//! A Futures channel-like utility to signal when a value is wanted.
//!
//! Futures are supposed to be lazy, and only starting work if `Future::poll`
//! is called. The same is true of `Stream`s, but when using a channel as
//! a `Stream`, it can be hard to know if the receiver is ready for the next
//! value.
//!
//! Put another way, given a `(tx, rx)` from `futures::sync::mpsc::channel()`,
//! how can the sender (`tx`) know when the receiver (`rx`) actually wants more
//! work to be produced? Just because there is room in the channel buffer
//! doesn't mean the work would be used by the receiver.
//!
//! This is where something like `want` comes in. Added to a channel, you can
//! make sure that the `tx` only creates the message and sends it when the `rx`
//! has `poll()` for it, and the buffer was empty.

extern crate futures;
#[macro_use]
extern crate log;
extern crate try_lock;

use std::fmt;
use std::mem;
use std::sync::Arc;
use std::sync::atomic::AtomicUsize;
// SeqCst is the only ordering used to ensure accessing the state and
// TryLock are never re-ordered.
use std::sync::atomic::Ordering::SeqCst;

use futures::{Async, Poll};
use futures::task::{self, Task};

use try_lock::TryLock;

/// Create a new `want` channel.
pub fn new() -> (Giver, Taker) {
    let inner = Arc::new(Inner {
        state: AtomicUsize::new(State::Idle.into()),
        task: TryLock::new(None),
    });
    let inner2 = inner.clone();
    (
        Giver {
            inner: inner,
        },
        Taker {
            inner: inner2,
        },
    )
}

/// An entity that gives a value when wanted.
pub struct Giver {
    inner: Arc<Inner>,
}

/// An entity that wants a value.
pub struct Taker {
    inner: Arc<Inner>,
}

/// A cloneable `Giver`.
///
/// It differs from `Giver` in that you cannot poll for `want`. It's only
/// usable as a cancellation watcher.
#[derive(Clone)]
pub struct SharedGiver {
    inner: Arc<Inner>,
}

/// The `Taker` has canceled its interest in a value.
pub struct Closed {
    _inner: (),
}

#[derive(Clone, Copy, Debug)]
enum State {
    Idle,
    Want,
    Give,
    Closed,
}

impl From<State> for usize {
    fn from(s: State) -> usize {
        match s {
            State::Idle => 0,
            State::Want => 1,
            State::Give => 2,
            State::Closed => 3,
        }
    }
}

impl From<usize> for State {
    fn from(num: usize) -> State {
        match num {
            0 => State::Idle,
            1 => State::Want,
            2 => State::Give,
            3 => State::Closed,
            _ => unreachable!("unknown state: {}", num),
        }
    }
}

struct Inner {
    state: AtomicUsize,
    task: TryLock<Option<Task>>,
}

// ===== impl Giver ======

impl Giver {
    /// Poll whether the `Taker` has registered interest in another value.
    ///
    /// - If the `Taker` has called `want()`, this returns `Async::Ready(())`.
    /// - If the `Taker` has not called `want()` since last poll, this
    ///   returns `Async::NotReady`, and parks the current task to be notified
    ///   when the `Taker` does call `want()`.
    /// - If the `Taker` has canceled (or dropped), this returns `Closed`.
    ///
    /// After knowing that the Taker is wanting, the state can be reset by
    /// calling [`give`](Giver::give).
    pub fn poll_want(&mut self) -> Poll<(), Closed> {
        loop {
            let state = self.inner.state.load(SeqCst).into();
            match state {
                State::Want => {
                    trace!("poll_want: taker wants!");
                    return Ok(Async::Ready(()));
                },
                State::Closed => {
                    trace!("poll_want: closed");
                    return Err(Closed { _inner: () });
                },
                State::Idle | State::Give => {
                    // Taker doesn't want anything yet, so park.
                    if let Some(mut locked) = self.inner.task.try_lock_order(SeqCst, SeqCst) {

                        // While we have the lock, try to set to GIVE.
                        let old = self.inner.state.compare_and_swap(
                            state.into(),
                            State::Give.into(),
                            SeqCst,
                        );
                        // If it's still the first state (Idle or Give), park current task.
                        if old == state.into() {
                            let park = locked.as_ref()
                                .map(|t| !t.will_notify_current())
                                .unwrap_or(true);
                            if park {
                                let old = mem::replace(&mut *locked, Some(task::current()));
                                drop(locked);
                                old.map(|prev_task| {
                                    // there was an old task parked here.
                                    // it might be waiting to be notified,
                                    // so poke it before dropping.
                                    prev_task.notify();
                                });
                            }
                            return Ok(Async::NotReady)
                        }
                        // Otherwise, something happened! Go around the loop again.
                    } else {
                        // if we couldn't take the lock, then a Taker has it.
                        // The *ONLY* reason is because it is in the process of notifying us
                        // of its want.
                        //
                        // We need to loop again to see what state it was changed to.
                    }
                },
            }
        }
    }

    /// Mark the state as idle, if the Taker currently is wanting.
    ///
    /// Returns true if Taker was wanting, false otherwise.
    #[inline]
    pub fn give(&self) -> bool {
        // only set to IDLE if it is still Want
        self.inner.state.compare_and_swap(
            State::Want.into(),
            State::Idle.into(),
            SeqCst,
        ) == State::Want.into()
    }

    /// Check if the `Taker` has called `want()` without parking a task.
    ///
    /// This is safe to call outside of a futures task context, but other
    /// means of being notified is left to the user.
    #[inline]
    pub fn is_wanting(&self) -> bool {
        self.inner.state.load(SeqCst) == State::Want.into()
    }


    /// Check if the `Taker` has canceled interest without parking a task.
    #[inline]
    pub fn is_canceled(&self) -> bool {
        self.inner.state.load(SeqCst) == State::Closed.into()
    }

    /// Converts this into a `SharedGiver`.
    #[inline]
    pub fn shared(self) -> SharedGiver {
        SharedGiver {
            inner: self.inner,
        }
    }
}

impl fmt::Debug for Giver {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("Giver")
            .field("state", &self.inner.state())
            .finish()
    }
}

// ===== impl SharedGiver ======

impl SharedGiver {
    /// Check if the `Taker` has called `want()` without parking a task.
    ///
    /// This is safe to call outside of a futures task context, but other
    /// means of being notified is left to the user.
    #[inline]
    pub fn is_wanting(&self) -> bool {
        self.inner.state.load(SeqCst) == State::Want.into()
    }


    /// Check if the `Taker` has canceled interest without parking a task.
    #[inline]
    pub fn is_canceled(&self) -> bool {
        self.inner.state.load(SeqCst) == State::Closed.into()
    }
}

impl fmt::Debug for SharedGiver {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("SharedGiver")
            .field("state", &self.inner.state())
            .finish()
    }
}

// ===== impl Taker ======

impl Taker {
    /// Signal to the `Giver` that the want is canceled.
    ///
    /// This is useful to tell that the channel is closed if you cannot
    /// drop the value yet.
    #[inline]
    pub fn cancel(&mut self) {
        trace!("signal: {:?}", State::Closed);
        self.signal(State::Closed)
    }

    /// Signal to the `Giver` that a value is wanted.
    #[inline]
    pub fn want(&mut self) {
        debug_assert!(
            self.inner.state.load(SeqCst) != State::Closed.into(),
            "want called after cancel"
        );
        trace!("signal: {:?}", State::Want);
        self.signal(State::Want)
    }

    #[inline]
    fn signal(&mut self, state: State) {
        let old_state = self.inner.state.swap(state.into(), SeqCst).into();
        match old_state {
            State::Idle | State::Want | State::Closed => (),
            State::Give => {
                loop {
                    if let Some(mut locked) = self.inner.task.try_lock_order(SeqCst, SeqCst) {
                        if let Some(task) = locked.take() {
                            drop(locked);
                            trace!("signal found waiting giver, notifying");
                            task.notify();
                        }
                        return;
                    } else {
                        // if we couldn't take the lock, then a Giver has it.
                        // The *ONLY* reason is because it is in the process of parking.
                        //
                        // We need to loop and take the lock so we can notify this task.
                    }
                }
            },
        }
    }
}

impl Drop for Taker {
    #[inline]
    fn drop(&mut self) {
        self.signal(State::Closed);
    }
}

impl fmt::Debug for Taker {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("Taker")
            .field("state", &self.inner.state())
            .finish()
    }
}

// ===== impl Closed ======

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

// ===== impl Inner ======

impl Inner {
    #[inline]
    fn state(&self) -> State {
        self.state.load(SeqCst).into()
    }
}

#[cfg(test)]
mod tests {
    use std::thread;
    use futures::{Async, Stream};
    use futures::future::{poll_fn, Future};
    use futures::sync::{mpsc, oneshot};
    use super::*;

    #[test]
    fn want_ready() {
        let (mut gv, mut tk) = new();
        tk.want();
        assert!(gv.poll_want().unwrap().is_ready());
    }

    #[test]
    fn want_notify_0() {
        let (mut gv, mut tk) = new();
        let (tx, rx) = oneshot::channel();

        thread::spawn(move || {
            tk.want();
            // use a oneshot to keep this thread alive
            // until other thread was notified of want
            rx.wait().expect("rx");
        });

        poll_fn(|| {
            gv.poll_want()
        }).wait().expect("wait");

        assert!(gv.is_wanting(), "still wanting after poll_want success");
        assert!(gv.give(), "give is true when wanting");

        assert!(!gv.is_wanting(), "no longer wanting after give");
        assert!(!gv.is_canceled(), "give doesn't cancel");

        assert!(!gv.give(), "give is false if not wanting");

        tx.send(()).expect("tx");
    }

    /// This tests that if the Giver moves tasks after parking,
    /// it will still wake up the correct task.
    #[test]
    fn want_notify_moving_tasks() {
        use std::sync::Arc;
        use futures::executor::{spawn, Notify, NotifyHandle};

        struct WantNotify;

        impl Notify for WantNotify {
            fn notify(&self, _id: usize) {
            }
        }

        fn n() -> NotifyHandle {
            Arc::new(WantNotify).into()
        }

        let (mut gv, mut tk) = new();

        let mut s = spawn(poll_fn(move || {
            gv.poll_want()
        }));

        // Register with t1 as the task::current()
        let t1 = n();
        assert!(s.poll_future_notify(&t1, 1).unwrap().is_not_ready());

        thread::spawn(move || {
            thread::sleep(::std::time::Duration::from_millis(100));
            tk.want();
        });

        // And now, move to a ThreadNotify task.
        s.into_inner().wait().expect("poll_want");
    }

    #[test]
    fn cancel() {
        // explicit
        let (mut gv, mut tk) = new();

        assert!(!gv.is_canceled());

        tk.cancel();

        assert!(gv.is_canceled());
        assert!(gv.poll_want().is_err());

        // implicit
        let (mut gv, tk) = new();

        assert!(!gv.is_canceled());

        drop(tk);

        assert!(gv.is_canceled());
        assert!(gv.poll_want().is_err());

        // notifies
        let (mut gv, tk) = new();

        thread::spawn(move || {
            let _tk = tk;
            // and dropped
        });

        poll_fn(move || {
            gv.poll_want()
        }).wait().expect_err("wait");
    }

    #[test]
    fn stress() {
        let nthreads = 5;
        let nwants = 100;

        for _ in 0..nthreads {
            let (mut gv, mut tk) = new();
            let (mut tx, mut rx) = mpsc::channel(0);

            // rx thread
            thread::spawn(move || {
                let mut cnt = 0;
                poll_fn(move || {
                    while cnt < nwants {
                        let n = match rx.poll().expect("rx poll") {
                            Async::Ready(n) => n.expect("rx opt"),
                            Async::NotReady => {
                                tk.want();
                                return Ok(Async::NotReady);
                            },
                        };
                        assert_eq!(cnt, n);
                        cnt += 1;
                    }
                    Ok::<_, ()>(Async::Ready(()))
                }).wait().expect("rx wait");
            });

            // tx thread
            thread::spawn(move || {
                let mut cnt = 0;
                let nsent = poll_fn(move || {
                    loop {
                        while let Ok(()) = tx.try_send(cnt) {
                            cnt += 1;
                        }
                        match gv.poll_want() {
                            Ok(Async::Ready(_)) => (),
                            Ok(Async::NotReady) => return Ok::<_, ()>(Async::NotReady),
                            Err(_) => return Ok(Async::Ready(cnt)),
                        }
                    }
                }).wait().expect("tx wait");

                assert_eq!(nsent, nwants);
            }).join().expect("thread join");
        }
    }
}