486 lines
17 KiB
Rust
486 lines
17 KiB
Rust
// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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/// Synchronous channels/ports
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///
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/// This channel implementation differs significantly from the asynchronous
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/// implementations found next to it (oneshot/stream/share). This is an
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/// implementation of a synchronous, bounded buffer channel.
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///
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/// Each channel is created with some amount of backing buffer, and sends will
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/// *block* until buffer space becomes available. A buffer size of 0 is valid,
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/// which means that every successful send is paired with a successful recv.
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///
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/// This flavor of channels defines a new `send_opt` method for channels which
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/// is the method by which a message is sent but the task does not panic if it
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/// cannot be delivered.
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///
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/// Another major difference is that send() will *always* return back the data
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/// if it couldn't be sent. This is because it is deterministically known when
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/// the data is received and when it is not received.
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///
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/// Implementation-wise, it can all be summed up with "use a mutex plus some
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/// logic". The mutex used here is an OS native mutex, meaning that no user code
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/// is run inside of the mutex (to prevent context switching). This
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/// implementation shares almost all code for the buffered and unbuffered cases
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/// of a synchronous channel. There are a few branches for the unbuffered case,
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/// but they're mostly just relevant to blocking senders.
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use core::prelude::*;
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pub use self::Failure::*;
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use self::Blocker::*;
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use vec::Vec;
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use core::mem;
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use core::ptr;
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use sync::atomic::{Ordering, AtomicUsize};
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use sync::mpsc::blocking::{self, WaitToken, SignalToken};
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use sync::mpsc::select::StartResult::{self, Installed, Abort};
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use sync::{Mutex, MutexGuard};
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pub struct Packet<T> {
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/// Only field outside of the mutex. Just done for kicks, but mainly because
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/// the other shared channel already had the code implemented
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channels: AtomicUsize,
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lock: Mutex<State<T>>,
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}
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unsafe impl<T:Send> Send for Packet<T> { }
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unsafe impl<T:Send> Sync for Packet<T> { }
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struct State<T> {
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disconnected: bool, // Is the channel disconnected yet?
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queue: Queue, // queue of senders waiting to send data
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blocker: Blocker, // currently blocked task on this channel
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buf: Buffer<T>, // storage for buffered messages
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cap: uint, // capacity of this channel
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/// A curious flag used to indicate whether a sender failed or succeeded in
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/// blocking. This is used to transmit information back to the task that it
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/// must dequeue its message from the buffer because it was not received.
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/// This is only relevant in the 0-buffer case. This obviously cannot be
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/// safely constructed, but it's guaranteed to always have a valid pointer
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/// value.
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canceled: Option<&'static mut bool>,
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}
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unsafe impl<T: Send> Send for State<T> {}
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/// Possible flavors of threads who can be blocked on this channel.
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enum Blocker {
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BlockedSender(SignalToken),
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BlockedReceiver(SignalToken),
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NoneBlocked
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}
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/// Simple queue for threading tasks together. Nodes are stack-allocated, so
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/// this structure is not safe at all
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struct Queue {
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head: *mut Node,
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tail: *mut Node,
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}
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struct Node {
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token: Option<SignalToken>,
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next: *mut Node,
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}
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unsafe impl Send for Node {}
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/// A simple ring-buffer
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struct Buffer<T> {
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buf: Vec<Option<T>>,
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start: uint,
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size: uint,
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}
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#[derive(Show)]
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pub enum Failure {
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Empty,
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Disconnected,
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}
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/// Atomically blocks the current thread, placing it into `slot`, unlocking `lock`
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/// in the meantime. This re-locks the mutex upon returning.
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fn wait<'a, 'b, T: Send>(lock: &'a Mutex<State<T>>,
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mut guard: MutexGuard<'b, State<T>>,
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f: fn(SignalToken) -> Blocker)
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-> MutexGuard<'a, State<T>>
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{
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let (wait_token, signal_token) = blocking::tokens();
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match mem::replace(&mut guard.blocker, f(signal_token)) {
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NoneBlocked => {}
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_ => unreachable!(),
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}
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drop(guard); // unlock
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wait_token.wait(); // block
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lock.lock().unwrap() // relock
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}
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/// Wakes up a thread, dropping the lock at the correct time
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fn wakeup<T>(token: SignalToken, guard: MutexGuard<State<T>>) {
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// We need to be careful to wake up the waiting task *outside* of the mutex
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// in case it incurs a context switch.
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drop(guard);
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token.signal();
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}
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impl<T: Send> Packet<T> {
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pub fn new(cap: uint) -> Packet<T> {
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Packet {
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channels: AtomicUsize::new(1),
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lock: Mutex::new(State {
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disconnected: false,
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blocker: NoneBlocked,
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cap: cap,
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canceled: None,
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queue: Queue {
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head: ptr::null_mut(),
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tail: ptr::null_mut(),
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},
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buf: Buffer {
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buf: range(0, cap + if cap == 0 {1} else {0}).map(|_| None).collect(),
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start: 0,
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size: 0,
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},
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}),
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}
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}
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// wait until a send slot is available, returning locked access to
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// the channel state.
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fn acquire_send_slot(&self) -> MutexGuard<State<T>> {
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let mut node = Node { token: None, next: ptr::null_mut() };
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loop {
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let mut guard = self.lock.lock().unwrap();
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// are we ready to go?
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if guard.disconnected || guard.buf.size() < guard.buf.cap() {
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return guard;
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}
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// no room; actually block
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let wait_token = guard.queue.enqueue(&mut node);
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drop(guard);
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wait_token.wait();
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}
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}
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pub fn send(&self, t: T) -> Result<(), T> {
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let mut guard = self.acquire_send_slot();
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if guard.disconnected { return Err(t) }
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guard.buf.enqueue(t);
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match mem::replace(&mut guard.blocker, NoneBlocked) {
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// if our capacity is 0, then we need to wait for a receiver to be
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// available to take our data. After waiting, we check again to make
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// sure the port didn't go away in the meantime. If it did, we need
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// to hand back our data.
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NoneBlocked if guard.cap == 0 => {
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let mut canceled = false;
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assert!(guard.canceled.is_none());
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guard.canceled = Some(unsafe { mem::transmute(&mut canceled) });
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let mut guard = wait(&self.lock, guard, BlockedSender);
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if canceled {Err(guard.buf.dequeue())} else {Ok(())}
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}
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// success, we buffered some data
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NoneBlocked => Ok(()),
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// success, someone's about to receive our buffered data.
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BlockedReceiver(token) => { wakeup(token, guard); Ok(()) }
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BlockedSender(..) => panic!("lolwut"),
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}
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}
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pub fn try_send(&self, t: T) -> Result<(), super::TrySendError<T>> {
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let mut guard = self.lock.lock().unwrap();
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if guard.disconnected {
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Err(super::TrySendError::Disconnected(t))
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} else if guard.buf.size() == guard.buf.cap() {
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Err(super::TrySendError::Full(t))
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} else if guard.cap == 0 {
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// With capacity 0, even though we have buffer space we can't
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// transfer the data unless there's a receiver waiting.
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match mem::replace(&mut guard.blocker, NoneBlocked) {
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NoneBlocked => Err(super::TrySendError::Full(t)),
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BlockedSender(..) => unreachable!(),
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BlockedReceiver(token) => {
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guard.buf.enqueue(t);
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wakeup(token, guard);
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Ok(())
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}
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}
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} else {
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// If the buffer has some space and the capacity isn't 0, then we
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// just enqueue the data for later retrieval, ensuring to wake up
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// any blocked receiver if there is one.
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assert!(guard.buf.size() < guard.buf.cap());
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guard.buf.enqueue(t);
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match mem::replace(&mut guard.blocker, NoneBlocked) {
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BlockedReceiver(token) => wakeup(token, guard),
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NoneBlocked => {}
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BlockedSender(..) => unreachable!(),
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}
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Ok(())
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}
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}
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// Receives a message from this channel
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//
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// When reading this, remember that there can only ever be one receiver at
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// time.
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pub fn recv(&self) -> Result<T, ()> {
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let mut guard = self.lock.lock().unwrap();
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// Wait for the buffer to have something in it. No need for a while loop
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// because we're the only receiver.
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let mut waited = false;
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if !guard.disconnected && guard.buf.size() == 0 {
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guard = wait(&self.lock, guard, BlockedReceiver);
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waited = true;
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}
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if guard.disconnected && guard.buf.size() == 0 { return Err(()) }
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// Pick up the data, wake up our neighbors, and carry on
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assert!(guard.buf.size() > 0);
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let ret = guard.buf.dequeue();
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self.wakeup_senders(waited, guard);
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return Ok(ret);
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}
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pub fn try_recv(&self) -> Result<T, Failure> {
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let mut guard = self.lock.lock().unwrap();
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// Easy cases first
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if guard.disconnected { return Err(Disconnected) }
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if guard.buf.size() == 0 { return Err(Empty) }
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// Be sure to wake up neighbors
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let ret = Ok(guard.buf.dequeue());
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self.wakeup_senders(false, guard);
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return ret;
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}
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// Wake up pending senders after some data has been received
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//
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// * `waited` - flag if the receiver blocked to receive some data, or if it
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// just picked up some data on the way out
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// * `guard` - the lock guard that is held over this channel's lock
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fn wakeup_senders(&self, waited: bool, mut guard: MutexGuard<State<T>>) {
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let pending_sender1: Option<SignalToken> = guard.queue.dequeue();
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// If this is a no-buffer channel (cap == 0), then if we didn't wait we
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// need to ACK the sender. If we waited, then the sender waking us up
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// was already the ACK.
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let pending_sender2 = if guard.cap == 0 && !waited {
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match mem::replace(&mut guard.blocker, NoneBlocked) {
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NoneBlocked => None,
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BlockedReceiver(..) => unreachable!(),
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BlockedSender(token) => {
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guard.canceled.take();
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Some(token)
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}
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}
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} else {
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None
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};
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mem::drop(guard);
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// only outside of the lock do we wake up the pending tasks
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pending_sender1.map(|t| t.signal());
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pending_sender2.map(|t| t.signal());
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}
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// Prepares this shared packet for a channel clone, essentially just bumping
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// a refcount.
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pub fn clone_chan(&self) {
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self.channels.fetch_add(1, Ordering::SeqCst);
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}
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pub fn drop_chan(&self) {
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// Only flag the channel as disconnected if we're the last channel
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match self.channels.fetch_sub(1, Ordering::SeqCst) {
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1 => {}
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_ => return
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}
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// Not much to do other than wake up a receiver if one's there
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let mut guard = self.lock.lock().unwrap();
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if guard.disconnected { return }
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guard.disconnected = true;
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match mem::replace(&mut guard.blocker, NoneBlocked) {
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NoneBlocked => {}
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BlockedSender(..) => unreachable!(),
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BlockedReceiver(token) => wakeup(token, guard),
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}
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}
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pub fn drop_port(&self) {
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let mut guard = self.lock.lock().unwrap();
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if guard.disconnected { return }
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guard.disconnected = true;
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// If the capacity is 0, then the sender may want its data back after
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// we're disconnected. Otherwise it's now our responsibility to destroy
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// the buffered data. As with many other portions of this code, this
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// needs to be careful to destroy the data *outside* of the lock to
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// prevent deadlock.
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let _data = if guard.cap != 0 {
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mem::replace(&mut guard.buf.buf, Vec::new())
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} else {
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Vec::new()
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};
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let mut queue = mem::replace(&mut guard.queue, Queue {
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head: ptr::null_mut(),
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tail: ptr::null_mut(),
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});
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let waiter = match mem::replace(&mut guard.blocker, NoneBlocked) {
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NoneBlocked => None,
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BlockedSender(token) => {
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*guard.canceled.take().unwrap() = true;
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Some(token)
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}
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BlockedReceiver(..) => unreachable!(),
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};
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mem::drop(guard);
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loop {
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match queue.dequeue() {
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Some(token) => { token.signal(); }
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None => break,
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}
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}
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waiter.map(|t| t.signal());
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}
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////////////////////////////////////////////////////////////////////////////
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// select implementation
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////////////////////////////////////////////////////////////////////////////
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// If Ok, the value is whether this port has data, if Err, then the upgraded
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// port needs to be checked instead of this one.
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pub fn can_recv(&self) -> bool {
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let guard = self.lock.lock().unwrap();
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guard.disconnected || guard.buf.size() > 0
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}
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// Attempts to start selection on this port. This can either succeed or fail
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// because there is data waiting.
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pub fn start_selection(&self, token: SignalToken) -> StartResult {
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let mut guard = self.lock.lock().unwrap();
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if guard.disconnected || guard.buf.size() > 0 {
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Abort
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} else {
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match mem::replace(&mut guard.blocker, BlockedReceiver(token)) {
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NoneBlocked => {}
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BlockedSender(..) => unreachable!(),
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BlockedReceiver(..) => unreachable!(),
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}
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Installed
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}
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}
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// Remove a previous selecting task from this port. This ensures that the
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// blocked task will no longer be visible to any other threads.
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//
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// The return value indicates whether there's data on this port.
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pub fn abort_selection(&self) -> bool {
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let mut guard = self.lock.lock().unwrap();
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match mem::replace(&mut guard.blocker, NoneBlocked) {
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NoneBlocked => true,
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BlockedSender(token) => {
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guard.blocker = BlockedSender(token);
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true
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}
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BlockedReceiver(token) => { drop(token); false }
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}
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}
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}
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#[unsafe_destructor]
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impl<T: Send> Drop for Packet<T> {
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fn drop(&mut self) {
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assert_eq!(self.channels.load(Ordering::SeqCst), 0);
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let mut guard = self.lock.lock().unwrap();
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assert!(guard.queue.dequeue().is_none());
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assert!(guard.canceled.is_none());
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}
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}
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////////////////////////////////////////////////////////////////////////////////
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// Buffer, a simple ring buffer backed by Vec<T>
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////////////////////////////////////////////////////////////////////////////////
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impl<T> Buffer<T> {
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fn enqueue(&mut self, t: T) {
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let pos = (self.start + self.size) % self.buf.len();
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self.size += 1;
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let prev = mem::replace(&mut self.buf[pos], Some(t));
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assert!(prev.is_none());
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}
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fn dequeue(&mut self) -> T {
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let start = self.start;
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self.size -= 1;
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self.start = (self.start + 1) % self.buf.len();
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let result = &mut self.buf[start];
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result.take().unwrap()
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}
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fn size(&self) -> uint { self.size }
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fn cap(&self) -> uint { self.buf.len() }
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}
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////////////////////////////////////////////////////////////////////////////////
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// Queue, a simple queue to enqueue tasks with (stack-allocated nodes)
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////////////////////////////////////////////////////////////////////////////////
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impl Queue {
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fn enqueue(&mut self, node: &mut Node) -> WaitToken {
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let (wait_token, signal_token) = blocking::tokens();
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node.token = Some(signal_token);
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node.next = ptr::null_mut();
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if self.tail.is_null() {
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self.head = node as *mut Node;
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self.tail = node as *mut Node;
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} else {
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unsafe {
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(*self.tail).next = node as *mut Node;
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self.tail = node as *mut Node;
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}
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}
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wait_token
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}
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fn dequeue(&mut self) -> Option<SignalToken> {
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if self.head.is_null() {
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return None
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}
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let node = self.head;
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self.head = unsafe { (*node).next };
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if self.head.is_null() {
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self.tail = ptr::null_mut();
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}
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unsafe {
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(*node).next = ptr::null_mut();
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Some((*node).token.take().unwrap())
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}
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}
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}
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