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// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//! A concurrent queue used to signal remote event loops
//!
//! This queue implementation is used to send tasks among event loops. This is
//! backed by a multi-producer/single-consumer queue from libstd and uv_async_t
//! handles (to wake up a remote event loop).
//!
//! The uv_async_t is stored next to the event loop, so in order to not keep the
//! event loop alive we use uv_ref and uv_unref in order to control when the
//! async handle is active or not.
#![allow(dead_code)]
use std::cast;
use std::libc::{c_void, c_int};
use std::rt::task::BlockedTask;
use std::unstable::mutex::NativeMutex;
use std::sync::arc::UnsafeArc;
use mpsc = std::sync::mpsc_queue;
use async::AsyncWatcher;
use super::{Loop, UvHandle};
use uvll;
enum Message {
Task(BlockedTask),
Increment,
Decrement,
}
struct State {
handle: *uvll::uv_async_t,
lock: NativeMutex, // see comments in async_cb for why this is needed
queue: mpsc::Queue<Message>,
}
/// This structure is intended to be stored next to the event loop, and it is
/// used to create new `Queue` structures.
pub struct QueuePool {
priv queue: UnsafeArc<State>,
priv refcnt: uint,
}
/// This type is used to send messages back to the original event loop.
pub struct Queue {
priv queue: UnsafeArc<State>,
}
extern fn async_cb(handle: *uvll::uv_async_t, status: c_int) {
assert_eq!(status, 0);
let pool: &mut QueuePool = unsafe {
cast::transmute(uvll::get_data_for_uv_handle(handle))
};
let state: &mut State = unsafe { cast::transmute(pool.queue.get()) };
// Remember that there is no guarantee about how many times an async
// callback is called with relation to the number of sends, so process the
// entire queue in a loop.
loop {
match state.queue.pop() {
mpsc::Data(Task(task)) => {
let _ = task.wake().map(|t| t.reawaken());
}
mpsc::Data(Increment) => unsafe {
if pool.refcnt == 0 {
uvll::uv_ref(state.handle);
}
pool.refcnt += 1;
},
mpsc::Data(Decrement) => unsafe {
pool.refcnt -= 1;
if pool.refcnt == 0 {
uvll::uv_unref(state.handle);
}
},
mpsc::Empty | mpsc::Inconsistent => break
};
}
// If the refcount is now zero after processing the queue, then there is no
// longer a reference on the async handle and it is possible that this event
// loop can exit. What we're not guaranteed, however, is that a producer in
// the middle of dropping itself is yet done with the handle. It could be
// possible that we saw their Decrement message but they have yet to signal
// on the async handle. If we were to return immediately, the entire uv loop
// could be destroyed meaning the call to uv_async_send would abort()
//
// In order to fix this, an OS mutex is used to wait for the other end to
// finish before we continue. The drop block on a handle will acquire a
// mutex and then drop it after both the push and send have been completed.
// If we acquire the mutex here, then we are guaranteed that there are no
// longer any senders which are holding on to their handles, so we can
// safely allow the event loop to exit.
if pool.refcnt == 0 {
unsafe {
let _l = state.lock.lock();
}
}
}
impl QueuePool {
pub fn new(loop_: &mut Loop) -> ~QueuePool {
let handle = UvHandle::alloc(None::<AsyncWatcher>, uvll::UV_ASYNC);
let state = UnsafeArc::new(State {
handle: handle,
lock: unsafe {NativeMutex::new()},
queue: mpsc::Queue::new(),
});
let q = ~QueuePool {
refcnt: 0,
queue: state,
};
unsafe {
assert_eq!(uvll::uv_async_init(loop_.handle, handle, async_cb), 0);
uvll::uv_unref(handle);
let data = &*q as *QueuePool as *c_void;
uvll::set_data_for_uv_handle(handle, data);
}
return q;
}
pub fn queue(&mut self) -> Queue {
unsafe {
if self.refcnt == 0 {
uvll::uv_ref((*self.queue.get()).handle);
}
self.refcnt += 1;
}
Queue { queue: self.queue.clone() }
}
pub fn handle(&self) -> *uvll::uv_async_t {
unsafe { (*self.queue.get()).handle }
}
}
impl Queue {
pub fn push(&mut self, task: BlockedTask) {
unsafe {
(*self.queue.get()).queue.push(Task(task));
uvll::uv_async_send((*self.queue.get()).handle);
}
}
}
impl Clone for Queue {
fn clone(&self) -> Queue {
// Push a request to increment on the queue, but there's no need to
// signal the event loop to process it at this time. We're guaranteed
// that the count is at least one (because we have a queue right here),
// and if the queue is dropped later on it'll see the increment for the
// decrement anyway.
unsafe {
(*self.queue.get()).queue.push(Increment);
}
Queue { queue: self.queue.clone() }
}
}
impl Drop for Queue {
fn drop(&mut self) {
// See the comments in the async_cb function for why there is a lock
// that is acquired only on a drop.
unsafe {
let state = self.queue.get();
let _l = (*state).lock.lock();
(*state).queue.push(Decrement);
uvll::uv_async_send((*state).handle);
}
}
}
impl Drop for State {
fn drop(&mut self) {
unsafe {
uvll::uv_close(self.handle, cast::transmute(0));
// Note that this does *not* free the handle, that is the
// responsibility of the caller because the uv loop must be closed
// before we deallocate this uv handle.
}
}
}
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