rust/src/librustuv/async.rs

// 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.

use alloc::arc::Arc;
use std::mem;
use std::rt::exclusive::Exclusive;
use std::rt::rtio::{Callback, RemoteCallback};

use uvll;
use super::{Loop, UvHandle};

// The entire point of async is to call into a loop from other threads so it
// does not need to home.
pub struct AsyncWatcher {
    handle: *mut uvll::uv_async_t,

    // A flag to tell the callback to exit, set from the dtor. This is
    // almost never contested - only in rare races with the dtor.
    exit_flag: Arc<Exclusive<bool>>,
}

struct Payload {
    callback: Box<Callback + Send>,
    exit_flag: Arc<Exclusive<bool>>,
}

impl AsyncWatcher {
    pub fn new(loop_: &mut Loop, cb: Box<Callback + Send>) -> AsyncWatcher {
        let handle = UvHandle::alloc(None::<AsyncWatcher>, uvll::UV_ASYNC);
        assert_eq!(unsafe {
            uvll::uv_async_init(loop_.handle, handle, async_cb)
        }, 0);
        let flag = Arc::new(Exclusive::new(false));
        let payload = box Payload { callback: cb, exit_flag: flag.clone() };
        unsafe {
            let payload: *mut u8 = mem::transmute(payload);
            uvll::set_data_for_uv_handle(handle, payload);
        }
        return AsyncWatcher { handle: handle, exit_flag: flag, };
    }
}

impl UvHandle<uvll::uv_async_t> for AsyncWatcher {
    fn uv_handle(&self) -> *mut uvll::uv_async_t { self.handle }
    unsafe fn from_uv_handle<'a>(_: &'a *mut uvll::uv_async_t) -> &'a mut AsyncWatcher {
        fail!("async watchers can't be built from their handles");
    }
}

extern fn async_cb(handle: *mut uvll::uv_async_t) {
    let payload: &mut Payload = unsafe {
        mem::transmute(uvll::get_data_for_uv_handle(handle))
    };

    // The synchronization logic here is subtle. To review,
    // the uv async handle type promises that, after it is
    // triggered the remote callback is definitely called at
    // least once. UvRemoteCallback needs to maintain those
    // semantics while also shutting down cleanly from the
    // dtor. In our case that means that, when the
    // UvRemoteCallback dtor calls `async.send()`, here `f` is
    // always called later.

    // In the dtor both the exit flag is set and the async
    // callback fired under a lock.  Here, before calling `f`,
    // we take the lock and check the flag. Because we are
    // checking the flag before calling `f`, and the flag is
    // set under the same lock as the send, then if the flag
    // is set then we're guaranteed to call `f` after the
    // final send.

    // If the check was done after `f()` then there would be a
    // period between that call and the check where the dtor
    // could be called in the other thread, missing the final
    // callback while still destroying the handle.

    let should_exit = unsafe { *payload.exit_flag.lock() };

    payload.callback.call();

    if should_exit {
        unsafe { uvll::uv_close(handle, close_cb) }
    }
}

extern fn close_cb(handle: *mut uvll::uv_handle_t) {
    // drop the payload
    let _payload: Box<Payload> = unsafe {
        mem::transmute(uvll::get_data_for_uv_handle(handle))
    };
    // and then free the handle
    unsafe { uvll::free_handle(handle) }
}

impl RemoteCallback for AsyncWatcher {
    fn fire(&mut self) {
        unsafe { uvll::uv_async_send(self.handle) }
    }
}

impl Drop for AsyncWatcher {
    fn drop(&mut self) {
        let mut should_exit = unsafe { self.exit_flag.lock() };
        // NB: These two things need to happen atomically. Otherwise
        // the event handler could wake up due to a *previous*
        // signal and see the exit flag, destroying the handle
        // before the final send.
        *should_exit = true;
        unsafe { uvll::uv_async_send(self.handle) }
    }
}

#[cfg(test)]
mod test_remote {
    use std::rt::rtio::{Callback, RemoteCallback};
    use std::rt::thread::Thread;

    use super::AsyncWatcher;
    use super::super::local_loop;

    // Make sure that we can fire watchers in remote threads and that they
    // actually trigger what they say they will.
    #[test]
    fn smoke_test() {
        struct MyCallback(Option<Sender<int>>);
        impl Callback for MyCallback {
            fn call(&mut self) {
                // this can get called more than once, but we only want to send
                // once
                let MyCallback(ref mut s) = *self;
                if s.is_some() {
                    s.take_unwrap().send(1);
                }
            }
        }

        let (tx, rx) = channel();
        let cb = box MyCallback(Some(tx));
        let watcher = AsyncWatcher::new(&mut local_loop().loop_, cb);

        let thread = Thread::start(proc() {
            let mut watcher = watcher;
            watcher.fire();
        });

        assert_eq!(rx.recv(), 1);
        thread.join();
    }
}