执行器和系统 IO

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2023-12-01

在The Future Trait 的上一章节中，我们讨论了这个 Future 在套接字上，执行异步读取的示例：

pub struct SocketRead<'a> {
    socket: &'a Socket,
}

impl SimpleFuture for SocketRead<'_> {
    type Output = Vec<u8>;

    fn poll(&mut self, wake: fn()) -> Poll<Self::Output> {
        if self.socket.has_data_to_read() {
            // The socket has data-- read it into a buffer and return it.
            Poll::Ready(self.socket.read_buf())
        } else {
            // The socket does not yet have data.
            //
            // Arrange for `wake` to be called once data is available.
            // When data becomes available, `wake` will be called, and the
            // user of this `Future` will know to call `poll` again and
            // receive data.
            self.socket.set_readable_callback(wake);
            Poll::Pending
        }
    }
}

这个 Future 将读取套接字上的可用数据，如果没有可用数据，它将交还给 executor，要求在套接字再次变得可读时，唤醒这个任务。但是，根据此示例尚不清楚，这个Socket类型是怎么实现的，尤其是set_readable_callback函数是如何工作的。我们如何安排lw.wake()，在一旦套接字变得可读时，就被调用？一种选择是，让一个线程不断检查socket是否可读，在适当的时候调用wake()。但是，这将是非常低效的，需要为每个阻塞的 IO Future 使用一个单独的线程。这将大大降低我们异步代码的效率。

实际上，此问题是通过与 IO-感知系统阻塞原语交互来解决。例如，epoll在 Linux 上，kqueue在 FreeBSD 和 Mac OS 上，在 Windows 上为 IOCP，以及 Fuchsia 的port（所有这些都通过跨平台的 Rust 箱子mio揭露）。这些原语都允许一个线程，在多个异步 IO 事件上阻塞，并在事件的其中一个完成后返回。实际上，这些 API 通常如下所示：

struct IoBlocker {
    ...
}

struct Event {
    // An ID uniquely identifying the event that occurred and was listened for.
    id: usize,

    // A set of signals to wait for, or which occurred.
    signals: Signals,
}

impl IoBlocker {
    /// Create a new collection of asynchronous IO events to block on.
    fn new() -> Self { ... }

    /// Express an interest in a particular IO event.
    fn add_io_event_interest(
        &self,

        /// The object on which the event will occur
        io_object: &IoObject,

        /// A set of signals that may appear on the `io_object` for
        /// which an event should be triggered, paired with
        /// an ID to give to events that result from this interest.
        event: Event,
    ) { ... }

    /// Block until one of the events occurs.
    fn block(&self) -> Event { ... }
}

let mut io_blocker = IoBlocker::new();
io_blocker.add_io_event_interest(
    &socket_1,
    Event { id: 1, signals: READABLE },
);
io_blocker.add_io_event_interest(
    &socket_2,
    Event { id: 2, signals: READABLE | WRITABLE },
);
let event = io_blocker.block();

// prints e.g. "Socket 1 is now READABLE" if socket one became readable.
println!("Socket {:?} is now {:?}", event.id, event.signals);

Future executor 可以使用这些原语来提供异步 IO 对象（例如套接字），这些对象可以配置，在发生特定 IO 事件时，运行的回调。在我们上面例子的SocketRead情况下Socket::set_readable_callback函数可能类似于以下伪代码：

impl Socket {
    fn set_readable_callback(&self, waker: Waker) {
        // `local_executor` is a reference to the local executor.
        // this could be provided at creation of the socket, but in practice
        // many executor implementations pass it down through thread local
        // storage for convenience.
        let local_executor = self.local_executor;

        // Unique ID for this IO object.
        let id = self.id;

        // Store the local waker in the executor's map so that it can be called
        // once the IO event arrives.
        local_executor.event_map.insert(id, waker);
        local_executor.add_io_event_interest(
            &self.socket_file_descriptor,
            Event { id, signals: READABLE },
        );
    }
}

现在，我们只有一个 executor 线程，该线程可以接收任何 IO 事件，并将 IO 事件分配给相应的Waker，这将唤醒相应的任务，从而使 executor 在返回以检查更多 IO 事件之前，可以驱使更多任务驶向完成，（且该循环会继续...）。