io/driver/registration.rs

#![cfg_attr(not(feature = "net"), allow(dead_code))]

use crate::io::driver::{Direction, Handle, Interest, ReadyEvent, ScheduledIo};
use crate::util::slab;

use mio::event::Source;
use std::io;
use std::task::{Context, Poll};

cfg_io_driver! {
    /// Associates an I/O resource with the reactor instance that drives it.
    ///
    /// A registration represents an I/O resource registered with a Reactor such
    /// that it will receive task notifications on readiness. This is the lowest
    /// level API for integrating with a reactor.
    ///
    /// The association between an I/O resource is made by calling
    /// [`new_with_interest_and_handle`].
    /// Once the association is established, it remains established until the
    /// registration instance is dropped.
    ///
    /// A registration instance represents two separate readiness streams. One
    /// for the read readiness and one for write readiness. These streams are
    /// independent and can be consumed from separate tasks.
    ///
    /// **Note**: while `Registration` is `Sync`, the caller must ensure that
    /// there are at most two tasks that use a registration instance
    /// concurrently. One task for [`poll_read_ready`] and one task for
    /// [`poll_write_ready`]. While violating this requirement is "safe" from a
    /// Rust memory safety point of view, it will result in unexpected behavior
    /// in the form of lost notifications and tasks hanging.
    ///
    /// ## Platform-specific events
    ///
    /// `Registration` also allows receiving platform-specific `mio::Ready`
    /// events. These events are included as part of the read readiness event
    /// stream. The write readiness event stream is only for `Ready::writable()`
    /// events.
    ///
    /// [`new_with_interest_and_handle`]: method@Self::new_with_interest_and_handle
    /// [`poll_read_ready`]: method@Self::poll_read_ready`
    /// [`poll_write_ready`]: method@Self::poll_write_ready`
    #[derive(Debug)]
    pub(crate) struct Registration {
        /// Handle to the associated driver.
        handle: Handle,

        /// Reference to state stored by the driver.
        shared: slab::Ref<ScheduledIo>,
    }
}

unsafe impl Send for Registration {}
unsafe impl Sync for Registration {}

// ===== impl Registration =====

impl Registration {
    /// Registers the I/O resource with the default reactor, for a specific
    /// `Interest`. `new_with_interest` should be used over `new` when you need
    /// control over the readiness state, such as when a file descriptor only
    /// allows reads. This does not add `hup` or `error` so if you are
    /// interested in those states, you will need to add them to the readiness
    /// state passed to this function.
    ///
    /// # Return
    ///
    /// - `Ok` if the registration happened successfully
    /// - `Err` if an error was encountered during registration
    pub(crate) fn new_with_interest_and_handle(
        io: &mut impl Source,
        interest: Interest,
        handle: Handle,
    ) -> io::Result<Registration> {
        let shared = if let Some(inner) = handle.inner() {
            inner.add_source(io, interest)?
        } else {
            return Err(io::Error::new(
                io::ErrorKind::Other,
                "failed to find event loop",
            ));
        };

        Ok(Registration { handle, shared })
    }

    /// Deregisters the I/O resource from the reactor it is associated with.
    ///
    /// This function must be called before the I/O resource associated with the
    /// registration is dropped.
    ///
    /// Note that deregistering does not guarantee that the I/O resource can be
    /// registered with a different reactor. Some I/O resource types can only be
    /// associated with a single reactor instance for their lifetime.
    ///
    /// # Return
    ///
    /// If the deregistration was successful, `Ok` is returned. Any calls to
    /// `Reactor::turn` that happen after a successful call to `deregister` will
    /// no longer result in notifications getting sent for this registration.
    ///
    /// `Err` is returned if an error is encountered.
    pub(crate) fn deregister(&mut self, io: &mut impl Source) -> io::Result<()> {
        let inner = match self.handle.inner() {
            Some(inner) => inner,
            None => return Err(io::Error::new(io::ErrorKind::Other, "reactor gone")),
        };
        inner.deregister_source(io)
    }

    pub(crate) fn clear_readiness(&self, event: ReadyEvent) {
        self.shared.clear_readiness(event);
    }

    // Uses the poll path, requiring the caller to ensure mutual exclusion for
    // correctness. Only the last task to call this function is notified.
    pub(crate) fn poll_read_ready(&self, cx: &mut Context<'_>) -> Poll<io::Result<ReadyEvent>> {
        self.poll_ready(cx, Direction::Read)
    }

    // Uses the poll path, requiring the caller to ensure mutual exclusion for
    // correctness. Only the last task to call this function is notified.
    pub(crate) fn poll_write_ready(&self, cx: &mut Context<'_>) -> Poll<io::Result<ReadyEvent>> {
        self.poll_ready(cx, Direction::Write)
    }

    // Uses the poll path, requiring the caller to ensure mutual exclusion for
    // correctness. Only the last task to call this function is notified.
    pub(crate) fn poll_read_io<R>(
        &self,
        cx: &mut Context<'_>,
        f: impl FnMut() -> io::Result<R>,
    ) -> Poll<io::Result<R>> {
        self.poll_io(cx, Direction::Read, f)
    }

    // Uses the poll path, requiring the caller to ensure mutual exclusion for
    // correctness. Only the last task to call this function is notified.
    pub(crate) fn poll_write_io<R>(
        &self,
        cx: &mut Context<'_>,
        f: impl FnMut() -> io::Result<R>,
    ) -> Poll<io::Result<R>> {
        self.poll_io(cx, Direction::Write, f)
    }

    /// Polls for events on the I/O resource's `direction` readiness stream.
    ///
    /// If called with a task context, notify the task when a new event is
    /// received.
    fn poll_ready(
        &self,
        cx: &mut Context<'_>,
        direction: Direction,
    ) -> Poll<io::Result<ReadyEvent>> {
        // Keep track of task budget
        let coop = ready!(crate::coop::poll_proceed(cx));
        let ev = ready!(self.shared.poll_readiness(cx, direction));

        if self.handle.inner().is_none() {
            return Poll::Ready(Err(gone()));
        }

        coop.made_progress();
        Poll::Ready(Ok(ev))
    }

    fn poll_io<R>(
        &self,
        cx: &mut Context<'_>,
        direction: Direction,
        mut f: impl FnMut() -> io::Result<R>,
    ) -> Poll<io::Result<R>> {
        loop {
            let ev = ready!(self.poll_ready(cx, direction))?;

            match f() {
                Ok(ret) => {
                    return Poll::Ready(Ok(ret));
                }
                Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
                    self.clear_readiness(ev);
                }
                Err(e) => return Poll::Ready(Err(e)),
            }
        }
    }

    pub(crate) fn try_io<R>(
        &self,
        interest: Interest,
        f: impl FnOnce() -> io::Result<R>,
    ) -> io::Result<R> {
        let ev = self.shared.ready_event(interest);

        // Don't attempt the operation if the resource is not ready.
        if ev.ready.is_empty() {
            return Err(io::ErrorKind::WouldBlock.into());
        }

        match f() {
            Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
                self.clear_readiness(ev);
                Err(io::ErrorKind::WouldBlock.into())
            }
            res => res,
        }
    }
}

impl Drop for Registration {
    fn drop(&mut self) {
        // It is possible for a cycle to be created between wakers stored in
        // `ScheduledIo` instances and `Arc<driver::Inner>`. To break this
        // cycle, wakers are cleared. This is an imperfect solution as it is
        // possible to store a `Registration` in a waker. In this case, the
        // cycle would remain.
        //
        // See tokio-rs/tokio#3481 for more details.
        self.shared.clear_wakers();
    }
}

fn gone() -> io::Error {
    io::Error::new(io::ErrorKind::Other, "IO driver has terminated")
}

cfg_io_readiness! {
    impl Registration {
        pub(crate) async fn readiness(&self, interest: Interest) -> io::Result<ReadyEvent> {
            use std::future::Future;
            use std::pin::Pin;

            let fut = self.shared.readiness(interest);
            pin!(fut);

            crate::future::poll_fn(|cx| {
                if self.handle.inner().is_none() {
                    return Poll::Ready(Err(io::Error::new(
                        io::ErrorKind::Other,
                        crate::util::error::RUNTIME_SHUTTING_DOWN_ERROR
                    )));
                }

                Pin::new(&mut fut).poll(cx).map(Ok)
            }).await
        }

        pub(crate) async fn async_io<R>(&self, interest: Interest, mut f: impl FnMut() -> io::Result<R>) -> io::Result<R> {
            loop {
                let event = self.readiness(interest).await?;

                match f() {
                    Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
                        self.clear_readiness(event);
                    }
                    x => return x,
                }
            }
        }
    }
}