async_write.rs

use std::io::{self, IoSlice};
use std::ops::DerefMut;
use std::pin::Pin;
use std::task::{Context, Poll};

/// Writes bytes asynchronously.
///
/// The trait inherits from [`std::io::Write`] and indicates that an I/O object is
/// **nonblocking**. All non-blocking I/O objects must return an error when
/// bytes cannot be written instead of blocking the current thread.
///
/// Specifically, this means that the [`poll_write`] function will return one of
/// the following:
///
/// * `Poll::Ready(Ok(n))` means that `n` bytes of data was immediately
///   written.
///
/// * `Poll::Pending` means that no data was written from the buffer
///   provided. The I/O object is not currently writable but may become writable
///   in the future. Most importantly, **the current future's task is scheduled
///   to get unparked when the object is writable**. This means that like
///   `Future::poll` you'll receive a notification when the I/O object is
///   writable again.
///
/// * `Poll::Ready(Err(e))` for other errors are standard I/O errors coming from the
///   underlying object.
///
/// This trait importantly means that the [`write`][stdwrite] method only works in
/// the context of a future's task. The object may panic if used outside of a task.
///
/// Note that this trait also represents that the  [`Write::flush`][stdflush] method
/// works very similarly to the `write` method, notably that `Ok(())` means that the
/// writer has successfully been flushed, a "would block" error means that the
/// current task is ready to receive a notification when flushing can make more
/// progress, and otherwise normal errors can happen as well.
///
/// Utilities for working with `AsyncWrite` values are provided by
/// [`AsyncWriteExt`].
///
/// [`std::io::Write`]: std::io::Write
/// [`poll_write`]: AsyncWrite::poll_write()
/// [stdwrite]: std::io::Write::write()
/// [stdflush]: std::io::Write::flush()
/// [`AsyncWriteExt`]: crate::io::AsyncWriteExt
pub trait AsyncWrite {
    /// Attempt to write bytes from `buf` into the object.
    ///
    /// On success, returns `Poll::Ready(Ok(num_bytes_written))`. If successful,
    /// then it must be guaranteed that `n <= buf.len()`. A return value of `0`
    /// typically means that the underlying object is no longer able to accept
    /// bytes and will likely not be able to in the future as well, or that the
    /// buffer provided is empty.
    ///
    /// If the object is not ready for writing, the method returns
    /// `Poll::Pending` and arranges for the current task (via
    /// `cx.waker()`) to receive a notification when the object becomes
    /// writable or is closed.
    fn poll_write(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<Result<usize, io::Error>>;

    /// Attempts to flush the object, ensuring that any buffered data reach
    /// their destination.
    ///
    /// On success, returns `Poll::Ready(Ok(()))`.
    ///
    /// If flushing cannot immediately complete, this method returns
    /// `Poll::Pending` and arranges for the current task (via
    /// `cx.waker()`) to receive a notification when the object can make
    /// progress towards flushing.
    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>>;

    /// Initiates or attempts to shut down this writer, returning success when
    /// the I/O connection has completely shut down.
    ///
    /// This method is intended to be used for asynchronous shutdown of I/O
    /// connections. For example this is suitable for implementing shutdown of a
    /// TLS connection or calling `TcpStream::shutdown` on a proxied connection.
    /// Protocols sometimes need to flush out final pieces of data or otherwise
    /// perform a graceful shutdown handshake, reading/writing more data as
    /// appropriate. This method is the hook for such protocols to implement the
    /// graceful shutdown logic.
    ///
    /// This `shutdown` method is required by implementers of the
    /// `AsyncWrite` trait. Wrappers typically just want to proxy this call
    /// through to the wrapped type, and base types will typically implement
    /// shutdown logic here or just return `Ok(().into())`. Note that if you're
    /// wrapping an underlying `AsyncWrite` a call to `shutdown` implies that
    /// transitively the entire stream has been shut down. After your wrapper's
    /// shutdown logic has been executed you should shut down the underlying
    /// stream.
    ///
    /// Invocation of a `shutdown` implies an invocation of `flush`. Once this
    /// method returns `Ready` it implies that a flush successfully happened
    /// before the shutdown happened. That is, callers don't need to call
    /// `flush` before calling `shutdown`. They can rely that by calling
    /// `shutdown` any pending buffered data will be written out.
    ///
    /// # Return value
    ///
    /// This function returns a `Poll<io::Result<()>>` classified as such:
    ///
    /// * `Poll::Ready(Ok(()))` - indicates that the connection was
    ///   successfully shut down and is now safe to deallocate/drop/close
    ///   resources associated with it. This method means that the current task
    ///   will no longer receive any notifications due to this method and the
    ///   I/O object itself is likely no longer usable.
    ///
    /// * `Poll::Pending` - indicates that shutdown is initiated but could
    ///   not complete just yet. This may mean that more I/O needs to happen to
    ///   continue this shutdown operation. The current task is scheduled to
    ///   receive a notification when it's otherwise ready to continue the
    ///   shutdown operation. When woken up this method should be called again.
    ///
    /// * `Poll::Ready(Err(e))` - indicates a fatal error has happened with shutdown,
    ///   indicating that the shutdown operation did not complete successfully.
    ///   This typically means that the I/O object is no longer usable.
    ///
    /// # Errors
    ///
    /// This function can return normal I/O errors through `Err`, described
    /// above. Additionally this method may also render the underlying
    /// `Write::write` method no longer usable (e.g. will return errors in the
    /// future). It's recommended that once `shutdown` is called the
    /// `write` method is no longer called.
    ///
    /// # Panics
    ///
    /// This function will panic if not called within the context of a future's
    /// task.
    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>>;

    /// Like [`poll_write`], except that it writes from a slice of buffers.
    ///
    /// Data is copied from each buffer in order, with the final buffer
    /// read from possibly being only partially consumed. This method must
    /// behave as a call to [`write`] with the buffers concatenated would.
    ///
    /// The default implementation calls [`poll_write`] with either the first nonempty
    /// buffer provided, or an empty one if none exists.
    ///
    /// On success, returns `Poll::Ready(Ok(num_bytes_written))`.
    ///
    /// If the object is not ready for writing, the method returns
    /// `Poll::Pending` and arranges for the current task (via
    /// `cx.waker()`) to receive a notification when the object becomes
    /// writable or is closed.
    ///
    /// # Note
    ///
    /// This should be implemented as a single "atomic" write action. If any
    /// data has been partially written, it is wrong to return an error or
    /// pending.
    ///
    /// [`poll_write`]: AsyncWrite::poll_write
    fn poll_write_vectored(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        bufs: &[IoSlice<'_>],
    ) -> Poll<Result<usize, io::Error>> {
        let buf = bufs
            .iter()
            .find(|b| !b.is_empty())
            .map_or(&[][..], |b| &**b);
        self.poll_write(cx, buf)
    }

    /// Determines if this writer has an efficient [`poll_write_vectored`]
    /// implementation.
    ///
    /// If a writer does not override the default [`poll_write_vectored`]
    /// implementation, code using it may want to avoid the method all together
    /// and coalesce writes into a single buffer for higher performance.
    ///
    /// The default implementation returns `false`.
    ///
    /// [`poll_write_vectored`]: AsyncWrite::poll_write_vectored
    fn is_write_vectored(&self) -> bool {
        false
    }
}

macro_rules! deref_async_write {
    () => {
        fn poll_write(
            mut self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            buf: &[u8],
        ) -> Poll<io::Result<usize>> {
            Pin::new(&mut **self).poll_write(cx, buf)
        }

        fn poll_write_vectored(
            mut self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            bufs: &[IoSlice<'_>],
        ) -> Poll<io::Result<usize>> {
            Pin::new(&mut **self).poll_write_vectored(cx, bufs)
        }

        fn is_write_vectored(&self) -> bool {
            (**self).is_write_vectored()
        }

        fn poll_flush(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
            Pin::new(&mut **self).poll_flush(cx)
        }

        fn poll_shutdown(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
            Pin::new(&mut **self).poll_shutdown(cx)
        }
    };
}

impl<T: ?Sized + AsyncWrite + Unpin> AsyncWrite for Box<T> {
    deref_async_write!();
}

impl<T: ?Sized + AsyncWrite + Unpin> AsyncWrite for &mut T {
    deref_async_write!();
}

impl<P> AsyncWrite for Pin<P>
where
    P: DerefMut + Unpin,
    P::Target: AsyncWrite,
{
    fn poll_write(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<io::Result<usize>> {
        self.get_mut().as_mut().poll_write(cx, buf)
    }

    fn poll_write_vectored(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        bufs: &[IoSlice<'_>],
    ) -> Poll<io::Result<usize>> {
        self.get_mut().as_mut().poll_write_vectored(cx, bufs)
    }

    fn is_write_vectored(&self) -> bool {
        (**self).is_write_vectored()
    }

    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        self.get_mut().as_mut().poll_flush(cx)
    }

    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        self.get_mut().as_mut().poll_shutdown(cx)
    }
}

impl AsyncWrite for Vec<u8> {
    fn poll_write(
        self: Pin<&mut Self>,
        _cx: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<io::Result<usize>> {
        self.get_mut().extend_from_slice(buf);
        Poll::Ready(Ok(buf.len()))
    }

    fn poll_write_vectored(
        mut self: Pin<&mut Self>,
        _: &mut Context<'_>,
        bufs: &[IoSlice<'_>],
    ) -> Poll<io::Result<usize>> {
        Poll::Ready(io::Write::write_vectored(&mut *self, bufs))
    }

    fn is_write_vectored(&self) -> bool {
        true
    }

    fn poll_flush(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        Poll::Ready(Ok(()))
    }

    fn poll_shutdown(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        Poll::Ready(Ok(()))
    }
}

impl AsyncWrite for io::Cursor<&mut [u8]> {
    fn poll_write(
        mut self: Pin<&mut Self>,
        _: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<io::Result<usize>> {
        Poll::Ready(io::Write::write(&mut *self, buf))
    }

    fn poll_write_vectored(
        mut self: Pin<&mut Self>,
        _: &mut Context<'_>,
        bufs: &[IoSlice<'_>],
    ) -> Poll<io::Result<usize>> {
        Poll::Ready(io::Write::write_vectored(&mut *self, bufs))
    }

    fn is_write_vectored(&self) -> bool {
        true
    }

    fn poll_flush(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
        Poll::Ready(io::Write::flush(&mut *self))
    }

    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        self.poll_flush(cx)
    }
}

impl AsyncWrite for io::Cursor<&mut Vec<u8>> {
    fn poll_write(
        mut self: Pin<&mut Self>,
        _: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<io::Result<usize>> {
        Poll::Ready(io::Write::write(&mut *self, buf))
    }

    fn poll_write_vectored(
        mut self: Pin<&mut Self>,
        _: &mut Context<'_>,
        bufs: &[IoSlice<'_>],
    ) -> Poll<io::Result<usize>> {
        Poll::Ready(io::Write::write_vectored(&mut *self, bufs))
    }

    fn is_write_vectored(&self) -> bool {
        true
    }

    fn poll_flush(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
        Poll::Ready(io::Write::flush(&mut *self))
    }

    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        self.poll_flush(cx)
    }
}

impl AsyncWrite for io::Cursor<Vec<u8>> {
    fn poll_write(
        mut self: Pin<&mut Self>,
        _: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<io::Result<usize>> {
        Poll::Ready(io::Write::write(&mut *self, buf))
    }

    fn poll_write_vectored(
        mut self: Pin<&mut Self>,
        _: &mut Context<'_>,
        bufs: &[IoSlice<'_>],
    ) -> Poll<io::Result<usize>> {
        Poll::Ready(io::Write::write_vectored(&mut *self, bufs))
    }

    fn is_write_vectored(&self) -> bool {
        true
    }

    fn poll_flush(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
        Poll::Ready(io::Write::flush(&mut *self))
    }

    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        self.poll_flush(cx)
    }
}

impl AsyncWrite for io::Cursor<Box<[u8]>> {
    fn poll_write(
        mut self: Pin<&mut Self>,
        _: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<io::Result<usize>> {
        Poll::Ready(io::Write::write(&mut *self, buf))
    }

    fn poll_write_vectored(
        mut self: Pin<&mut Self>,
        _: &mut Context<'_>,
        bufs: &[IoSlice<'_>],
    ) -> Poll<io::Result<usize>> {
        Poll::Ready(io::Write::write_vectored(&mut *self, bufs))
    }

    fn is_write_vectored(&self) -> bool {
        true
    }

    fn poll_flush(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<io::Result<()>> {
        Poll::Ready(io::Write::flush(&mut *self))
    }

    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
        self.poll_flush(cx)
    }
}