tokio: add AsyncBufReadExt::read_line

tokio/src/io/async_buf_read_ext.rs

@@ -1,3 +1,4 @@
+use crate::io::read_line::{read_line, ReadLine};
 use crate::io::read_until::{read_until, ReadUntil};
 
 use tokio_io::AsyncBufRead;
@@ -29,6 +30,42 @@ pub trait AsyncBufReadExt: AsyncBufRead {
     {
         read_until(self, byte, buf)
     }
+
+    /// Creates a future which will read all the bytes associated with this I/O
+    /// object into `buf` until a newline (the 0xA byte) or EOF is reached,
+    /// This method is the async equivalent to [`BufRead::read_line`](std::io::BufRead::read_line).
+    ///
+    /// This function will read bytes from the underlying stream until the
+    /// newline delimiter (the 0xA byte) or EOF is found. Once found, all bytes
+    /// up to, and including, the delimiter (if found) will be appended to
+    /// `buf`.
+    ///
+    /// The returned future will resolve to the number of bytes read once the read
+    /// operation is completed.
+    ///
+    /// In the case of an error the buffer and the object will be discarded, with
+    /// the error yielded.
+    ///
+    /// # Errors
+    ///
+    /// This function has the same error semantics as [`read_until`] and will
+    /// also return an error if the read bytes are not valid UTF-8. If an I/O
+    /// error is encountered then `buf` may contain some bytes already read in
+    /// the event that all data read so far was valid UTF-8.
+    ///
+    /// [`read_until`]: AsyncBufReadExt::read_until
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// unimplemented!();
+    /// ```
+    fn read_line<'a>(&'a mut self, buf: &'a mut String) -> ReadLine<'a, Self>
+    where
+        Self: Unpin,
+    {
+        read_line(self, buf)
+    }
 }
 
 impl<R: AsyncBufRead + ?Sized> AsyncBufReadExt for R {}

tokio/src/io/mod.rs

@@ -42,6 +42,7 @@ mod async_write_ext;
 mod copy;
 mod read;
 mod read_exact;
+mod read_line;
 mod read_to_end;
 mod read_until;
 mod write;

tokio/src/io/read_line.rs

@@ -0,0 +1,70 @@
+use super::read_until::read_until_internal;
+use std::future::Future;
+use std::io;
+use std::mem;
+use std::pin::Pin;
+use std::str;
+use std::task::{Context, Poll};
+use tokio_io::AsyncBufRead;
+
+/// Future for the [`read_line`](crate::io::AsyncBufReadExt::read_line) method.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct ReadLine<'a, R: ?Sized + Unpin> {
+    reader: &'a mut R,
+    buf: &'a mut String,
+    bytes: Vec<u8>,
+    read: usize,
+}
+
+impl<R: ?Sized + Unpin> Unpin for ReadLine<'_, R> {}
+
+pub(crate) fn read_line<'a, R>(reader: &'a mut R, buf: &'a mut String) -> ReadLine<'a, R>
+where
+    R: AsyncBufRead + ?Sized + Unpin,
+{
+    ReadLine {
+        reader,
+        bytes: unsafe { mem::replace(buf.as_mut_vec(), Vec::new()) },
+        buf,
+        read: 0,
+    }
+}
+
+pub(super) fn read_line_internal<R: AsyncBufRead + ?Sized>(
+    reader: Pin<&mut R>,
+    cx: &mut Context<'_>,
+    buf: &mut String,
+    bytes: &mut Vec<u8>,
+    read: &mut usize,
+) -> Poll<io::Result<usize>> {
+    let ret = ready!(read_until_internal(reader, cx, b'\n', bytes, read));
+    if str::from_utf8(&bytes).is_err() {
+        Poll::Ready(ret.and_then(|_| {
+            Err(io::Error::new(
+                io::ErrorKind::InvalidData,
+                "stream did not contain valid UTF-8",
+            ))
+        }))
+    } else {
+        debug_assert!(buf.is_empty());
+        debug_assert_eq!(*read, 0);
+        // Safety: `bytes` is a valid UTF-8 because `str::from_utf8` returned `Ok`.
+        mem::swap(unsafe { buf.as_mut_vec() }, bytes);
+        Poll::Ready(ret)
+    }
+}
+
+impl<R: AsyncBufRead + ?Sized + Unpin> Future for ReadLine<'_, R> {
+    type Output = io::Result<usize>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let Self {
+            reader,
+            buf,
+            bytes,
+            read,
+        } = &mut *self;
+        read_line_internal(Pin::new(reader), cx, buf, bytes, read)
+    }
+}

tokio/tests/io_read_line.rs

@@ -0,0 +1,60 @@
+#![deny(warnings, rust_2018_idioms)]
+#![feature(async_await)]
+
+use tokio::io::{AsyncBufRead, AsyncBufReadExt, AsyncRead};
+use tokio_test::assert_ok;
+
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+#[tokio::test]
+async fn read_line() {
+    struct Rd {
+        val: &'static [u8],
+    }
+
+    impl AsyncRead for Rd {
+        fn poll_read(
+            self: Pin<&mut Self>,
+            _: &mut Context<'_>,
+            _: &mut [u8],
+        ) -> Poll<io::Result<usize>> {
+            unimplemented!()
+        }
+    }
+
+    impl AsyncBufRead for Rd {
+        fn poll_fill_buf<'a>(
+            self: Pin<&'a mut Self>,
+            _: &mut Context<'_>,
+        ) -> Poll<io::Result<&'a [u8]>> {
+            Poll::Ready(Ok(self.val))
+        }
+
+        fn consume(mut self: Pin<&mut Self>, amt: usize) {
+            self.val = &self.val[amt..];
+        }
+    }
+
+    let mut buf = String::new();
+    let mut rd = Rd {
+        val: b"hello\nworld\n\n",
+    };
+
+    let n = assert_ok!(rd.read_line(&mut buf).await);
+    assert_eq!(n, 6);
+    assert_eq!(buf, "hello\n");
+    buf.clear();
+    let n = assert_ok!(rd.read_line(&mut buf).await);
+    assert_eq!(n, 6);
+    assert_eq!(buf, "world\n");
+    buf.clear();
+    let n = assert_ok!(rd.read_line(&mut buf).await);
+    assert_eq!(n, 1);
+    assert_eq!(buf, "\n");
+    buf.clear();
+    let n = assert_ok!(rd.read_line(&mut buf).await);
+    assert_eq!(n, 0);
+    assert_eq!(buf, "");
+}