tokio: add AsyncReadExt::read_to_string

tokio/src/io/async_read_ext.rs

@@ -2,6 +2,7 @@ use crate::io::copy::{copy, Copy};
 use crate::io::read::{read, Read};
 use crate::io::read_exact::{read_exact, ReadExact};
 use crate::io::read_to_end::{read_to_end, ReadToEnd};
+use crate::io::read_to_string::{read_to_string, ReadToString};
 
 use tokio_io::{AsyncRead, AsyncWrite};
 
@@ -63,12 +64,36 @@ pub trait AsyncReadExt: AsyncRead {
     }
 
     /// Read all bytes until EOF in this source, placing them into `dst`.
+    ///
+    /// On success the total number of bytes read is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// unimplemented!();
+    /// ```
     fn read_to_end<'a>(&'a mut self, dst: &'a mut Vec<u8>) -> ReadToEnd<'a, Self>
     where
         Self: Unpin,
     {
         read_to_end(self, dst)
     }
+
+    /// Read all bytes until EOF in this source, placing them into `dst`.
+    ///
+    /// On success the total number of bytes read is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// unimplemented!();
+    /// ```
+    fn read_to_string<'a>(&'a mut self, dst: &'a mut String) -> ReadToString<'a, Self>
+    where
+        Self: Unpin,
+    {
+        read_to_string(self, dst)
+    }
 }
 
 impl<R: AsyncRead + ?Sized> AsyncReadExt for R {}

tokio/src/io/mod.rs

@@ -45,6 +45,7 @@ mod read;
 mod read_exact;
 mod read_line;
 mod read_to_end;
+mod read_to_string;
 mod read_until;
 mod write;
 mod write_all;

tokio/src/io/read_to_string.rs

@@ -0,0 +1,72 @@
+use super::read_to_end::read_to_end_internal;
+use futures_core::ready;
+use std::future::Future;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+use std::{io, mem, str};
+use tokio_io::AsyncRead;
+
+/// Future for the [`read_to_string`](super::AsyncReadExt::read_to_string) method.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct ReadToString<'a, R: ?Sized + Unpin> {
+    reader: &'a mut R,
+    buf: &'a mut String,
+    bytes: Vec<u8>,
+    start_len: usize,
+}
+
+impl<R: ?Sized + Unpin> Unpin for ReadToString<'_, R> {}
+
+pub(crate) fn read_to_string<'a, R>(reader: &'a mut R, buf: &'a mut String) -> ReadToString<'a, R>
+where
+    R: AsyncRead + ?Sized + Unpin,
+{
+    let start_len = buf.len();
+    ReadToString {
+        reader,
+        bytes: unsafe { mem::replace(buf.as_mut_vec(), Vec::new()) },
+        buf,
+        start_len,
+    }
+}
+
+fn read_to_string_internal<R: AsyncRead + ?Sized>(
+    reader: Pin<&mut R>,
+    cx: &mut Context<'_>,
+    buf: &mut String,
+    bytes: &mut Vec<u8>,
+    start_len: usize,
+) -> Poll<io::Result<usize>> {
+    let ret = ready!(read_to_end_internal(reader, cx, bytes, start_len));
+    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());
+        // 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<A> Future for ReadToString<'_, A>
+where
+    A: AsyncRead + ?Sized + Unpin,
+{
+    type Output = io::Result<usize>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let Self {
+            reader,
+            buf,
+            bytes,
+            start_len,
+        } = &mut *self;
+        read_to_string_internal(Pin::new(reader), cx, buf, bytes, *start_len)
+    }
+}

tokio/tests/io_read_to_string.rs

@@ -0,0 +1,40 @@
+#![deny(warnings, rust_2018_idioms)]
+#![feature(async_await)]
+
+use tokio::io::{AsyncRead, AsyncReadExt};
+use tokio_test::assert_ok;
+
+use std::pin::Pin;
+use std::task::{Context, Poll};
+use std::{cmp, io};
+
+#[tokio::test]
+async fn read_to_string() {
+    struct Rd {
+        val: &'static [u8],
+    }
+
+    impl AsyncRead for Rd {
+        fn poll_read(
+            mut self: Pin<&mut Self>,
+            _cx: &mut Context<'_>,
+            buf: &mut [u8],
+        ) -> Poll<io::Result<usize>> {
+            let me = &mut *self;
+            let len = cmp::min(buf.len(), me.val.len());
+
+            buf[..len].copy_from_slice(&me.val[..len]);
+            me.val = &me.val[len..];
+            Poll::Ready(Ok(len))
+        }
+    }
+
+    let mut buf = String::new();
+    let mut rd = Rd {
+        val: b"hello world",
+    };
+
+    let n = assert_ok!(rd.read_to_string(&mut buf).await);
+    assert_eq!(n, 11);
+    assert_eq!(buf[..], "hello world"[..]);
+}