Fix issues with AsyncBufRead::read_line and AsyncBufReadExt::lines`

Fixes the following issues in `AsyncBufRead::read_line`:
* When the future is dropped the previous string contents are not restored so the string is empty.
* If invalid UTF-8 is encountered the previous string contents are not restored.
* If an IO error occurs after `read_until_internal` already read a couple of bytes a debug assertion fails.
* Performance: The whole string to which read contents are appended is check for UTF-8 validity instead of just the added bytes.

* Fixes the following issues in `AsyncBufRead::read_line`
* If an IO error occurs after `read_until_internal` already read a couple of bytes a debug assertion fails. (#2862)

Fixes #2862.

futures-util/src/io/lines.rs

@@ -35,6 +35,7 @@ impl<R: AsyncBufRead> Stream for Lines<R> {
     fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
         let this = self.project();
         let n = ready!(read_line_internal(this.reader, cx, this.buf, this.bytes, this.read))?;
+        *this.read = 0;
         if n == 0 && this.buf.is_empty() {
             return Poll::Ready(None);
         }

futures-util/src/io/read_line.rs

@@ -18,13 +18,14 @@ pub struct ReadLine<'a, R: ?Sized> {
     buf: &'a mut String,
     bytes: Vec<u8>,
     read: usize,
+    finished: bool,
 }
 
 impl<R: ?Sized + Unpin> Unpin for ReadLine<'_, R> {}
 
 impl<'a, R: AsyncBufRead + ?Sized + Unpin> ReadLine<'a, R> {
     pub(super) fn new(reader: &'a mut R, buf: &'a mut String) -> Self {
-        Self { reader, bytes: mem::take(buf).into_bytes(), buf, read: 0 }
+        Self { reader, bytes: mem::take(buf).into_bytes(), buf, read: 0, finished: false }
     }
 }
 
@@ -35,26 +36,42 @@ pub(super) fn read_line_internal<R: AsyncBufRead + ?Sized>(
     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() {
-        bytes.clear();
-        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)
+    let mut ret = ready!(read_until_internal(reader, cx, b'\n', bytes, read));
+    if str::from_utf8(&bytes[bytes.len() - *read..bytes.len()]).is_err() {
+        bytes.truncate(bytes.len() - *read);
+        if ret.is_ok() {
+            ret = Err(io::Error::new(
+                io::ErrorKind::InvalidData,
+                "stream did not contain valid UTF-8",
+            ));
+        }
     }
+    *read = 0;
+    // Safety: `bytes` is valid UTF-8 because it was taken from a String
+    // and the newly read bytes are either valid UTF-8 or have been removed.
+    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)
+        let Self { reader, buf, bytes, read, finished: _ } = &mut *self;
+        let ret = ready!(read_line_internal(Pin::new(reader), cx, buf, bytes, read));
+        self.finished = true;
+        Poll::Ready(ret)
+    }
+}
+
+impl<R: ?Sized> Drop for ReadLine<'_, R> {
+    fn drop(&mut self) {
+        // restore old string contents
+        if !self.finished {
+            self.bytes.truncate(self.bytes.len() - self.read);
+            // Safety: `bytes` is valid UTF-8 because it was taken from a String
+            // and the newly read bytes have been removed.
+            mem::swap(unsafe { self.buf.as_mut_vec() }, &mut self.bytes);
+        }
     }
 }

futures-util/src/io/read_until.rs

@@ -3,7 +3,6 @@ use futures_core::ready;
 use futures_core::task::{Context, Poll};
 use futures_io::AsyncBufRead;
 use std::io;
-use std::mem;
 use std::pin::Pin;
 use std::vec::Vec;
 
@@ -46,7 +45,7 @@ pub(super) fn read_until_internal<R: AsyncBufRead + ?Sized>(
         reader.as_mut().consume(used);
         *read += used;
         if done || used == 0 {
-            return Poll::Ready(Ok(mem::replace(read, 0)));
+            return Poll::Ready(Ok(*read));
         }
     }
 }