1 files changed, 130 insertions, 0 deletions
diff --git a/tokio/src/io/util/take.rs b/tokio/src/io/util/take.rs
new file mode 100644
index 00000000..e96009e2
--- /dev/null
+++ b/tokio/src/io/util/take.rs
@@ -0,0 +1,130 @@
+use crate::io::{AsyncBufRead, AsyncRead};
+
+use pin_project::{pin_project, project};
+use std::pin::Pin;
+use std::task::{Context, Poll};
+use std::{cmp, io};
+
+/// Stream for the [`take`](super::AsyncReadExt::take) method.
+#[pin_project]
+#[derive(Debug)]
+#[must_use = "streams do nothing unless you `.await` or poll them"]
+pub struct Take<R> {
+    #[pin]
+    inner: R,
+    // Add '_' to avoid conflicts with `limit` method.
+    limit_: u64,
+}
+
+pub(super) fn take<R: AsyncRead>(inner: R, limit: u64) -> Take<R> {
+    Take {
+        inner,
+        limit_: limit,
+    }
+}
+
+impl<R: AsyncRead> Take<R> {
+    /// Returns the remaining number of bytes that can be
+    /// read before this instance will return EOF.
+    ///
+    /// # Note
+    ///
+    /// This instance may reach `EOF` after reading fewer bytes than indicated by
+    /// this method if the underlying [`AsyncRead`] instance reaches EOF.
+    pub fn limit(&self) -> u64 {
+        self.limit_
+    }
+
+    /// Sets the number of bytes that can be read before this instance will
+    /// return EOF. This is the same as constructing a new `Take` instance, so
+    /// the amount of bytes read and the previous limit value don't matter when
+    /// calling this method.
+    pub fn set_limit(&mut self, limit: u64) {
+        self.limit_ = limit
+    }
+
+    /// Gets a reference to the underlying reader.
+    pub fn get_ref(&self) -> &R {
+        &self.inner
+    }
+
+    /// Gets a mutable reference to the underlying reader.
+    ///
+    /// Care should be taken to avoid modifying the internal I/O state of the
+    /// underlying reader as doing so may corrupt the internal limit of this
+    /// `Take`.
+    pub fn get_mut(&mut self) -> &mut R {
+        &mut self.inner
+    }
+
+    /// Gets a pinned mutable reference to the underlying reader.
+    ///
+    /// Care should be taken to avoid modifying the internal I/O state of the
+    /// underlying reader as doing so may corrupt the internal limit of this
+    /// `Take`.
+    pub fn get_pin_mut(self: Pin<&mut Self>) -> Pin<&mut R> {
+        self.project().inner
+    }
+
+    /// Consumes the `Take`, returning the wrapped reader.
+    pub fn into_inner(self) -> R {
+        self.inner
+    }
+}
+
+impl<R: AsyncRead> AsyncRead for Take<R> {
+    unsafe fn prepare_uninitialized_buffer(&self, buf: &mut [u8]) -> bool {
+        self.inner.prepare_uninitialized_buffer(buf)
+    }
+
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<Result<usize, io::Error>> {
+        if self.limit_ == 0 {
+            return Poll::Ready(Ok(0));
+        }
+
+        let me = self.project();
+        let max = std::cmp::min(buf.len() as u64, *me.limit_) as usize;
+        let n = ready!(me.inner.poll_read(cx, &mut buf[..max]))?;
+        *me.limit_ -= n as u64;
+        Poll::Ready(Ok(n))
+    }
+}
+
+impl<R: AsyncBufRead> AsyncBufRead for Take<R> {
+    #[project]
+    fn poll_fill_buf(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<&[u8]>> {
+        #[project]
+        let Take { inner, limit_ } = self.project();
+
+        // Don't call into inner reader at all at EOF because it may still block
+        if *limit_ == 0 {
+            return Poll::Ready(Ok(&[]));
+        }
+
+        let buf = ready!(inner.poll_fill_buf(cx)?);
+        let cap = cmp::min(buf.len() as u64, *limit_) as usize;
+        Poll::Ready(Ok(&buf[..cap]))
+    }
+
+    fn consume(self: Pin<&mut Self>, amt: usize) {
+        let me = self.project();
+        // Don't let callers reset the limit by passing an overlarge value
+        let amt = cmp::min(amt as u64, *me.limit_) as usize;
+        *me.limit_ -= amt as u64;
+        me.inner.consume(amt);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn assert_unpin() {
+        crate::is_unpin::<Take<()>>();
+    }
+}