Add specialized Buf::chunks_vectored for Take (#617)

Co-authored-by: Alice Ryhl <aliceryhl@google.com>
Co-authored-by: Michal 'vorner' Vaner <vorner@vorner.cz>

Author: 3 people

src/buf/take.rs

@@ -2,6 +2,9 @@ use crate::{Buf, Bytes};
 
 use core::cmp;
 
+#[cfg(feature = "std")]
+use std::io::IoSlice;
+
 /// A `Buf` adapter which limits the bytes read from an underlying buffer.
 ///
 /// This struct is generally created by calling `take()` on `Buf`. See
@@ -152,4 +155,50 @@ impl<T: Buf> Buf for Take<T> {
         self.limit -= len;
         r
     }
+
+    #[cfg(feature = "std")]
+    fn chunks_vectored<'a>(&'a self, dst: &mut [IoSlice<'a>]) -> usize {
+        if self.limit == 0 {
+            return 0;
+        }
+
+        const LEN: usize = 16;
+        let mut slices: [IoSlice<'a>; LEN] = [
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+        ];
+
+        let cnt = self
+            .inner
+            .chunks_vectored(&mut slices[..dst.len().min(LEN)]);
+        let mut limit = self.limit;
+        for (i, (dst, slice)) in dst[..cnt].iter_mut().zip(slices.iter()).enumerate() {
+            if let Some(buf) = slice.get(..limit) {
+                // SAFETY: We could do this safely with `IoSlice::advance` if we had a larger MSRV.
+                let buf = unsafe { std::mem::transmute::<&[u8], &'a [u8]>(buf) };
+                *dst = IoSlice::new(buf);
+                return i + 1;
+            } else {
+                // SAFETY: We could do this safely with `IoSlice::advance` if we had a larger MSRV.
+                let buf = unsafe { std::mem::transmute::<&[u8], &'a [u8]>(slice) };
+                *dst = IoSlice::new(buf);
+                limit -= slice.len();
+            }
+        }
+        cnt
+    }
 }

tests/test_buf.rs

@@ -404,7 +404,7 @@ mod chain_limited_slices {
         Buf::take(Buf::chain(Buf::chain(a, b), Buf::chain(c, d)), buf.len())
     }
 
-    buf_tests!(make_input, /* `Limit` does not forward `chucks_vectored */ false);
+    buf_tests!(make_input, true);
 }
 
 #[allow(unused_allocation)] // This is intentional.

tests/test_take.rs

@@ -30,3 +30,55 @@ fn take_copy_to_bytes_panics() {
     let abcd = Bytes::copy_from_slice(b"abcd");
     abcd.take(2).copy_to_bytes(3);
 }
+
+#[cfg(feature = "std")]
+#[test]
+fn take_chunks_vectored() {
+    fn chain() -> impl Buf {
+        Bytes::from([1, 2, 3].to_vec()).chain(Bytes::from([4, 5, 6].to_vec()))
+    }
+
+    {
+        let mut dst = [std::io::IoSlice::new(&[]); 2];
+        let take = chain().take(0);
+        assert_eq!(take.chunks_vectored(&mut dst), 0);
+    }
+
+    {
+        let mut dst = [std::io::IoSlice::new(&[]); 2];
+        let take = chain().take(1);
+        assert_eq!(take.chunks_vectored(&mut dst), 1);
+        assert_eq!(&*dst[0], &[1]);
+    }
+
+    {
+        let mut dst = [std::io::IoSlice::new(&[]); 2];
+        let take = chain().take(3);
+        assert_eq!(take.chunks_vectored(&mut dst), 1);
+        assert_eq!(&*dst[0], &[1, 2, 3]);
+    }
+
+    {
+        let mut dst = [std::io::IoSlice::new(&[]); 2];
+        let take = chain().take(4);
+        assert_eq!(take.chunks_vectored(&mut dst), 2);
+        assert_eq!(&*dst[0], &[1, 2, 3]);
+        assert_eq!(&*dst[1], &[4]);
+    }
+
+    {
+        let mut dst = [std::io::IoSlice::new(&[]); 2];
+        let take = chain().take(6);
+        assert_eq!(take.chunks_vectored(&mut dst), 2);
+        assert_eq!(&*dst[0], &[1, 2, 3]);
+        assert_eq!(&*dst[1], &[4, 5, 6]);
+    }
+
+    {
+        let mut dst = [std::io::IoSlice::new(&[]); 2];
+        let take = chain().take(7);
+        assert_eq!(take.chunks_vectored(&mut dst), 2);
+        assert_eq!(&*dst[0], &[1, 2, 3]);
+        assert_eq!(&*dst[1], &[4, 5, 6]);
+    }
+}