perf(body): specialize BufList::copy_to_bytes (#2413)

Some implementations of the Buf trait have an optimized version (for
example Bytes) of copy_to_bytes, opportunistically use that one.

src/common/buf.rs

@@ -1,7 +1,7 @@
 use std::collections::VecDeque;
 use std::io::IoSlice;
 
-use bytes::Buf;
+use bytes::{Buf, BufMut, Bytes, BytesMut};
 
 pub(crate) struct BufList<T> {
     bufs: VecDeque<T>,
@@ -70,4 +70,82 @@ impl<T: Buf> Buf for BufList<T> {
         }
         vecs
     }
+
+    #[inline]
+    fn copy_to_bytes(&mut self, len: usize) -> Bytes {
+        // Our inner buffer may have an optimized version of copy_to_bytes, and if the whole
+        // request can be fulfilled by the front buffer, we can take advantage.
+        match self.bufs.front_mut() {
+            Some(front) if front.remaining() == len => {
+                let b = front.copy_to_bytes(len);
+                self.bufs.pop_front();
+                b
+            }
+            Some(front) if front.remaining() > len => front.copy_to_bytes(len),
+            _ => {
+                assert!(len <= self.remaining(), "`len` greater than remaining");
+                let mut bm = BytesMut::with_capacity(len);
+                bm.put(self.take(len));
+                bm.freeze()
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::ptr;
+
+    use super::*;
+
+    fn hello_world_buf() -> BufList<Bytes> {
+        BufList {
+            bufs: vec![Bytes::from("Hello"), Bytes::from(" "), Bytes::from("World")].into(),
+        }
+    }
+
+    #[test]
+    fn to_bytes_shorter() {
+        let mut bufs = hello_world_buf();
+        let old_ptr = bufs.chunk().as_ptr();
+        let start = bufs.copy_to_bytes(4);
+        assert_eq!(start, "Hell");
+        assert!(ptr::eq(old_ptr, start.as_ptr()));
+        assert_eq!(bufs.chunk(), b"o");
+        assert!(ptr::eq(old_ptr.wrapping_add(4), bufs.chunk().as_ptr()));
+        assert_eq!(bufs.remaining(), 7);
+    }
+
+    #[test]
+    fn to_bytes_eq() {
+        let mut bufs = hello_world_buf();
+        let old_ptr = bufs.chunk().as_ptr();
+        let start = bufs.copy_to_bytes(5);
+        assert_eq!(start, "Hello");
+        assert!(ptr::eq(old_ptr, start.as_ptr()));
+        assert_eq!(bufs.chunk(), b" ");
+        assert_eq!(bufs.remaining(), 6);
+    }
+
+    #[test]
+    fn to_bytes_longer() {
+        let mut bufs = hello_world_buf();
+        let start = bufs.copy_to_bytes(7);
+        assert_eq!(start, "Hello W");
+        assert_eq!(bufs.remaining(), 4);
+    }
+
+    #[test]
+    fn one_long_buf_to_bytes() {
+        let mut buf = BufList::new();
+        buf.push(b"Hello World" as &[_]);
+        assert_eq!(buf.copy_to_bytes(5), "Hello");
+        assert_eq!(buf.chunk(), b" World");
+    }
+
+    #[test]
+    #[should_panic(expected = "`len` greater than remaining")]
+    fn buf_to_bytes_too_many() {
+        hello_world_buf().copy_to_bytes(42);
+    }
 }