BytesMut: Reuse buffer when data fully consumed via Buf

Closes tokio-rs#412

src/bytes_mut.rs

@@ -559,9 +559,8 @@ impl BytesMut {
             unsafe {
                 let (off, prev) = self.get_vec_pos();
 
-                // Only reuse space if we stand to gain at least capacity/2
-                // bytes of space back
-                if off >= additional && off >= (self.cap / 2) {
+                // Only reuse space if we can satisfy the requested additional space.
+                if self.capacity() - self.len() + off >= additional {
                     // There's space - reuse it
                     //
                     // Just move the pointer back to the start after copying

tests/test_bytes.rs

@@ -929,6 +929,22 @@ fn bytes_buf_mut_advance() {
     }
 }
 
+#[test]
+fn bytes_buf_mut_reuse_when_fully_consumed() {
+    use bytes::{Buf, BytesMut};
+    let mut buf = BytesMut::new();
+    buf.reserve(8192);
+    buf.extend_from_slice(&[0u8; 100][..]);
+
+    let p = &buf[0] as *const u8;
+    buf.advance(100);
+
+    buf.reserve(8192);
+    buf.extend_from_slice(b" ");
+
+    assert_eq!(&buf[0] as *const u8, p);
+}
+
 #[test]
 #[should_panic]
 fn bytes_reserve_overflow() {