Optimize BytesMut::reserve: Reuse vec if possible (#529)

* Optimize `BytesMut::reserve`: Reuse vec if possible

If the `BytesMut` holds a unqiue reference to `KIND_ARC` while the
capacity of the `Vec` is not big enough , reuse the existing `Vec`
instead of allocating a new one.

Signed-off-by: Jiahao XU <[email protected]>

src/bytes_mut.rs

@@ -644,9 +644,15 @@ impl BytesMut {
 
                     self.ptr = vptr(ptr);
                     self.cap = v.capacity();
+                } else {
+                    // calculate offset
+                    let off = v.capacity() - self.cap;
 
-                    return;
+                    // new_cap is calculated in terms of `BytesMut`, not the underlying
-                }
+                    // `Vec`, so it does not take the offset into account.
+                    //
+                    // Thus we have to manually add it here.
+                    new_cap = new_cap.checked_add(off).expect("overflow");
 
                     // The vector capacity is not sufficient. The reserve request is
                     // asking for more than the initial buffer capacity. Allocate more
@@ -658,7 +664,17 @@ impl BytesMut {
                     // check.
                     let double = v.capacity().checked_shl(1).unwrap_or(new_cap);
 
-                new_cap = cmp::max(cmp::max(double, new_cap), original_capacity);
+                    new_cap = cmp::max(double, new_cap);
+
+                    // No space - allocate more
+                    v.reserve(new_cap - v.len());
+
+                    // Update the info
+                    self.ptr = vptr(v.as_mut_ptr().add(off));
+                    self.cap = v.capacity() - off;
+                }
+
+                return;
             } else {
                 new_cap = cmp::max(new_cap, original_capacity);
             }

tests/test_bytes.rs

@@ -443,7 +443,7 @@ fn reserve_growth() {
     let _ = bytes.split();
 
     bytes.reserve(65);
-    assert_eq!(bytes.capacity(), 128);
+    assert_eq!(bytes.capacity(), 117);
 }
 
 #[test]