@Override
        public void onCompletion(Long result, Exception exception) {
          buf.release();
          if (callback != null) {
            callback.onCompletion(result, exception);
          }
        }
Constructs a {@link ReadableStreamChannel} whose read operations return data from the provided {@code buf}.
@param buf the {@link ByteBuf} that is used to retrieve data from on invocation of read operations.
ambry-test-utils/src/main/java/com/github/ambry/commons/ByteBufReadableStreamChannel.java