@Override
    public boolean completeExceptionally(Throwable cause) {
        if (causeUpdater.compareAndSet(this, null, requireNonNull(cause, "cause"))) {
            return super.completeExceptionally(cause);
        }
        return false;
    }
An {@link RpcResponse} implementation which is initially incomplete and can be completed later.
core/src/main/java/com/linecorp/armeria/common/CompletableRpcResponse.java