lock.withLock {
            if (q.isNotEmpty()) {
                return takeNextGroup()
            }
            if (notEmpty.await(timeoutMillis, TimeUnit.MILLISECONDS)) {
                return takeNextGroup()
            }
        }
        return emptyList()
    }

    private
    fun takeNextGroup(): List<T> {
        require(q.isNotEmpty())
        val group = mutableListOf<T>()