class BufferUntilReady<T>(source: Observable<T>) {
    @Volatile
    private var ready = false
    private val cache = LinkedList<T>()
    private val subscription = source.takeWhile { !ready }.subscribe { cache += it }
    private val merged = Observable.merge(Observable.from(cache).doOnCompleted { cache.clear() }, source)
    val buffered = merged.doOnSubscribe { ready = true }
}
fun <T> Observable<T>.bufferUntilSubscribed(): Observable<T> = BufferUntilReady(this).buffered