RetryCallEnqueueOnSubscribe.kt

class RetryCallEnqueueOnSubscribe<T>(
    private val originalCall: Call<T>
) : OnSubscribe<Response<T>> {

    private var retries = 0

    override fun call(subscriber: Subscriber<in Response<T>>) {
        val call = originalCall.clone()

        val headers = call.request()
                .headers()
                .newBuilder()
                .set(RETRY_HEADER, retries++)
                .build()

        // Dirty hack
        Request::class.java.getDeclaredField(HEADERS_FIELD).apply {
            isAccessible = true
            set(call.request(), headers)
            isAccessible = false
        }

        call.enqueue(...)
    }

}