I first thought I encountered a race condition and a bug related to the caching mechanism. But it's likely due to the same issue.

Observations:

1. When I start my app and run "raceconditionTest()" sometimes it returns 10.2.2.2 for "test", which is the correct local IP, but most of the times it returns null. Which means the result of 8.8.8.8 is used, and not from 10.1.1.1 (8.8.8.8 seems to be faster than my local DNS).
2. When "test" failed to resolve, "test2" will always be null. This is often the case for raceconditionTest() and always the case for cacheTest().

My input:

1. I don't think a cached HOST_NOT_FOUND result should take precedence. I created a new Resolver with another server to query, using the cached result from another server in this situation seems illogical.
2. I haven't look closely at the code, but I assume an ExtendedResolver queries all servers and uses the first result it gets. In such a situation I don't think a HOST_NOT_FOUND result should be used when the other server(s) might return the correct value.

Kotlin code (running on Android 8. 10.1.1.1 is my local OPNsense instance, running Unbound DNS):

    private fun raceconditionTest(): {
            val test = resolveDomain("someserver.mydomain.it", listOf("10.1.1.1", "8.8.8.8"))
            val test2 = resolveDomain("someserver.mydomain.it", listOf("10.1.1.1", ))
    }

    private fun cacheTest(): {
            val test = resolveDomain("someserver.mydomain.it", listOf("8.8.8.8"))
            val test2 = resolveDomain("someserver.mydomain.it", listOf("10.1.1.1", ))
    }

    private fun resolveDomain(domain: String, serverNames: List<String>): String? {
        try {
            val resolver: Resolver = if (serverNames.size > 1) {
                ExtendedResolver(serverNames.toTypedArray())
            } else {
                SimpleResolver(serverNames[0])
            }
            
            val lookup = Lookup(domain)
            lookup.setResolver(resolver)
            
            val recs: Array<Record> = lookup.run() ?: return null
            for (rec in recs) {
                return rec.rdataToString()
            }
        } catch (e: Exception) { }
        return null
    }