fix(android): use system DNS for wide-area discovery

apps/android/app/src/main/java/com/steipete/clawdis/node/bridge/BridgeDiscovery.kt

@@ -2,13 +2,16 @@ package com.steipete.clawdis.node.bridge
 
 import android.content.Context
 import android.net.ConnectivityManager
+import android.net.DnsResolver
 import android.net.NetworkCapabilities
 import android.net.nsd.NsdManager
 import android.net.nsd.NsdServiceInfo
 import android.os.Build
-import java.net.InetSocketAddress
-import java.time.Duration
+import android.os.CancellationSignal
+import java.io.IOException
 import java.util.concurrent.ConcurrentHashMap
+import java.util.concurrent.Executor
+import java.util.concurrent.Executors
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.Job
@@ -17,15 +20,20 @@ import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.asStateFlow
 import kotlinx.coroutines.launch
-import org.xbill.DNS.ExtendedResolver
-import org.xbill.DNS.Lookup
-import org.xbill.DNS.SimpleResolver
+import kotlinx.coroutines.suspendCancellableCoroutine
 import org.xbill.DNS.AAAARecord
 import org.xbill.DNS.ARecord
+import org.xbill.DNS.DClass
+import org.xbill.DNS.Message
+import org.xbill.DNS.Name
 import org.xbill.DNS.PTRRecord
 import org.xbill.DNS.SRVRecord
+import org.xbill.DNS.Section
+import org.xbill.DNS.TextParseException
 import org.xbill.DNS.TXTRecord
 import org.xbill.DNS.Type
+import kotlin.coroutines.resume
+import kotlin.coroutines.resumeWithException
 
 class BridgeDiscovery(
   context: Context,
@@ -33,6 +41,7 @@ class BridgeDiscovery(
 ) {
   private val nsd = context.getSystemService(NsdManager::class.java)
   private val connectivity = context.getSystemService(ConnectivityManager::class.java)
+  private val dns = DnsResolver.getInstance()
   private val serviceType = "_clawdis-bridge._tcp."
   private val wideAreaDomain = "clawdis.internal."
 
@@ -42,6 +51,7 @@ class BridgeDiscovery(
   val bridges: StateFlow<List<BridgeEndpoint>> = _bridges.asStateFlow()
 
   private var unicastJob: Job? = null
+  private val dnsExecutor: Executor = Executors.newCachedThreadPool()
 
   private val discoveryListener =
     object : NsdManager.DiscoveryListener {
@@ -144,22 +154,21 @@ class BridgeDiscovery(
   }
 
   private suspend fun refreshUnicast(domain: String) {
-    val resolver = createUnicastResolver()
     val ptrName = "${serviceType}${domain}"
-    val ptrRecords = lookup(ptrName, Type.PTR, resolver).mapNotNull { it as? PTRRecord }
+    val ptrRecords = lookupUnicast(ptrName, Type.PTR).mapNotNull { it as? PTRRecord }
 
     val next = LinkedHashMap<String, BridgeEndpoint>()
     for (ptr in ptrRecords) {
       val instanceFqdn = ptr.target.toString()
       val srv =
-        lookup(instanceFqdn, Type.SRV, resolver).firstOrNull { it is SRVRecord } as? SRVRecord ?: continue
+        lookupUnicast(instanceFqdn, Type.SRV).firstOrNull { it is SRVRecord } as? SRVRecord ?: continue
       val port = srv.port
       if (port <= 0) continue
 
       val targetFqdn = srv.target.toString()
-      val host = resolveHost(targetFqdn, resolver) ?: continue
+      val host = resolveHostUnicast(targetFqdn) ?: continue
 
-      val txt = lookup(instanceFqdn, Type.TXT, resolver).mapNotNull { it as? TXTRecord }
+      val txt = lookupUnicast(instanceFqdn, Type.TXT).mapNotNull { it as? TXTRecord }
       val instanceName = BonjourEscapes.decode(decodeInstanceName(instanceFqdn, domain))
       val displayName = BonjourEscapes.decode(txtValue(txt, "displayName") ?: instanceName)
       val id = stableId(instanceName, domain)
@@ -186,64 +195,71 @@ class BridgeDiscovery(
     return raw.removeSuffix(".")
   }
 
-  private fun lookup(name: String, type: Int, resolver: org.xbill.DNS.Resolver?): List<org.xbill.DNS.Record> {
-    return try {
-      val lookup = Lookup(name, type)
-      if (resolver != null) {
-        lookup.setResolver(resolver)
-        lookup.setCache(null)
+  private suspend fun lookupUnicast(name: String, type: Int): List<org.xbill.DNS.Record> {
+    val query =
+      try {
+        Message.newQuery(
+          org.xbill.DNS.Record.newRecord(
+            Name.fromString(name),
+            type,
+            DClass.IN,
+          ),
+        )
+      } catch (_: TextParseException) {
+        return emptyList()
       }
-      val out = lookup.run() ?: return emptyList()
-      out.toList()
-    } catch (_: Throwable) {
+
+    val network = preferredDnsNetwork()
+    val bytes =
+      try {
+        rawQuery(network, query.toWire())
+      } catch (_: Throwable) {
+        return emptyList()
+      }
+
+    return try {
+      val msg = Message(bytes)
+      msg.getSectionArray(Section.ANSWER)?.toList() ?: emptyList()
+    } catch (_: IOException) {
       emptyList()
     }
   }
 
-  private fun createUnicastResolver(): org.xbill.DNS.Resolver? {
+  private fun preferredDnsNetwork(): android.net.Network? {
     val cm = connectivity ?: return null
 
-    // Prefer VPN DNS (Tailscale) when present; fall back to active network DNS.
-    val candidateNetworks =
-      buildList {
-        cm.allNetworks
-          .firstOrNull { n ->
-            val caps = cm.getNetworkCapabilities(n) ?: return@firstOrNull false
-            caps.hasTransport(NetworkCapabilities.TRANSPORT_VPN)
-          }?.let(::add)
-        cm.activeNetwork?.let(::add)
-      }.distinct()
+    // Prefer VPN (Tailscale) when present; otherwise use the active network.
+    cm.allNetworks.firstOrNull { n ->
+      val caps = cm.getNetworkCapabilities(n) ?: return@firstOrNull false
+      caps.hasTransport(NetworkCapabilities.TRANSPORT_VPN)
+    }?.let { return it }
 
-    val servers =
-      candidateNetworks
-        .asSequence()
-        .flatMap { n ->
-          cm.getLinkProperties(n)?.dnsServers?.asSequence() ?: emptySequence()
-        }
-        .distinctBy { it.hostAddress ?: it.toString() }
-        .toList()
-    if (servers.isEmpty()) return null
-
-    return try {
-      val resolvers =
-        servers.mapNotNull { addr ->
-          try {
-            SimpleResolver().apply { setAddress(InetSocketAddress(addr, 53)) }
-          } catch (_: Throwable) {
-            null
-          }
-        }
-      if (resolvers.isEmpty()) return null
-
-      ExtendedResolver(resolvers.toTypedArray()).apply {
-        // Vienna -> London via tailnet: allow a bit more headroom than LAN mDNS.
-        setTimeout(Duration.ofMillis(3000))
-      }
-    } catch (_: Throwable) {
-      null
-    }
+    return cm.activeNetwork
   }
 
+  private suspend fun rawQuery(network: android.net.Network?, wireQuery: ByteArray): ByteArray =
+    suspendCancellableCoroutine { cont ->
+      val signal = CancellationSignal()
+      cont.invokeOnCancellation { signal.cancel() }
+
+      dns.rawQuery(
+        network,
+        wireQuery,
+        0,
+        dnsExecutor,
+        signal,
+        object : DnsResolver.Callback<ByteArray> {
+          override fun onAnswer(answer: ByteArray, rcode: Int) {
+            cont.resume(answer)
+          }
+
+          override fun onError(error: DnsResolver.DnsException) {
+            cont.resumeWithException(error)
+          }
+        },
+      )
+    }
+
   private fun txtValue(records: List<TXTRecord>, key: String): String? {
     val prefix = "$key="
     for (r in records) {
@@ -263,13 +279,13 @@ class BridgeDiscovery(
     return null
   }
 
-  private fun resolveHost(hostname: String, resolver: org.xbill.DNS.Resolver?): String? {
+  private suspend fun resolveHostUnicast(hostname: String): String? {
     val a =
-      lookup(hostname, Type.A, resolver)
+      lookupUnicast(hostname, Type.A)
         .mapNotNull { it as? ARecord }
         .mapNotNull { it.address?.hostAddress }
     val aaaa =
-      lookup(hostname, Type.AAAA, resolver)
+      lookupUnicast(hostname, Type.AAAA)
         .mapNotNull { it as? AAAARecord }
         .mapNotNull { it.address?.hostAddress }