Fix AppTP unregister crash

Author: joshliebe

app-tracking-protection/vpn-impl/src/main/java/com/duckduckgo/mobile/android/vpn/service/DnsChangeCallback.kt

@@ -45,6 +45,10 @@ class DnsChangeCallback @Inject constructor(
     }
     private var lastDns: List<InetAddress>? = null
 
+    // Guards against unregistering an unregistered callback.
+    // Note: Not synchronized, so register()/unregister() must always run on TrackerBlockingVpnService's serviceDispatcher.
+    private var registered: Boolean = false
+
     override fun onLinkPropertiesChanged(network: Network, linkProperties: LinkProperties) {
         logcat { "onLinkPropertiesChanged: $linkProperties" }
         coroutineScope.launch(dispatcherProvider.io()) {
@@ -68,22 +72,27 @@ class DnsChangeCallback @Inject constructor(
     }
 
     internal fun register() {
+        if (registered) return
         kotlin.runCatching {
             val request = NetworkRequest.Builder().apply {
                 addTransportType(NetworkCapabilities.TRANSPORT_CELLULAR)
                 addTransportType(NetworkCapabilities.TRANSPORT_WIFI)
             }.build()
             connectivityManager?.registerNetworkCallback(request, this)
+            registered = true
         }.onFailure {
             logcat(ERROR) { it.asLog() }
         }
     }
 
     internal fun unregister() {
+        if (!registered) return
         kotlin.runCatching {
             connectivityManager?.unregisterNetworkCallback(this)
         }.onFailure {
             logcat(ERROR) { it.asLog() }
+        }.also {
+            registered = false
         }
     }