Use Android DNS resolver (experimental)

app/src/main/java/eu/faircode/email/DnsHelper.java

@@ -21,6 +21,7 @@ package eu.faircode.email;
 
 import android.content.Context;
 import android.net.ConnectivityManager;
+import android.net.DnsResolver;
 import android.net.LinkProperties;
 import android.net.Network;
 import android.net.NetworkInfo;
@@ -30,16 +31,20 @@ import androidx.annotation.NonNull;
 
 import org.xbill.DNS.Lookup;
 import org.xbill.DNS.MXRecord;
+import org.xbill.DNS.Message;
 import org.xbill.DNS.Record;
 import org.xbill.DNS.SRVRecord;
 import org.xbill.DNS.SimpleResolver;
 import org.xbill.DNS.TextParseException;
 import org.xbill.DNS.Type;
 
+import java.io.IOException;
 import java.net.InetAddress;
 import java.net.UnknownHostException;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.concurrent.Semaphore;
+import java.util.concurrent.TimeUnit;
 
 import javax.mail.Address;
 import javax.mail.internet.InternetAddress;
@@ -115,7 +120,60 @@ public class DnsHelper {
         }
 
         try {
-            SimpleResolver resolver = new SimpleResolver(getDnsServer(context));
+            SimpleResolver resolver = new SimpleResolver(getDnsServer(context)) {
+                private IOException ex;
+                private Message result;
+
+                @Override
+                public Message send(Message query) throws IOException {
+                    if (Build.VERSION.SDK_INT < Build.VERSION_CODES.Q || !BuildConfig.DEBUG)
+                        return super.send(query);
+                    else {
+                        Log.i("Using Android DNS resolver");
+                        Semaphore sem = new Semaphore(0);
+                        DnsResolver resolver = DnsResolver.getInstance();
+                        resolver.rawQuery(
+                                null,
+                                query.toWire(),
+                                DnsResolver.FLAG_EMPTY,
+                                Helper.getBackgroundExecutor(1, "dns"),
+                                null,
+                                new DnsResolver.Callback<byte[]>() {
+                                    @Override
+                                    public void onAnswer(@NonNull byte[] answer, int rcode) {
+                                        Log.i("DNS rcode=" + rcode);
+                                        try {
+                                            if (rcode == 0)
+                                                result = new Message(answer);
+                                            else
+                                                throw new IOException("rcode=" + rcode);
+                                        } catch (Throwable e) {
+                                            ex = new IOException(e.getMessage());
+                                        }
+                                        sem.release();
+                                    }
+
+                                    @Override
+                                    public void onError(@NonNull DnsResolver.DnsException e) {
+                                        Log.w(e);
+                                        ex = new IOException(e.getMessage());
+                                        sem.release();
+                                    }
+                                });
+                        try {
+                            if (!sem.tryAcquire(LOOKUP_TIMEOUT, TimeUnit.SECONDS))
+                                throw new IOException("timeout");
+                        } catch (InterruptedException e) {
+                            ex = new IOException("interrupted");
+                        }
+
+                        if (ex == null)
+                            return result;
+                        else
+                            throw ex;
+                    }
+                }
+            };
             resolver.setTimeout(LOOKUP_TIMEOUT);
             Lookup lookup = new Lookup(name, rtype);
             lookup.setResolver(resolver);