Hey there!

We recently run into the following issue, after upgrading to 3.0.2:

Stacktrace

java.lang.IndexOutOfBoundsException: fromIndex = -1
	at java.base/java.util.AbstractList.subListRangeCheck(AbstractList.java:505)
	at java.base/java.util.ArrayList.subList(ArrayList.java:1137)
	at org.xbill.DNS.RRset.rrs(RRset.java:150)
	at org.xbill.DNS.Lookup.processResponse(Lookup.java:445)
	at org.xbill.DNS.Lookup.lookup(Lookup.java:484)
	at org.xbill.DNS.Lookup.resolve(Lookup.java:543)
	at org.xbill.DNS.Lookup.run(Lookup.java:561)

Cause:

This happens because of:

  private short position;

  ...

    int start = position++ % rrs.size();
    l.addAll(rrs.subList(start, rrs.size()));

Eventually (when position > 32.767) it overflows and goes negative. This leads to a negative modulo because... that's how Java wills it. Sublist then blows up because it always expects positive

Reproducing:

• Create an RRset
• Add 2 or more elements
• call rrs() or rrs(true) more than 32.767 times

Test (uses Junit 5 but feel free to switch it to 4):

package dns;

import org.junit.jupiter.api.Test;
import org.xbill.DNS.Name;
import org.xbill.DNS.RRset;
import org.xbill.DNS.Record;

import java.lang.reflect.Field;
import java.util.ArrayList;

public class RRsetTest {

    @Test
    void cycleBelowShort() throws Exception {
        runSim(100);
    }

    @Test
    void cycleAboveShort() throws Exception {
        runSim(50_000);
    }

    private void runSim(int numOfCalls) throws Exception {
        RRset rRset = new RRset();

        // Sorry for the dirtiness here. I hacked this because
        // I couldn't figure out an easy second sample that would get through validation
        Field rrsField = RRset.class.getDeclaredField("rrs");
        rrsField.setAccessible(true);
        ((ArrayList<Record>) rrsField.get(rRset)).add(Record.newRecord(Name.root, 1, 1));
        ((ArrayList<Record>) rrsField.get(rRset)).add(Record.newRecord(Name.root, 1, 1));

        for (int i = 0; i < numOfCalls; i++) {
            rRset.rrs(true);
        }
    }

}

The cycleAboveShort() method eventually throws.

Suggested solution:

You can either use one of the suggestions in the link for always positive modulo, or an iterator that resets. I like the latter solution much more

Other notices:

• Cycling does not work as expected on lists > 32.767 elements (though honestly, whatever)

Have a great day!