LinkedBlockingQueue's iterator can return null if drainTo(c) executes concurrently

Martin Buchholz reports:

The following test program should never throw,
but it quickly throws NullPointerException.

import java.util.*;
import java.util.concurrent.*;

public class LBQRace {
    static void checkNotContainsNull(Iterable it) {
        for (Object x : it)
            if (x == null)
                throw new NullPointerException();
    }

    public static void main(String[] args) throws Throwable {
        final BlockingQueue q = new LinkedBlockingQueue();

        new Thread() { public void run() {
            for (;;) {
                q.offer(1);
            }}}.start();

        new Thread() { public void run() {
            for (;;) {
                List list = new ArrayList();
                q.drainTo(list);
                checkNotContainsNull(list);
            }}}.start();

        new Thread() { public void run() {
            for (;;) {
                checkNotContainsNull(q);
            }}}.start();
    }
}