两人没改同一行，为什么值班表还是空了？

import java.util.Arrays;

/** JDK 17. A fixed two-row model, not a database isolation implementation. */
public class WriteSkewDemo {
    record Snapshot(boolean[] onDuty, int[] versions) {
        int count() {
            int total = 0;
            for (boolean value : onDuty) if (value) total++;
            return total;
        }
    }

    static final class Roster {
        private final boolean[] onDuty = {true, true};
        private final int[] versions = {0, 0};

        synchronized Snapshot read() {
            return new Snapshot(onDuty.clone(), versions.clone());
        }

        synchronized String leave(int person, Snapshot read, boolean checkAllReads) {
            if (read.count() <= 1) return "rule-rejected";
            for (int i = 0; i < versions.length; i++) {
                if ((checkAllReads || i == person) && versions[i] != read.versions()[i]) {
                    return "stale-read";
                }
            }
            onDuty[person] = false;
            versions[person]++;
            return "committed";
        }
    }

    static void check(boolean ok) {
        if (!ok) throw new AssertionError("unexpected model result");
    }

    public static void main(String[] args) {
        Roster rowOnly = new Roster();
        Snapshot first = rowOnly.read(), second = rowOnly.read();
        check(rowOnly.leave(0, first, false).equals("committed"));
        check(rowOnly.leave(1, second, false).equals("committed"));
        check(rowOnly.read().count() == 0);
        System.out.println("check written row only: " + Arrays.toString(rowOnly.read().onDuty()));

        Roster allReads = new Roster();
        first = allReads.read(); second = allReads.read();
        String a = allReads.leave(0, first, true);
        String b = allReads.leave(1, second, true);
        String retry = allReads.leave(1, allReads.read(), true);
        check(a.equals("committed") && b.equals("stale-read"));
        check(retry.equals("rule-rejected") && allReads.read().count() == 1);
        System.out.println("check both read rows: " + a + ", " + b);
        System.out.println("retry with a fresh read: " + retry);
        System.out.println("remaining on duty: " + allReads.read().count());

        Roster sameRow = new Roster();
        first = sameRow.read(); second = sameRow.read();
        check(sameRow.leave(0, first, false).equals("committed"));
        check(sameRow.leave(0, second, false).equals("stale-read"));
        System.out.println("same-row version conflict is still detected: true");
        System.out.println("Deterministic model only; no database or parallel threads.");
    }
}