CachedFields are not reset when references-flag changes

[thomas-z96]

Describe the bug
When changing the references flag from false to true on a kryo instance that has already been used for deserializing, subsequent deserialization does not work correctly any more (wrong results / exceptions).

To Reproduce

@Test
public void testBug() {
    Output output1 = new Output(512, -1);
    Kryo kryo = getKryoInstance(false);
    kryo.writeObject(output1, new TestString("abc"));

    Output output2 = new Output(512, -1);
    kryo = getKryoInstance(true);
    kryo.writeObject(output2, new TestString("abc"));

    Input input1 = new Input(output1.getBuffer());
    kryo = getKryoInstance(false);
    assertEquals("abc", kryo.readObject(input1, TestString.class).str);

    Input input2 = new Input(output2.getBuffer());
    kryo.setReferences(true);
    assertEquals("abc", kryo.readObject(input2, TestString.class).str);
}

static class TestString {
    String str;
    TestString() {
    }
    TestString(String str) {
        this.str = str;
    }
    @Override
    public String toString() {
        return "TestString [str=" + str + "]";
    }
}

private Kryo getKryoInstance(boolean references) {
    Kryo kryo = new Kryo();
    kryo.setRegistrationRequired(false);
    kryo.setReferences(references);
    return kryo;
}

Environment:

• OS: Windows 10
• JDK Version: 11
• Kryo Version: 5.5.0

Additional context
The issue is that CachedFields.newUnsafeField() / CachedFields.newAsmField() check the references flag of the kryo instance and return different CachedField instances depending on that flag. This CachedField is then kept in the FieldSerializer's cachedFields and is not removed if the flag changes, so the wrong CachedField type is used for subsequent deserializations.

[theigl]

What is your use-case for changing the reference flag on the fly? Could you work around this limitation by switching between two separate Kryo instances instead of changing the flag?

[thomas-z96]

Yes, creating a new Kryo instance is the workaround that I used to fix this.
The use-case is that I changed the reference flag to true by default but still need to be able to read older files (that were written with references=false). So when I encounter an old file, I wanted to switch the Kryo instance to reference=false temporarily.

[theigl]

Ok thanks for clarifying. Please continue using two separate instances.

It would be rather difficult to reset the cached fields of all field serializers when the flag changes. I think I'll document that the flag cannot be changed on the fly instead of trying to address this.