Expected behavior of FakeTimeProvider when timer changes period during callback

[egil]

Hi @sebastienros, @geeknoid, @danmoseley, @RussKie, et al.

The general scenario I am trying to understand is this: Suppose a ITimer has been scheduled to invoke a callback every 1. second, but callbacks are delayed e.g., by 3. seconds, such that the timer will invoke the callback 3 times when it finally gets allocated processor time (i.e. emulated by calling FakeTimeProvider.Advance(3 seconds). Normally, the callback would just be invoked 3 times and the next invocation will then get scheduled.

But what if the timer period is changed by the callback, e.g. during the first of the three callbacks? Does it influence the remaining two callbacks that have been scheduled to happen or does it just have an effect after the three callbacks are done? What does the real System.Threading.Timer do in this case?

Consider this test case. What should the expectedCallbackCount be set to in each of the scenarios?

[Theory]
[InlineData(Timeout.Infinite,Timeout.Infinite, ???)]
[InlineData(1000,  500, ???)]
[InlineData(1000, 2000, ???)]
[InlineData(2000, 1000, ???)]
[InlineData( 500, 1000, ???)]
public void Timer_changed_during_callback_while_advancing(int dueTimeMs, int periodMs, int expectedCallbackCount)
{
    var timeProvider = new FakeTimeProvider();
    var callbackCount = 0;
    ITimer timer = default!;
    timer = timeProvider.CreateTimer(
        _ =>
        {
            callbackCount++;
            if (callbackCount == 1)
            {
                timer.Change(TimeSpan.FromMilliseconds(dueTimeMs), TimeSpan.FromMilliseconds(periodMs));
            }
        }, 
        null, 
        TimeSpan.FromMilliseconds(1000), 
        TimeSpan.FromMilliseconds(1000));

    timeProvider.Advance(TimeSpan.FromMilliseconds(3000));

    Assert.Equal(callbackCount, expectedCallbackCount);
}