Context switching can affect timer triggering

[mapogolions]

Description

There is a possibility that a registered timer callback will never be called, and this can happen if context switching between threads occurs in a certain way.

Reproduction Steps

Please use the following snippet to reproduce the problem.

DateTimeOffset now = new(2000, 1, 1, 0, 0, 0, 0, TimeSpan.Zero);
var provider = new FakeTimeProvider(now) { AutoAdvanceAmount = TimeSpan.FromSeconds(2) };
using var timer = provider.CreateTimer(
    callback: Console.WriteLine,
    state: "waiter-1",
    dueTime: TimeSpan.FromSeconds(3),
    period: TimeSpan.Zero);

var threads = Enumerable.Range(start: 0, count: 5)
    .Select(x => new Thread(() =>
    {
        if (x != 0) Thread.Sleep(TimeSpan.FromMilliseconds(400));
        provider.GetUtcNow();
    }))
    .ToList();
threads.ForEach(t => t.Start());
threads.ForEach(t => t.Join());
Console.WriteLine(provider); // Print `_now` to confirm that 10 seconds have passed

Important: Please add some kind of delay to the FakeTimeProvider before the following line of code to emulate context switching.

Thread.Sleep(TimeSpan.FromSeconds(2));
if (candidate == null)
{
    // didn't find a candidate to wake, we're done
    _wakeWaitersGate = 0;
    return;
}

Expected behavior

The timer callback should be triggered, and the message 'waiter-1' should be printed to the console.

Actual behavior

Despite making 5 calls to advance the clock by 10 seconds, the timer callback has not been called.

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response