ScadaBridge/tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Scripts/ScriptExecutionSchedulerTests.cs

using ZB.MOM.WW.ScadaBridge.SiteRuntime;
using ZB.MOM.WW.ScadaBridge.SiteRuntime.Scripts;

namespace ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests.Scripts;

/// <summary>
/// The dedicated script-execution scheduler must run script bodies on
/// its own dedicated threads, not on the shared .NET thread pool, so blocking script
/// I/O cannot starve the global pool.
/// </summary>
public class ScriptExecutionSchedulerTests
{
    /// <summary>Verifies that the scheduler does not inline work on another scheduler's worker thread.</summary>
    /// <returns>A task that represents the asynchronous operation.</returns>
    [Fact]
    public async Task Scheduler_DoesNotInlineOnAnotherSchedulersWorker()
    {
        using var first = new ScriptExecutionScheduler(1);
        using var second = new ScriptExecutionScheduler(1);
        await Task.Factory.StartNew(() =>
        {
            var firstThread = Environment.CurrentManagedThreadId;
            var nested = new Task<int>(() => Environment.CurrentManagedThreadId);
            nested.RunSynchronously(second);
            Assert.NotEqual(firstThread, nested.Result);
        }, CancellationToken.None, TaskCreationOptions.None, first);
    }

    /// <summary>Verifies that the scheduler runs work off the shared thread pool.</summary>
    /// <returns>A task that represents the asynchronous operation.</returns>
    [Fact]
    public async Task Scheduler_RunsWork_OffTheThreadPool()
    {
        using var scheduler = new ScriptExecutionScheduler(2);

        bool wasThreadPoolThread = true;
        string? threadName = null;

        await Task.Factory.StartNew(() =>
        {
            wasThreadPoolThread = Thread.CurrentThread.IsThreadPoolThread;
            threadName = Thread.CurrentThread.Name;
        }, CancellationToken.None, TaskCreationOptions.DenyChildAttach, scheduler);

        Assert.False(wasThreadPoolThread,
            "Script work must not run on a shared thread-pool thread.");
        Assert.StartsWith("script-execution-", threadName);
    }

    /// <summary>Verifies that the scheduler respects the configured thread count.</summary>
    [Fact]
    public void Scheduler_RespectsConfiguredThreadCount()
    {
        using var scheduler = new ScriptExecutionScheduler(4);
        Assert.Equal(4, scheduler.MaximumConcurrencyLevel);
    }

    /// <summary>Verifies that the shared scheduler returns the same instance for the same options.</summary>
    [Fact]
    public void Scheduler_Shared_ReturnsSameInstanceForOptions()
    {
        var options = new SiteRuntimeOptions { ScriptExecutionThreadCount = 3 };
        var a = ScriptExecutionScheduler.Shared(options);
        var b = ScriptExecutionScheduler.Shared(options);
        Assert.Same(a, b);
    }

    // IsDisposed backs the Shared() recreate guard so a disposed scheduler
    // can never be handed back and silently poison every later execution. Verified on a
    // local instance — deliberately NOT by disposing the process-wide singleton, which
    // would strand any script another test class queued on it (the very static hazard
    // this test guards against).
    /// <summary>Verifies that <c>IsDisposed</c> flips to true when the scheduler is disposed.</summary>
    [Fact]
    public void IsDisposed_FlipsOnDispose()
    {
        var scheduler = new ScriptExecutionScheduler(1);
        Assert.False(scheduler.IsDisposed);

        scheduler.Dispose();
        Assert.True(scheduler.IsDisposed);

        scheduler.Dispose(); // idempotent — stays disposed, does not throw
        Assert.True(scheduler.IsDisposed);
    }

    // SiteRuntime-S2/UA5: the scheduler exposes observability gauges so a stuck /
    // saturated script-execution pool is visible to operators via site health.
    /// <summary>Verifies that the gauges reflect the busy thread count and the queue depth.</summary>
    /// <returns>A task that represents the asynchronous operation.</returns>
    [Fact]
    public async Task Gauges_ReflectBusyThreadAndQueueDepth()
    {
        using var scheduler = new ScriptExecutionScheduler(1);
        using var gate = new ManualResetEventSlim(false);

        var blocking = Task.Factory.StartNew(() => gate.Wait(),
            CancellationToken.None, TaskCreationOptions.None, scheduler);
        var queued = Task.Factory.StartNew(() => { },
            CancellationToken.None, TaskCreationOptions.None, scheduler);

        await WaitUntilAsync(() => scheduler.BusyThreadCount == 1);
        Assert.Equal(1, scheduler.QueueDepth);          // second task waiting
        // A busy thread reports a non-null age. Not asserting a strictly-positive
        // value: Environment.TickCount64 has ~15 ms granularity, so under load the
        // busy-timestamp and this read can fall in the same tick (age == Zero).
        Assert.NotNull(scheduler.OldestBusyAge);

        gate.Set();
        await Task.WhenAll(blocking, queued);
        await WaitUntilAsync(() => scheduler.BusyThreadCount == 0);
        Assert.Null(scheduler.OldestBusyAge);
    }

    private static async Task WaitUntilAsync(Func<bool> condition)
    {
        for (var i = 0; i < 100 && !condition(); i++)
            await Task.Delay(50);
        Assert.True(condition(), "condition not met within timeout");
    }

    /// <summary>
    /// Repeatedly wedging + detaching a worker must reuse the vacated slot rather than grow the
    /// slot table without bound (review 2026-10-21 01/SR-M1).
    /// </summary>
    [Fact]
    public async Task DetachedWorkerSlot_IsReused_SlotTableStaysBounded()
    {
        using var scheduler = new ScriptExecutionScheduler(1);

        for (var round = 0; round < 6; round++)
        {
            var gate = new ManualResetEventSlim(false);
            var started = new TaskCompletionSource<(int Slot, long Stamp)>();
            var wedged = Task.Factory.StartNew(() =>
            {
                var slot = ScriptExecutionScheduler.CurrentWorkerSlot!.Value;
                started.SetResult((slot, scheduler.CurrentRunStamp(slot)));
                gate.Wait();
            }, CancellationToken.None, TaskCreationOptions.None, scheduler);

            var (s, stamp) = await started.Task;
            Assert.Equal(WorkerDetachOutcome.Detached, scheduler.TryDetachWorker(s, stamp));
            gate.Set();
            await wedged;

            // Wait for the detached worker to exit and hand its slot back.
            var deadline = DateTime.UtcNow.AddSeconds(5);
            while (scheduler.DetachedThreadCount != 0 && DateTime.UtcNow < deadline)
                await Task.Delay(10);
            Assert.Equal(0, scheduler.DetachedThreadCount);
            await Task.Delay(50);
        }

        // 1 live + at most 1 free/in-flight detached slot, never 1 + 6.
        Assert.True(scheduler.SlotCount <= 2, $"slot table grew to {scheduler.SlotCount}");
        // The pool still works afterwards.
        var ran = await Task.Factory.StartNew(() => 42, CancellationToken.None, TaskCreationOptions.None, scheduler);
        Assert.Equal(42, ran);
    }
}