using System.Collections.Concurrent;
using System.Runtime.CompilerServices;
namespace ZB.MOM.WW.OtOpcUa.Driver.Ab.Transport.Tests.Fakes;

/// <summary>
///     Collects <see cref="TaskScheduler.UnobservedTaskException"/>s raised after some code abandons work, with a
///     positive control (Gitea #583): after the abandon, a task is deliberately faulted and dropped, and garbage is
///     collected until THAT task's exception has been reported. Only then is the collection returned — so "nothing
///     of ours was reported" means the finalizer pass that would have reported it really ran, not merely that a
///     fixed number of GC cycles went by before the runtime got round to it.
/// </summary>
internal static class UnobservedTaskProbe
{
    public static async Task<IReadOnlyList<Exception>> CollectAsync(Func<Task> abandon, TimeSpan timeout)
    {
        var marker = $"unobserved-task-probe-control-{Guid.NewGuid():N}";
        var seen = new ConcurrentQueue<Exception>();
        var controlReported = 0;
        EventHandler<UnobservedTaskExceptionEventArgs> handler = (_, e) =>
        {
            foreach (var ex in e.Exception.Flatten().InnerExceptions)
            {
                if (ex.Message == marker) Volatile.Write(ref controlReported, 1);
                else seen.Enqueue(ex);
            }
        };
        TaskScheduler.UnobservedTaskException += handler;
        try
        {
            await abandon();
            AbandonControl(marker);
            var giveUp = DateTime.UtcNow + timeout;
            while (Volatile.Read(ref controlReported) == 0)
            {
                if (DateTime.UtcNow > giveUp)
                    throw new TimeoutException("The positive control's unobserved exception was never reported.");
                GC.Collect();
                GC.WaitForPendingFinalizers();
                await Task.Delay(10);
            }
            // One more full pass, so anything that became garbage in the same cycle as the control is reported too.
            GC.Collect();
            GC.WaitForPendingFinalizers();
        }
        finally
        {
            TaskScheduler.UnobservedTaskException -= handler;
        }
        return [.. seen];
    }

    [MethodImpl(MethodImplOptions.NoInlining)]
    private static void AbandonControl(string marker) => _ = Task.FromException(new InvalidOperationException(marker));
}