cursor-lang/CursorLang.Core/Services/SingleInstanceGate.cs

using CursorLang.Core.Interop;

namespace CursorLang.Core.Services;

/// <summary>
/// Lets only one instance of the application run: a second launch does not bring up
/// a second window but shows the window of the one already running.
/// </summary>
/// <remarks>
/// The kernel object names are left without the Global prefix, that is, they live in
/// the session namespace. A single instance for the whole machine would make for an
/// odd picture with fast user switching: the second user would be left without the
/// application, and showing them the window of the first one is impossible anyway —
/// windows belong to a session.
///
/// Two processes use this now, and each guards its own slot: the agent so that one
/// background process runs, the settings window so that a second "Settings" from the
/// tray raises the window already open instead of a second one. Hence the name part.
/// </remarks>
public sealed class SingleInstanceGate : IDisposable
{
    /// <summary>The agent's slot — one background process per session.</summary>
    public const string AgentName = ".Agent";

    /// <summary>The settings window's slot — one window per session.</summary>
    public const string SettingsName = ".Settings";

    private const string MutexName = "CursorLang.SingleInstance";
    private const string ActivationEventName = "CursorLang.ActivationRequest";

    private readonly string _mutexName;
    private readonly string _activationEventName;

    private Mutex? _mutex;
    private EventWaitHandle? _activationRequest;
    private RegisteredWaitHandle? _activationWait;
    private bool _isOwner;

    /// <summary>
    /// Takes a named slot. The name tells the agent's slot from the settings window's,
    /// and the tests use one of their own: otherwise they would share a slot with the
    /// running application and get in its way.
    /// </summary>
    public SingleInstanceGate(string nameSuffix)
    {
        _mutexName = MutexName + nameSuffix;
        _activationEventName = ActivationEventName + nameSuffix;
    }

    /// <summary>
    /// Another launch asks for the window to be shown.
    /// </summary>
    /// <remarks>
    /// Raised on a thread pool thread, wherever the wait happened to be answered. The
    /// two hosts get back to their own thread differently — one through the dispatcher,
    /// one by posting to its window — so neither is assumed here.
    /// </remarks>
    public event EventHandler? ActivationRequested;

    /// <summary>
    /// Takes the single-instance slot. When the application is already running, asks
    /// it to show itself and returns <c>false</c> — the caller is left to exit.
    /// </summary>
    public bool TryAcquire() => TryAcquire(showRunningInstance: true);

    /// <summary>
    /// The same, with a say in what is to happen to the application already running.
    /// </summary>
    /// <param name="showRunningInstance">
    /// Whether the running application is to be brought up. A launch by Windows
    /// itself passes <c>false</c>: it was not asked for a window, and the
    /// application already in the tray is answer enough.
    /// </param>
    public bool TryAcquire(bool showRunningInstance)
    {
        _mutex = new Mutex(initiallyOwned: false, _mutexName);

        try
        {
            _isOwner = _mutex.WaitOne(TimeSpan.Zero, exitContext: false);
        }
        catch (AbandonedMutexException)
        {
            // The previous instance crashed and did not release the mutex.
            // It has no owner now, which means the slot is free
            _isOwner = true;
        }

        // The event is opened by both instances: the first one to wait for a request,
        // the second one to make it. Which of them creates the object depends on who
        // came first and does not affect the work
        _activationRequest = new EventWaitHandle(false, EventResetMode.AutoReset, _activationEventName);

        if (!_isOwner)
        {
            if (showRunningInstance)
            {
                ForegroundPermissionNative.GrantToAnyProcess();
                _activationRequest.Set();
            }

            return false;
        }

        // The wait is handed over to the thread pool: there is no reason to hold a
        // thread of our own for it, and the request may never come
        _activationWait = ThreadPool.RegisterWaitForSingleObject(
            _activationRequest,
            OnActivationSignalled,
            state: null,
            Timeout.Infinite,
            executeOnlyOnce: false);

        return true;
    }

    public void Dispose()
    {
        _activationWait?.Unregister(null);
        _activationWait = null;

        _activationRequest?.Dispose();
        _activationRequest = null;

        // The mutex is released by the same thread that took it: both happen
        // on the user interface thread
        if (_isOwner)
        {
            _mutex?.ReleaseMutex();
            _isOwner = false;
        }

        _mutex?.Dispose();
        _mutex = null;
    }

    private void OnActivationSignalled(object? state, bool timedOut) =>
        ActivationRequested?.Invoke(this, EventArgs.Empty);
}