feat(m4): single instance IPC

- LaunchCommand: ShowMain / OpenWorkspace(path), doubles as IPC message
- LaunchArguments: first non-blank arg wins, blank-only args -> ShowMain
- IpcProtocol: strict newline-line codec (show / open+path), tolerant
  of trailing-newline framing only
- SingleInstanceService: per-session named mutex decides primary;
  primary serves a named-pipe loop (decode -> dispatch -> ack) with
  handler failures contained; secondary retries send, exits on ack
- Program/App wiring: secondary forwards command and exits; primary
  activates the main window for forwarded commands (workspace switch
  deferred to the workspace UI milestone)
- 15 new tests (protocol codec, argument parsing); smoke-verified:
  instance 1 stays alive, instance 2 self-exits with code 0

YKanBan.Tests/Ipc/IpcProtocolTests.cs

@@ -0,0 +1,87 @@
+using YKanBan.Ipc;
+
+namespace YKanBan.Tests.Ipc;
+
+/// <summary>Tests for the IPC wire protocol codec.</summary>
+[TestClass]
+public class IpcProtocolTests
+{
+    [TestMethod]
+    public void ShowMainEncodesAsSingleLine()
+    {
+        Assert.AreEqual("show\n", IpcProtocol.Encode(new LaunchCommand.ShowMain()));
+    }
+
+    [TestMethod]
+    public void OpenWorkspaceEncodesCommandAndPathLines()
+    {
+        Assert.AreEqual(
+            "open\nD:\\my repo\n",
+            IpcProtocol.Encode(new LaunchCommand.OpenWorkspace(@"D:\my repo")));
+    }
+
+    [TestMethod]
+    public void RoundTripPreservesCommands()
+    {
+        LaunchCommand[] commands =
+        [
+            new LaunchCommand.ShowMain(),
+            new LaunchCommand.OpenWorkspace(@"D:\my repo"),
+            new LaunchCommand.OpenWorkspace("/home/user/仓库"),
+        ];
+
+        foreach (LaunchCommand command in commands)
+        {
+            Assert.AreEqual(command, IpcProtocol.Decode(IpcProtocol.Encode(command)));
+        }
+    }
+
+    [TestMethod]
+    public void DecodeToleratesMissingTrailingNewline()
+    {
+        Assert.AreEqual(new LaunchCommand.ShowMain(), IpcProtocol.Decode("show"));
+        Assert.AreEqual(
+            new LaunchCommand.OpenWorkspace(@"C:\x"),
+            IpcProtocol.Decode("open\nC:\\x"));
+    }
+
+    [TestMethod]
+    public void DecodePreservesPathsWithSpacesAndUnicode()
+    {
+        LaunchCommand command = IpcProtocol.Decode("open\nD:\\我的 仓库\n");
+        Assert.AreEqual(new LaunchCommand.OpenWorkspace(@"D:\我的 仓库"), command);
+    }
+
+    [TestMethod]
+    public void DecodeRejectsEmptyPayload()
+    {
+        Assert.ThrowsExactly<FormatException>(() => IpcProtocol.Decode(string.Empty));
+        Assert.ThrowsExactly<FormatException>(() => IpcProtocol.Decode("\n"));
+    }
+
+    [TestMethod]
+    public void DecodeRejectsUnknownCommand()
+    {
+        Assert.ThrowsExactly<FormatException>(() => IpcProtocol.Decode("frobnicate\n"));
+    }
+
+    [TestMethod]
+    public void DecodeRejectsOpenWithoutPath()
+    {
+        Assert.ThrowsExactly<FormatException>(() => IpcProtocol.Decode("open\n"));
+        Assert.ThrowsExactly<FormatException>(() => IpcProtocol.Decode("open"));
+        Assert.ThrowsExactly<FormatException>(() => IpcProtocol.Decode("open\n\n"));
+    }
+
+    [TestMethod]
+    public void DecodeRejectsContentAfterShow()
+    {
+        Assert.ThrowsExactly<FormatException>(() => IpcProtocol.Decode("show\nextra\n"));
+    }
+
+    [TestMethod]
+    public void EncodeRejectsNullCommand()
+    {
+        Assert.ThrowsExactly<InvalidOperationException>(() => IpcProtocol.Encode(null!));
+    }
+}

YKanBan.Tests/Ipc/LaunchArgumentsTests.cs

@@ -0,0 +1,47 @@
+using YKanBan.Ipc;
+
+namespace YKanBan.Tests.Ipc;
+
+/// <summary>Tests for command-line argument parsing.</summary>
+[TestClass]
+public class LaunchArgumentsTests
+{
+    [TestMethod]
+    public void NoArgumentsYieldShowMain()
+    {
+        Assert.AreEqual(new LaunchCommand.ShowMain(), LaunchArguments.Parse([]));
+    }
+
+    [TestMethod]
+    public void BlankArgumentsOnlyYieldShowMain()
+    {
+        Assert.AreEqual(new LaunchCommand.ShowMain(), LaunchArguments.Parse(["", "  "]));
+    }
+
+    [TestMethod]
+    public void SingleArgumentBecomesWorkspacePath()
+    {
+        Assert.AreEqual(
+            new LaunchCommand.OpenWorkspace(@"D:\repo"),
+            LaunchArguments.Parse([@"D:\repo"]));
+    }
+
+    [TestMethod]
+    public void PathWithSpacesIsPreserved()
+    {
+        Assert.AreEqual(
+            new LaunchCommand.OpenWorkspace(@"D:\my repo\ws"),
+            LaunchArguments.Parse([@"D:\my repo\ws"]));
+    }
+
+    [TestMethod]
+    public void FirstMeaningfulArgumentWins()
+    {
+        Assert.AreEqual(
+            new LaunchCommand.OpenWorkspace(@"D:\a"),
+            LaunchArguments.Parse([@"D:\a", @"D:\b"]));
+        Assert.AreEqual(
+            new LaunchCommand.OpenWorkspace(@"D:\b"),
+            LaunchArguments.Parse(["", @"D:\b", @"D:\c"]));
+    }
+}

YKanBan/App.axaml.cs

@@ -4,6 +4,8 @@ using Avalonia.Controls.ApplicationLifetimes;
 using Avalonia.Data.Core;
 using Avalonia.Data.Core.Plugins;
 using Avalonia.Markup.Xaml;
+using Avalonia.Threading;
+using YKanBan.Ipc;
 using YKanBan.ViewModels;
 using YKanBan.Views;
 
@@ -25,12 +27,33 @@ public partial class App : Application
             {
                 DataContext = new MainWindowViewModel(),
             };
+
+            // Primary instance: serve launch commands forwarded by secondary instances.
+            Program.SingleInstance?.StartServer(command => HandleLaunchCommand(desktop, command));
         }
 
         base.OnFrameworkInitializationCompleted();
     }
 
-    private void DisableAvaloniaDataAnnotationValidation()
+    /// <summary>
+    /// Brings the main window to front for a forwarded launch command. Both
+    /// ShowMain and OpenWorkspace activate the window; switching to the
+    /// requested workspace lands together with the workspace UI.
+    /// </summary>
+    private static void HandleLaunchCommand(IClassicDesktopStyleApplicationLifetime desktop, LaunchCommand command)
+    {
+        // IPC delivery happens on a background thread; window access must hop to the UI thread.
+        Dispatcher.UIThread.Post(() =>
+        {
+            if (desktop.MainWindow is { } window)
+            {
+                window.Show();
+                window.Activate();
+            }
+        });
+    }
+
+    private static void DisableAvaloniaDataAnnotationValidation()
     {
         var dataValidationPluginsToRemove =
             BindingPlugins.DataValidators.OfType<DataAnnotationsValidationPlugin>().ToArray();

YKanBan/Ipc/ISingleInstanceService.cs

@@ -0,0 +1,25 @@
+namespace YKanBan.Ipc;
+
+/// <summary>
+/// Single-instance detection and inter-instance messaging, built on a named
+/// mutex (detection) and a named pipe (command forwarding). The primary
+/// instance holds the mutex for its lifetime and serves the pipe; secondary
+/// instances forward their launch command to the primary and exit.
+/// </summary>
+public interface ISingleInstanceService : IDisposable
+{
+    /// <summary>Attempts to register this process as the single YKanBan instance.</summary>
+    /// <returns>True when this process is the primary instance; false when another one exists.</returns>
+    bool TryAcquireSingleInstance();
+
+    /// <summary>Starts serving launch commands on a background task (primary instance only).</summary>
+    /// <param name="handler">Invoked for each decoded command; runs on a background thread.</param>
+    /// <exception cref="InvalidOperationException">
+    /// Not called on a primary instance, or called a second time.
+    /// </exception>
+    void StartServer(Action<LaunchCommand> handler);
+
+    /// <summary>Delivers a command to the primary instance and waits for its acknowledgement.</summary>
+    /// <returns>True when the primary instance acknowledged the command.</returns>
+    bool TrySend(LaunchCommand command);
+}

YKanBan/Ipc/IpcProtocol.cs

@@ -0,0 +1,62 @@
+namespace YKanBan.Ipc;
+
+/// <summary>
+/// Text encoding of launch commands on the IPC pipe. A message is one or
+/// two newline-terminated lines: "show", or "open" followed by the
+/// workspace path taken verbatim (paths may contain spaces; paths
+/// containing newlines are not supported). Decoding is strict and throws
+/// FormatException on malformed payloads.
+/// </summary>
+public static class IpcProtocol
+{
+    /// <summary>Encodes a command into its wire text.</summary>
+    /// <exception cref="InvalidOperationException">The command type is unknown.</exception>
+    public static string Encode(LaunchCommand command) => command switch
+    {
+        LaunchCommand.ShowMain => "show\n",
+        LaunchCommand.OpenWorkspace open => $"open\n{open.Path}\n",
+        _ => throw new InvalidOperationException($"Unsupported launch command: {command?.GetType().Name ?? "null"}"),
+    };
+
+    /// <summary>Decodes wire text into a command; trailing newlines are tolerated.</summary>
+    /// <exception cref="FormatException">The payload is not a valid message.</exception>
+    public static LaunchCommand Decode(string payload)
+    {
+        List<string> lines = payload.Split('\n').ToList();
+
+        // Trailing newlines are a framing detail; drop the empty lines they produce.
+        while (lines.Count > 0 && lines[^1].Length == 0)
+        {
+            lines.RemoveAt(lines.Count - 1);
+        }
+
+        if (lines.Count == 0)
+        {
+            throw new FormatException("Empty IPC payload.");
+        }
+
+        switch (lines[0])
+        {
+            case "show":
+                if (lines.Count != 1)
+                {
+                    throw new FormatException($"Unexpected content after 'show': '{lines[1]}'");
+                }
+                return new LaunchCommand.ShowMain();
+
+            case "open":
+                if (lines.Count != 2)
+                {
+                    throw new FormatException("'open' requires exactly one path line.");
+                }
+                if (lines[1].Length == 0)
+                {
+                    throw new FormatException("'open' requires a non-empty path.");
+                }
+                return new LaunchCommand.OpenWorkspace(lines[1]);
+
+            default:
+                throw new FormatException($"Unknown IPC command: '{lines[0]}'");
+        }
+    }
+}

YKanBan/Ipc/LaunchArguments.cs

@@ -0,0 +1,18 @@
+namespace YKanBan.Ipc;
+
+/// <summary>Parses command-line arguments into a launch command.</summary>
+public static class LaunchArguments
+{
+    /// <summary>
+    /// Parses raw command-line arguments: with no meaningful argument the
+    /// command is ShowMain; otherwise the first non-blank argument is taken
+    /// as the workspace path and any further arguments are ignored.
+    /// </summary>
+    public static LaunchCommand Parse(IReadOnlyList<string> arguments)
+    {
+        string? path = arguments.FirstOrDefault(argument => !string.IsNullOrWhiteSpace(argument));
+        return path is null
+            ? new LaunchCommand.ShowMain()
+            : new LaunchCommand.OpenWorkspace(path);
+    }
+}

YKanBan/Ipc/LaunchCommand.cs

@@ -0,0 +1,15 @@
+namespace YKanBan.Ipc;
+
+/// <summary>
+/// A launch command: what a YKanBan invocation asks the application to do.
+/// Doubles as the IPC message forwarded from a secondary instance to the
+/// primary one.
+/// </summary>
+public abstract record LaunchCommand
+{
+    /// <summary>No arguments: show the main window and bring it to front.</summary>
+    public sealed record ShowMain : LaunchCommand;
+
+    /// <summary>A workspace path: activate the window and switch to (or open) the workspace.</summary>
+    public sealed record OpenWorkspace(string Path) : LaunchCommand;
+}

YKanBan/Ipc/SingleInstanceService.cs

@@ -0,0 +1,188 @@
+using System.IO.Pipes;
+using System.Text;
+
+namespace YKanBan.Ipc;
+
+/// <summary>
+/// Mutex + named pipe implementation of <see cref="ISingleInstanceService"/>.
+/// The mutex name is per-user-session (Local\ prefix), so separate users on
+/// one machine run independently. A crash releases the mutex with the
+/// process — no stale state survives.
+/// </summary>
+public sealed class SingleInstanceService : ISingleInstanceService
+{
+    private const string MutexName = @"Local\YKanBan.SingleInstance";
+    private const string PipeName = "YKanBan.Ipc";
+    private const int ConnectTimeoutMilliseconds = 2000;
+    private const int ConnectAttempts = 3;
+    private const int RetryDelayMilliseconds = 300;
+
+    private readonly object _gate = new();
+    private Mutex? _mutex;
+    private CancellationTokenSource? _serverCancellation;
+    private bool _isPrimary;
+
+    /// <summary>Attempts to register this process as the single YKanBan instance.</summary>
+    /// <exception cref="InvalidOperationException">Acquired a second time.</exception>
+    public bool TryAcquireSingleInstance()
+    {
+        lock (_gate)
+        {
+            if (_mutex is not null)
+            {
+                throw new InvalidOperationException("Single instance already acquired.");
+            }
+
+            // Ownership is requested up front; createdNew tells primary from secondary.
+            _mutex = new Mutex(initiallyOwned: true, MutexName, out bool createdNew);
+            _isPrimary = createdNew;
+            return createdNew;
+        }
+    }
+
+    /// <summary>Starts serving launch commands on a background task (primary instance only).</summary>
+    /// <exception cref="InvalidOperationException">
+    /// Not called on a primary instance, or called a second time.
+    /// </exception>
+    public void StartServer(Action<LaunchCommand> handler)
+    {
+        CancellationTokenSource cancellation;
+        lock (_gate)
+        {
+            if (!_isPrimary || _mutex is null)
+            {
+                throw new InvalidOperationException("Only the primary instance can serve IPC.");
+            }
+            if (_serverCancellation is not null)
+            {
+                throw new InvalidOperationException("IPC server already started.");
+            }
+            _serverCancellation = new CancellationTokenSource();
+            cancellation = _serverCancellation;
+        }
+
+        _ = Task.Run(() => ServerLoopAsync(handler, cancellation.Token));
+    }
+
+    /// <summary>Delivers a command to the primary instance and waits for its acknowledgement.</summary>
+    public bool TrySend(LaunchCommand command)
+    {
+        string payload = IpcProtocol.Encode(command);
+
+        // The primary may still be starting its server; retry briefly before giving up.
+        for (int attempt = 0; attempt < ConnectAttempts; attempt++)
+        {
+            try
+            {
+                using var client = new NamedPipeClientStream(".", PipeName, PipeDirection.InOut);
+                client.Connect(ConnectTimeoutMilliseconds);
+
+                using (var writer = new StreamWriter(client, Encoding.UTF8, bufferSize: 64, leaveOpen: true)
+                {
+                    AutoFlush = true,
+                })
+                {
+                    writer.Write(payload);
+                }
+
+                using var reader = new StreamReader(client, Encoding.UTF8, detectEncodingFromByteOrderMarks: false, bufferSize: 64, leaveOpen: true);
+                string acknowledgement = reader.ReadLine() ?? string.Empty;
+                return acknowledgement == "ok";
+            }
+            catch (TimeoutException)
+            {
+                Thread.Sleep(RetryDelayMilliseconds);
+            }
+            catch (IOException)
+            {
+                Thread.Sleep(RetryDelayMilliseconds);
+            }
+        }
+        return false;
+    }
+
+    /// <summary>Releases the mutex and stops the IPC server.</summary>
+    public void Dispose()
+    {
+        lock (_gate)
+        {
+            _serverCancellation?.Cancel();
+            _serverCancellation?.Dispose();
+            _serverCancellation = null;
+            _mutex?.Dispose();
+            _mutex = null;
+            _isPrimary = false;
+        }
+    }
+
+    /// <summary>Accepts one client at a time: read, decode, dispatch, acknowledge, disconnect.</summary>
+    private static async Task ServerLoopAsync(Action<LaunchCommand> handler, CancellationToken cancellation)
+    {
+        while (!cancellation.IsCancellationRequested)
+        {
+            try
+            {
+                using var server = new NamedPipeServerStream(PipeName, PipeDirection.InOut, maxNumberOfServerInstances: 1);
+                await server.WaitForConnectionAsync(cancellation);
+
+                string payload = await ReadMessageAsync(server, cancellation);
+                LaunchCommand command = IpcProtocol.Decode(payload);
+
+                // Dispatch to the app; handler failures must not kill the server.
+                try
+                {
+                    handler(command);
+                }
+                catch (Exception ex) when (ex is not OperationCanceledException)
+                {
+                    // Swallow handler exceptions: one bad dispatch must not stop serving.
+                }
+
+                WriteAcknowledgement(server);
+                server.Disconnect();
+            }
+            catch (OperationCanceledException)
+            {
+                break;
+            }
+            catch (IOException)
+            {
+                // Client vanished mid-conversation: drop the connection and keep serving.
+            }
+            catch (FormatException)
+            {
+                // Malformed payload: no acknowledgement is written; keep serving.
+            }
+        }
+    }
+
+    /// <summary>Reads the command line plus, for open, the path line.</summary>
+    private static async Task<string> ReadMessageAsync(PipeStream pipe, CancellationToken cancellation)
+    {
+        using var reader = new StreamReader(pipe, Encoding.UTF8, detectEncodingFromByteOrderMarks: false, bufferSize: 1024, leaveOpen: true);
+
+        string firstLine = await reader.ReadLineAsync(cancellation)
+            ?? throw new IOException("Connection closed before the command line arrived.");
+        if (firstLine != "open")
+        {
+            return firstLine;
+        }
+
+        string? pathLine = await reader.ReadLineAsync(cancellation);
+        if (pathLine is null)
+        {
+            throw new IOException("Connection closed before the path line arrived.");
+        }
+        return $"open\n{pathLine}";
+    }
+
+    /// <summary>Writes the acknowledgement the sender waits for.</summary>
+    private static void WriteAcknowledgement(PipeStream pipe)
+    {
+        using var writer = new StreamWriter(pipe, Encoding.UTF8, bufferSize: 64, leaveOpen: true)
+        {
+            AutoFlush = true,
+        };
+        writer.Write("ok\n");
+    }
+}

YKanBan/Program.cs

@@ -1,13 +1,30 @@
 using System;
 using Avalonia;
+using YKanBan.Ipc;
 
 namespace YKanBan;
 
 sealed class Program
 {
+    /// <summary>The single-instance service held by this process while the app runs.</summary>
+    internal static ISingleInstanceService? SingleInstance { get; private set; }
+
     [STAThread]
-    public static void Main(string[] args) => BuildAvaloniaApp()
-        .StartWithClassicDesktopLifetime(args);
+    public static void Main(string[] args)
+    {
+        LaunchCommand command = LaunchArguments.Parse(args);
+        using ISingleInstanceService singleInstance = new SingleInstanceService();
+
+        // Secondary instance: forward the command to the primary, then exit immediately.
+        if (!singleInstance.TryAcquireSingleInstance())
+        {
+            singleInstance.TrySend(command);
+            return;
+        }
+
+        SingleInstance = singleInstance;
+        BuildAvaloniaApp().StartWithClassicDesktopLifetime(args);
+    }
 
     public static AppBuilder BuildAvaloniaApp()
         => AppBuilder.Configure<App>()