fix(launcher): harden installer transactions

src/AcDream.Launcher.Core/Installation/InstallerTransactionLease.cs

@@ -0,0 +1,60 @@
+namespace AcDream.Launcher.Core.Installation;
+
+/// <summary>
+/// Cross-process ownership for every mutation or recovery of one launcher
+/// DataDirectory. The persistent lock pathname is harmless; exclusivity is
+/// owned by the open OS handle and therefore disappears if the process dies.
+/// </summary>
+internal sealed class InstallerTransactionLease : IAsyncDisposable
+{
+    internal const string LockFileName = ".install.lock";
+    private static readonly TimeSpan RetryDelay = TimeSpan.FromMilliseconds(50);
+
+    private readonly FileStream _stream;
+
+    private InstallerTransactionLease(FileStream stream)
+    {
+        _stream = stream;
+    }
+
+    internal static string GetLockPath(string dataDirectory) =>
+        Path.Combine(Path.GetFullPath(dataDirectory), LockFileName);
+
+    internal static async ValueTask<InstallerTransactionLease> AcquireAsync(
+        string dataDirectory,
+        CancellationToken cancellationToken = default)
+    {
+        string lockPath = GetLockPath(dataDirectory);
+        Directory.CreateDirectory(
+            Path.GetDirectoryName(lockPath)
+            ?? throw new InvalidOperationException(
+                "The installer lock path has no parent directory."));
+
+        while (true)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+            try
+            {
+                var stream = new FileStream(
+                    lockPath,
+                    FileMode.OpenOrCreate,
+                    FileAccess.ReadWrite,
+                    FileShare.None,
+                    bufferSize: 1,
+                    FileOptions.None);
+                return new InstallerTransactionLease(stream);
+            }
+            catch (IOException)
+            {
+                await Task.Delay(RetryDelay, cancellationToken)
+                    .ConfigureAwait(false);
+            }
+        }
+    }
+
+    public ValueTask DisposeAsync()
+    {
+        _stream.Dispose();
+        return ValueTask.CompletedTask;
+    }
+}