Sandbox: Start aus einem langlebigen Thread - das Elternsterbe-Signal haengt am Thread (#654)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ClawdDotNet.Core/Sandbox/BwrapSandbox.cs

@@ -219,7 +219,7 @@ public sealed class BwrapSandbox : ISandboxRunner
 
     private async Task<SandboxResult> RunProcessAsync(ProcessStartInfo start, SandboxRequest request, CancellationToken ct)
     {
-        using var process = Process.Start(start) ?? throw new InvalidOperationException("The sandbox did not start.");
+        using var process = await Spawner.StartAsync(start);
         using var timeout = CancellationTokenSource.CreateLinkedTokenSource(ct);
         timeout.CancelAfter(request.Limits.Timeout);
 
@@ -411,6 +411,52 @@ public sealed class BwrapSandbox : ISandboxRunner
     }
 }
 
+/// <summary>
+/// #654: Starts every sandbox from one thread that lives as long as the process.
+///
+/// <para><b>Why.</b> <c>--die-with-parent</c> uses the kernel's parent-death signal, and that
+/// is bound to the <i>thread</i> that forked, not to the process. .NET forks on the calling
+/// thread — a thread-pool thread, and the pool retires idle threads. Seen on the test machine
+/// under load: the pool dropped a thread, bwrap got SIGKILL (exit 137), and when it happened
+/// while bwrap was still building the sandbox, the inside lived on as an orphan holding the
+/// output pipes. Started from a thread that never ends, the signal fires only when the
+/// instance itself dies — which is what it is for.</para>
+/// </summary>
+internal static class Spawner
+{
+    private static readonly System.Collections.Concurrent.BlockingCollection<(ProcessStartInfo Start, TaskCompletionSource<Process> Done)> Queue = new();
+
+    static Spawner()
+    {
+        var thread = new Thread(() =>
+        {
+            foreach (var (start, done) in Queue.GetConsumingEnumerable())
+            {
+                try
+                {
+                    done.SetResult(Process.Start(start) ?? throw new InvalidOperationException("The sandbox did not start."));
+                }
+                catch (Exception ex)
+                {
+                    done.SetException(ex);
+                }
+            }
+        })
+        {
+            IsBackground = true,
+            Name = "ClawdDotNet sandbox spawner"
+        };
+        thread.Start();
+    }
+
+    public static Task<Process> StartAsync(ProcessStartInfo start)
+    {
+        var done = new TaskCompletionSource<Process>(TaskCreationOptions.RunContinuationsAsynchronously);
+        Queue.Add((start, done));
+        return done.Task;
+    }
+}
+
 /// <summary>#654: What the installed bwrap can do beyond the basics.</summary>
 public sealed record BwrapFeatures(bool DisableUserns, bool ClearEnv)
 {