test(headless): cover K4 death and cancellation

tests/AcDream.Headless.Tests/HeadlessProcessSchedulerTests.cs

@@ -358,6 +358,76 @@ public sealed class HeadlessProcessSchedulerTests
             StringComparison.Ordinal);
     }
 
+    [Fact]
+    public async Task ThirtySessionsConvergeAcrossRandomizedCancellationEdges()
+    {
+        const int sessionCount = 30;
+        var random = new Random(0xAC2013);
+        for (int iteration = 0; iteration < 5; iteration++)
+        {
+            var configuration = new HeadlessConfiguration
+            {
+                Version = 1,
+                Sessions = Enumerable.Range(0, sessionCount)
+                    .Select(index => Descriptor(
+                        $"cancel-{iteration}-{index}",
+                        $"cancel-{iteration}-{index}-stdin"))
+                    .Cast<HeadlessSessionDescriptor?>()
+                    .ToList(),
+            };
+            var operations = new FixtureSessionOperations();
+            using var diagnostics = new StringWriter();
+            using var host = new HeadlessProcessHost(
+                configuration,
+                HeadlessPathSet.Resolve(
+                    new HeadlessPathOverrides()),
+                new System.IO.StringReader(string.Concat(
+                    Enumerable.Repeat(
+                        "random-cancel-password"
+                        + Environment.NewLine,
+                        sessionCount))),
+                diagnostics,
+                operations);
+            using var cancellation = new CancellationTokenSource(
+                TimeSpan.FromMilliseconds(random.Next(37, 144)));
+
+            HeadlessExitCode result =
+                await host.RunAsync(cancellation.Token);
+
+            Assert.True(cancellation.IsCancellationRequested);
+            Assert.Equal(HeadlessExitCode.Success, result);
+            Assert.Equal(sessionCount, operations.CreatedSessionCount);
+            Assert.All(
+                host.Sessions,
+                static session =>
+                    Assert.False(session.IsFaulted));
+
+            host.Dispose();
+
+            Assert.Equal(sessionCount, operations.DisposedSessionCount);
+            Assert.All(
+                host.Sessions,
+                static session =>
+                    Assert.True(
+                        session.Runtime
+                            .CaptureOwnership()
+                            .IsConverged));
+            string output = diagnostics.ToString();
+            Assert.Contains(
+                "\"state\":\"disposed\"",
+                output,
+                StringComparison.Ordinal);
+            Assert.Contains(
+                $"\"convergedRuntimeCount\":{sessionCount}",
+                output,
+                StringComparison.Ordinal);
+            Assert.DoesNotContain(
+                "random-cancel-password",
+                output,
+                StringComparison.Ordinal);
+        }
+    }
+
     [Fact]
     public void ThrowingPolicyQuarantinesOnlyItsOwnSession()
     {