feat(sessions): detach-grace retention window for reconnect

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Sessions/SessionManagerTests.cs

@@ -1,5 +1,6 @@
 using Google.Protobuf.WellKnownTypes;
 using Microsoft.Extensions.Options;
+using Microsoft.Extensions.Time.Testing;
 using ZB.MOM.WW.MxGateway.Contracts.Proto;
 using ZB.MOM.WW.MxGateway.Server.Configuration;
 using ZB.MOM.WW.MxGateway.Server.Metrics;
@@ -752,6 +753,42 @@ public sealed class SessionManagerTests
         Assert.Equal(0, workerClient.ShutdownCount);
     }
 
+    /// <summary>
+    ///     Task 11. With detach-grace enabled, a session whose last external subscriber dropped
+    ///     and whose detach-grace window has elapsed is closed by the lease sweep exactly like an
+    ///     expired-lease session — even though its normal lease is still far in the future.
+    /// </summary>
+    [Fact]
+    public async Task CloseExpiredLeasesAsync_ClosesSessionWhoseDetachGraceWindowExpired()
+    {
+        FakeWorkerClient workerClient = new();
+        FakeTimeProvider clock = new(DateTimeOffset.UtcNow);
+        SessionManager manager = CreateManager(
+            new FakeSessionWorkerClientFactory(workerClient),
+            options: CreateOptions(defaultLeaseSeconds: 1800, detachGraceSeconds: 30),
+            timeProvider: clock);
+        GatewaySession session = await manager.OpenSessionAsync(CreateOpenRequest(), "client-1", ownerKeyId: null, CancellationToken.None);
+
+        // Attach and drop an external subscriber to enter detach-grace. The normal lease is
+        // still 30 minutes out, so only the detach-grace window can close this session.
+        IDisposable subscriber = session.AttachEventSubscriber(maxSubscribers: 1);
+        subscriber.Dispose();
+        Assert.NotNull(session.DetachedAtUtc);
+
+        // Before the window elapses: not closed.
+        clock.Advance(TimeSpan.FromSeconds(29));
+        int closedBefore = await manager.CloseExpiredLeasesAsync(clock.GetUtcNow(), CancellationToken.None);
+        Assert.Equal(0, closedBefore);
+        Assert.Equal(SessionState.Ready, session.State);
+
+        // After the window elapses: the sweep closes it.
+        clock.Advance(TimeSpan.FromSeconds(1));
+        int closedAfter = await manager.CloseExpiredLeasesAsync(clock.GetUtcNow(), CancellationToken.None);
+        Assert.Equal(1, closedAfter);
+        Assert.Equal(SessionState.Closed, session.State);
+        Assert.Equal(1, workerClient.ShutdownCount);
+    }
+
     /// <summary>Verifies that shutdown closes all registered sessions.</summary>
     [Fact]
     public async Task ShutdownAsync_ClosesAllRegisteredSessions()
@@ -797,7 +834,8 @@ public sealed class SessionManagerTests
 
     private static GatewayOptions CreateOptions(
         int maxSessions = 64,
-        int defaultLeaseSeconds = 1800)
+        int defaultLeaseSeconds = 1800,
+        int detachGraceSeconds = 0)
     {
         return new GatewayOptions
         {
@@ -806,6 +844,7 @@ public sealed class SessionManagerTests
                 DefaultCommandTimeoutSeconds = 30,
                 MaxSessions = maxSessions,
                 DefaultLeaseSeconds = defaultLeaseSeconds,
+                DetachGraceSeconds = detachGraceSeconds,
             },
             Worker = new WorkerOptions
             {