diff --git a/docs/requirements/Component-DataConnectionLayer.md b/docs/requirements/Component-DataConnectionLayer.md
index 15f2966c..e203e4b1 100644
--- a/docs/requirements/Component-DataConnectionLayer.md
+++ b/docs/requirements/Component-DataConnectionLayer.md
@@ -402,7 +402,7 @@ The DCL manages connection lifecycle automatically:
3. **Connection state transitions**: The DCL tracks each connection's state as `connected`, `disconnected`, or `reconnecting`. All transitions are logged to Site Event Logging (event type `connection`): the initial connect (`Connection established on … endpoint`, Info — added by review 2026-10-21, it previously logged nothing), connection lost / reconfigured (`Warning` / `Info`), restored on reconnect (`Info`) and failover (`Warning`).
4. **Transparent re-subscribe**: On successful reconnection, the DCL automatically re-establishes all previously active subscriptions for that connection. Instance Actors require no action — they simply see quality return to `good` as fresh values arrive from restored subscriptions.
5. **Clean client replacement on (re)connect**: On every (re)connect the adapter detaches and disposes the previous protocol client (session, keep-alive timers, event loop) before creating a new one — a stale client can neither leak nor signal a disconnect against the new session.
-6. **Session generation on same-adapter reconnect** (review 2026-10-21, finding C1): a plain drop + reconnect reuses the adapter object, so `SwapAdapter`'s generation bump never runs. On a successful reconnect the actor therefore advances `_adapterGeneration` itself — **before** `ReSubscribeAll` — and re-hooks the adapter's `Disconnected` handler for the new generation (new handler hooked before the old one is removed, so a disconnect in between is never lost). Every subscribe/alarm completion, value callback and seed started against the dead session then takes the existing stale path. That path releases a stale completion's handles against the adapter that created them, **except** when that adapter is still the current one: the handles died with the dead session, and releasing by handle against the adapter's NEW session could tear down a live item. Before the fix a dead-session initial-subscribe completion (in flight up to `SeedOverallTimeout`) was applied against the new session — parking a dead handle that made the live re-subscribe row look redundant and get released (the tag read resolved and stayed silent), or releasing a handle number the new MxGateway session had reused, and a connection-level failure row in it knocked the fresh link back into Reconnecting.
+6. **Session generation on same-adapter reconnect** (review 2026-10-21, finding C1): a plain drop + reconnect reuses the adapter object, so `SwapAdapter`'s generation bump never runs. Each **reconnect attempt** therefore advances `_adapterGeneration` itself — before its `ConnectAsync` is issued — and re-hooks the adapter's `Disconnected` handler for the new generation (new handler hooked before the old one is removed). The attempt's `ConnectResult`, the new session's `Disconnected` and the `ReSubscribeAll` that follows a success all share that generation, so a disconnect queued right behind a successful result is still acted on (advancing the generation only when the result was applied would have dropped it as stale and wedged the actor Connected on a dead session — the adapter raises `Disconnected` once per session). The initial connect does not advance it: no session has existed yet. Every subscribe/alarm completion, value callback and seed started against the dead session then takes the existing stale path. That path releases a stale completion's handles against the adapter that created them, **except** when that adapter is still the current one: the handles died with the dead session, and releasing by handle against the adapter's NEW session could tear down a live item. Before the fix a dead-session initial-subscribe completion (in flight up to `SeedOverallTimeout`) was applied against the new session — parking a dead handle that made the live re-subscribe row look redundant and get released (the tag read resolved and stayed silent), or releasing a handle number the new MxGateway session had reused, and a connection-level failure row in it knocked the fresh link back into Reconnecting.
7. **Session-scoped MxGateway subscription ids** (findings C1 + L1): gateway item handles are plain ints that restart per session, so `MxGatewayDataConnection` hands out ids of the form `"{session}:{handle}"` and only ever sends a handle to the gateway when its session is the live one — a dead session's id is dropped silently instead of unadvising whatever the new session gave that number. The adapter's routing maps are cleared on every `ConnectAsync`, `DisconnectAsync` and `DisposeAsync` (they previously kept every dead session's entries, growing per flap), and a subscribe completing after a reconnect does not repopulate them. An alarm unsubscribe for an id the adapter does not know no longer decrements the shared alarm stream's reference count.
### Disconnect Detection Pattern
diff --git a/src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Actors/DataConnectionActor.cs b/src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Actors/DataConnectionActor.cs
index 54fd1376..0cc79f85 100644
--- a/src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Actors/DataConnectionActor.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Actors/DataConnectionActor.cs
@@ -269,10 +269,12 @@ public class DataConnectionActor : UntypedActor, IWithTimers
///
///
///
- /// Residual (accepted, and in the safe direction): a disconnect raised by the PREVIOUS
- /// session but delivered after the next attempt has already cleared the flag is attributed to
- /// the new attempt, costing one extra reconnect cycle. Being wedged Connected on a dead
- /// adapter is the failure this trades against.
+ /// Each attempt advances the session generation before it is issued, so a disconnect the
+ /// PREVIOUS session already raised (stamped with its generation) is dropped as stale rather
+ /// than attributed to the new attempt. Residual (accepted, and in the safe direction): a
+ /// previous-session disconnect raised only AFTER the next attempt re-hooked the handler is
+ /// attributed to the new attempt, costing one extra reconnect cycle. Being wedged Connected
+ /// on a dead adapter is the failure this trades against.
///
///
private bool _disconnectDuringConnectAttempt;
@@ -408,7 +410,7 @@ public class DataConnectionActor : UntypedActor, IWithTimers
switch (message)
{
case AttemptConnect:
- HandleAttemptConnect();
+ HandleAttemptConnect(advanceSession: false);
break;
case ConnectResult result:
HandleConnectResult(result);
@@ -736,6 +738,9 @@ public class DataConnectionActor : UntypedActor, IWithTimers
///
/// Advances for a new SESSION on the SAME adapter — a plain
/// drop + reconnect, where never runs (review 2026-10-21 02/C1).
+ /// Called by before each RECONNECT attempt is issued, so the
+ /// attempt's result, the new session's Disconnected and everything started after it
+ /// share one generation, and anything started against an earlier session is stale.
///
///
/// Without it, an initial subscribe issued against the dead session (in flight for up to
@@ -751,9 +756,9 @@ public class DataConnectionActor : UntypedActor, IWithTimers
///
/// The Disconnected handler stamps the generation it was hooked with, so it is
/// re-hooked for the new generation. The new handler is hooked BEFORE the old one is
- /// removed: a disconnect raised in between is then delivered twice (old stamp dropped as
- /// stale, new stamp acted on) rather than lost, which would wedge the actor Connected on a
- /// dead session.
+ /// removed, so no window exists with no handler at all. Because this runs before the
+ /// attempt's ConnectAsync, the only disconnects that can carry the OLD stamp belong
+ /// to the previous (already-dead) session and are correctly dropped as stale.
///
///
private void AdvanceSessionGeneration()
@@ -852,7 +857,7 @@ public class DataConnectionActor : UntypedActor, IWithTimers
switch (message)
{
case AttemptConnect:
- HandleAttemptConnect();
+ HandleAttemptConnect(advanceSession: true);
break;
case ConnectResult result:
HandleReconnectResult(result);
@@ -944,13 +949,27 @@ public class DataConnectionActor : UntypedActor, IWithTimers
// ── Connection Management ──
- private void HandleAttemptConnect()
+ ///
+ /// True from Reconnecting: a session has existed on this adapter, so the attempt opens a NEW
+ /// one and gets its own generation (see ). False from
+ /// the initial Connecting state, where no session has ever been established and nothing is
+ /// in flight against one, so the generation the handler and result share is already right.
+ ///
+ private void HandleAttemptConnect(bool advanceSession)
{
_log.Debug("[{0}] Attempting connection...", _connectionName);
// Scope the pending-disconnect flag to THIS attempt. Anything noted before now
// belongs to a session that is already over and must not be replayed against the one
// about to start.
_disconnectDuringConnectAttempt = false;
+ // Every reconnect attempt opens a NEW session, so it gets its own generation (review 2026-10-21
+ // 02/C1), advanced — and the Disconnected handler re-hooked — BEFORE ConnectAsync is
+ // issued. The new session's Disconnected therefore always carries the same generation
+ // as its ConnectResult: a disconnect that lands in the mailbox right behind a successful
+ // result is acted on, not dropped as stale (which would wedge the actor Connected on a
+ // dead session — the adapter raises Disconnected only once per session).
+ if (advanceSession)
+ AdvanceSessionGeneration();
var self = Self;
// Stamp the generation this attempt belongs to, exactly as every other piped
// completion does. A failover or an in-place reconfigure landing while ConnectAsync
@@ -1146,12 +1165,9 @@ public class DataConnectionActor : UntypedActor, IWithTimers
$"Connection restored on {_activeEndpoint} endpoint", null);
}
- // The adapter object survived the drop but its SESSION did not. Advance the
- // generation BEFORE re-subscribing so every completion, value and alarm callback
- // started against the dead session takes the existing stale path (review
- // 2026-10-21 02/C1) — exactly as it would after a swap.
- AdvanceSessionGeneration();
-
+ // The session generation was already advanced when this attempt was issued
+ // (HandleAttemptConnect), so ReSubscribeAll below runs on the NEW session's
+ // generation and every completion from the dead one takes the stale path.
// Transparent re-subscribe — re-establish all active subscriptions
ReSubscribeAll();
// Re-establish native alarm feeds (source replays a snapshot).
diff --git a/tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Actors/DataConnectionSameAdapterReconnectTests.cs b/tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Actors/DataConnectionSameAdapterReconnectTests.cs
index 84808ada..56c72b86 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Actors/DataConnectionSameAdapterReconnectTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Actors/DataConnectionSameAdapterReconnectTests.cs
@@ -187,6 +187,58 @@ public class DataConnectionSameAdapterReconnectTests : TestKit
Assert.DoesNotContain("live-2", adapter.ReleasedIds);
}
+ ///
+ /// The connect-result/disconnect race (adversarial review of the C1 fix). The (re)connect
+ /// succeeds and its ConnectResult is enqueued; before the actor processes it the new
+ /// session dies, so the adapter's once-only Disconnected lands in the mailbox AFTER the
+ /// result. That disconnect belongs to the session the result describes and must drive the
+ /// actor back to Reconnecting — if it were stamped with a generation the result's processing
+ /// then advanced past, it would be dropped as stale and the actor would sit Connected on a
+ /// dead session that never raises Disconnected again.
+ ///
+ /// False: the initial connect; true: a same-adapter reconnect.
+ [Theory]
+ [InlineData(false)]
+ [InlineData(true)]
+ public void DisconnectQueuedBehindASuccessfulConnectResult_IsNotLost(bool reconnect)
+ {
+ var name = reconnect ? "race-reconnect" : "race-initial";
+ var adapter = new GatedAdapter();
+ var connectGate = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+ adapter.NextConnectGate = connectGate.Task;
+ var actor = CreateConnectionActor(name, adapter);
+ var expectedConnectCalls = 1;
+
+ if (reconnect)
+ {
+ connectGate.SetResult();
+ AwaitConnectedCount(name, 1);
+ connectGate = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+ adapter.NextConnectGate = connectGate.Task;
+ adapter.RaiseDisconnected();
+ expectedConnectCalls = 2;
+ }
+
+ AwaitCondition(() => Volatile.Read(ref adapter.ConnectCalls) == expectedConnectCalls, TimeSpan.FromSeconds(5));
+ var disconnectsBefore = HealthTransitions(name, ConnectionHealth.Disconnected);
+
+ // Hold the actor inside a health-report read …
+ using var hold = new ManualResetEventSlim(false);
+ adapter.HoldStatus = hold;
+ actor.Tell(new DataConnectionActor.GetHealthReport());
+ Assert.True(adapter.StatusEntered.Wait(TimeSpan.FromSeconds(5)));
+
+ // … while the connect succeeds (its result queues behind the held message) …
+ connectGate.SetResult();
+ Thread.Sleep(300);
+ // … and the new session dies at once, queuing its disconnect behind the result.
+ adapter.RaiseDisconnected();
+ hold.Set();
+
+ AwaitCondition(() => HealthTransitions(name, ConnectionHealth.Disconnected) > disconnectsBefore,
+ TimeSpan.FromSeconds(5), TimeSpan.FromMilliseconds(20));
+ }
+
/// One gated SubscribeBatchAsync call.
internal sealed class PendingBatch
{
@@ -219,7 +271,33 @@ public class DataConnectionSameAdapterReconnectTests : TestKit
public readonly ConcurrentQueue ReleasedIds = new();
- public ConnectionHealth Status { get; private set; } = ConnectionHealth.Disconnected;
+ private ConnectionHealth _status = ConnectionHealth.Disconnected;
+
+ /// When set, the NEXT read of signals and blocks on it — a latch that holds the actor mid-message.
+ public ManualResetEventSlim? HoldStatus;
+
+ /// Set once a held read has started.
+ public readonly ManualResetEventSlim StatusEntered = new(false);
+
+ /// When set, the next returns this task instead of completing at once.
+ public Task? NextConnectGate;
+
+ /// Number of calls so far.
+ public int ConnectCalls;
+
+ public ConnectionHealth Status
+ {
+ get
+ {
+ if (Interlocked.Exchange(ref HoldStatus, null) is { } hold)
+ {
+ StatusEntered.Set();
+ hold.Wait(TimeSpan.FromSeconds(10));
+ }
+ return _status;
+ }
+ private set => _status = value;
+ }
public event Action? Disconnected;
@@ -244,8 +322,9 @@ public class DataConnectionSameAdapterReconnectTests : TestKit
public Task ConnectAsync(IDictionary connectionDetails, CancellationToken cancellationToken = default)
{
+ Interlocked.Increment(ref ConnectCalls);
Status = ConnectionHealth.Connected;
- return System.Threading.Tasks.Task.CompletedTask;
+ return Interlocked.Exchange(ref NextConnectGate, null) ?? System.Threading.Tasks.Task.CompletedTask;
}
public Task DisconnectAsync(CancellationToken cancellationToken = default) =>