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fix(SubscriptionManager): replace List with ConcurrentDictionary for abandoned subscriptions (#3617) #3618

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fix(SubscriptionManager): replace List with ConcurrentDictionary for abandoned subscriptions (#3617) #3618

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fix(SubscriptionManager): replace List with ConcurrentDictionary for …
cristipogacean Mar 13, 2026
189f720
merge main
cristipogacean Mar 13, 2026
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73 changes: 14 additions & 59 deletions Libraries/Opc.Ua.Server/Subscription/SubscriptionManager.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -75,6 +75,7 @@ public SubscriptionManager(IServerInternal server, ApplicationConfiguration conf
m_subscriptionStore = server.SubscriptionStore;

m_subscriptions = [];
m_abandonedSubscriptions = [];
m_publishQueues = [];
m_statusMessages = [];
m_lastSubscriptionId = BitConverter.ToUInt32(
Expand Down Expand Up @@ -528,18 +529,12 @@ public virtual async ValueTask SessionClosingAsync(
// mark the subscriptions as abandoned.
else
{
await m_semaphoreSlim.WaitAsync(cancellationToken).ConfigureAwait(false);
try
if (m_abandonedSubscriptions.TryAdd(subscription.Id, subscription))
{
(m_abandonedSubscriptions ??= []).Add(subscription);
m_logger.LogWarning(
"Subscription ABANDONED, Id={SubscriptionId}.",
subscription.Id);
}
finally
{
m_semaphoreSlim.Release();
}
}
}
}
Expand Down Expand Up @@ -699,19 +694,11 @@ public async ValueTask<StatusCode> DeleteSubscriptionAsync(OperationContext cont
}

// check for abandoned subscription.
if (m_abandonedSubscriptions != null)
if (m_abandonedSubscriptions.TryRemove(subscriptionId, out _))
{
for (int ii = 0; ii < m_abandonedSubscriptions.Count; ii++)
{
if (m_abandonedSubscriptions[ii].Id == subscriptionId)
{
m_abandonedSubscriptions.RemoveAt(ii);
m_logger.LogWarning(
"Subscription DELETED(ABANDONED), Id={SubscriptionId}.",
subscriptionId);
break;
}
}
m_logger.LogWarning(
"Subscription DELETED(ABANDONED), Id={SubscriptionId}.",
subscriptionId);
}

// remove subscription.
Expand Down Expand Up @@ -2061,34 +2048,10 @@ private async ValueTask PublishSubscriptionsAsync(int sleepCycle, CancellationTo

while (true)
{
DateTime start = HiResClock.UtcNow;

SessionPublishQueue[] queues = null;
ISubscription[] abandonedSubscriptions = null;

await m_semaphoreSlim.WaitAsync(cancellationToken).ConfigureAwait(false);
try
{
// collect active session queues.
queues = new SessionPublishQueue[m_publishQueues.Count];
m_publishQueues.Values.CopyTo(queues, 0);

// collect abandoned subscriptions.
if (m_abandonedSubscriptions != null && m_abandonedSubscriptions.Count > 0)
{
abandonedSubscriptions = new ISubscription[m_abandonedSubscriptions
.Count];

for (int ii = 0; ii < abandonedSubscriptions.Length; ii++)
{
abandonedSubscriptions[ii] = m_abandonedSubscriptions[ii];
}
}
}
finally
{
m_semaphoreSlim.Release();
}
// ConcurrentDictionary enumeration is thread-safe and provides a stable
// snapshot for the current pass without taking the manager semaphore.
SessionPublishQueue[] queues = [.. m_publishQueues.Values];
ISubscription[] abandonedSubscriptions = [.. m_abandonedSubscriptions.Values];

// check the publish timer for each subscription.
for (int ii = 0; ii < queues.Length; ii++)
Expand All @@ -2097,7 +2060,7 @@ private async ValueTask PublishSubscriptionsAsync(int sleepCycle, CancellationTo
}

// check the publish timer for each abandoned subscription.
if (abandonedSubscriptions != null)
if (abandonedSubscriptions.Length > 0)
{
var subscriptionsToDelete = new List<ISubscription>();

Expand All @@ -2120,17 +2083,9 @@ private async ValueTask PublishSubscriptionsAsync(int sleepCycle, CancellationTo
// schedule cleanup on a background thread.
if (subscriptionsToDelete.Count > 0)
{
await m_semaphoreSlim.WaitAsync(cancellationToken).ConfigureAwait(false);
try
{
for (int ii = 0; ii < subscriptionsToDelete.Count; ii++)
{
m_abandonedSubscriptions.Remove(subscriptionsToDelete[ii]);
}
}
finally
for (int ii = 0; ii < subscriptionsToDelete.Count; ii++)
{
m_semaphoreSlim.Release();
m_abandonedSubscriptions.TryRemove(subscriptionsToDelete[ii].Id, out _);
}

CleanupSubscriptions(m_server, subscriptionsToDelete, m_logger);
Expand Down Expand Up @@ -2327,7 +2282,7 @@ public override int GetHashCode()
private readonly int m_maxSubscriptionCount;
private readonly bool m_durableSubscriptionsEnabled;
private readonly ConcurrentDictionary<uint, ISubscription> m_subscriptions;
private List<ISubscription> m_abandonedSubscriptions;
private readonly ConcurrentDictionary<uint, ISubscription> m_abandonedSubscriptions;
private readonly NodeIdDictionary<Queue<StatusMessage>> m_statusMessages;
private readonly NodeIdDictionary<SessionPublishQueue> m_publishQueues;
private readonly ManualResetEvent m_shutdownEvent;
Expand Down
65 changes: 65 additions & 0 deletions Tests/Opc.Ua.Server.Tests/ReferenceServerTest.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -28,6 +28,7 @@
* ======================================================================*/

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
Expand Down Expand Up @@ -616,6 +617,70 @@ public async Task SubscriptionAsync()
await CommonTestWorkers.SubscriptionTestAsync(serverTestServices, m_requestHeader).ConfigureAwait(false);
}

/// <summary>
/// Create multiple sessions, each with a subscription.
/// Close all sessions without deleting subscriptions (abandoning them).
/// Concurrently delete the abandoned subscriptions from the main session.
/// Verifies that the concurrent dictionary backing abandoned subscriptions
/// handles parallel access correctly (fix for issue #3612).
/// </summary>
[Test]
public async Task DeleteAbandonedSubscriptionsConcurrentlyAsync()
{
const int sessionCount = 5;
var subscriptionIds = new UInt32Collection();

NamespaceTable namespaceUris = m_server.CurrentInstance.NamespaceUris;
NodeId[] testSet =
[
.. CommonTestWorkers.NodeIdTestSetStatic
.Select(n => ExpandedNodeId.ToNodeId(n, namespaceUris))
];

// Create multiple sessions, each with a subscription, then close
// the session without deleting subscriptions so they become abandoned.
for (int i = 0; i < sessionCount; i++)
{
(RequestHeader header, SecureChannelContext context) =
await m_server.CreateAndActivateSessionAsync($"AbandonSession_{i}")
.ConfigureAwait(false);

var services = new ServerTestServices(m_server, context);
header.Timestamp = DateTime.UtcNow;
UInt32Collection ids = await CommonTestWorkers.CreateSubscriptionForTransferAsync(
services, header, testSet, kQueueSize, -1).ConfigureAwait(false);
subscriptionIds.AddRange(ids);

// Close session without deleting subscriptions - makes them abandoned
header.Timestamp = DateTime.UtcNow;
await m_server.CloseSessionAsync(context, header, false, CancellationToken.None)
.ConfigureAwait(false);
}

// Concurrently delete all abandoned subscriptions from the main session.
var mainServices = new ServerTestServices(m_server, m_secureChannelContext);
var deleteTasks = new List<Task<DeleteSubscriptionsResponse>>();
foreach (uint id in subscriptionIds)
{
var singleId = new UInt32Collection { id };
m_requestHeader.Timestamp = DateTime.UtcNow;
deleteTasks.Add(
mainServices.DeleteSubscriptionsAsync(m_requestHeader, singleId)
.AsTask());
}

DeleteSubscriptionsResponse[] responses = await Task.WhenAll(deleteTasks)
.ConfigureAwait(false);

// All deletions should succeed.
foreach (DeleteSubscriptionsResponse response in responses)
{
Assert.AreEqual(StatusCodes.Good, response.ResponseHeader.ServiceResult);
Assert.AreEqual(1, response.Results.Count);
Assert.AreEqual(StatusCodes.Good, (uint)response.Results[0]);
}
}

/// <summary>
/// Create a secondary Session.
/// Create a subscription with a monitored item.
Expand Down
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