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Marc Schier 0fbc900919 Add opt-in SubscriptionRecoveryPolicy for unsolicited Good_SubscriptionTransferred (#3540) (#3847)
# Description

Adds an opt-in `SubscriptionRecoveryPolicy` for both the Classic V1 and
V2 subscription engines that lets the client recover from server-emitted
`Good_SubscriptionTransferred` notifications which arrive against a
freshly created subscription (observed against Kepware after a server
re-initialise, very likely a leak from the pre-restart session's pending
status notifications onto the new session — abetted by Kepware re-using
`SubscriptionId = 1` across generations).

Per [OPC UA Part 4
§5.14.7](https://reference.opcfoundation.org/Core/Part4/v105/docs/5.14.7),
`Good_SubscriptionTransferred` is supposed to be delivered to the *old*
Session whose subscription was transferred away via
`TransferSubscriptions`. The existing V1 behaviour of stopping the
per-subscription publish loop on receipt is therefore
**spec-compliant**; the issue is that some servers deliver the
notification on a Session that is still actively using the subscription,
leaving the per-subscription dispatch silently disabled.

The fix is **opt-in** and **non-breaking**:

- New `SubscriptionRecoveryPolicy` enum (`ReportOnly` default,
`RecreateOnUnsolicitedTransfer`).
- V1 Classic `Subscription.RecoveryPolicy` property; V2
`Subscriptions.SubscriptionOptions.RecoveryPolicy` init property.
- When the policy is `RecreateOnUnsolicitedTransfer`, the SDK drops
queued acks for the dead subscription id (avoiding
`BadSubscriptionIdInvalid` ack errors on servers that re-use ids) and
recreates the subscription in place on the same Session.

## What changed

**Public API additions (non-breaking, defaults preserve current
behaviour):**
- `SubscriptionRecoveryPolicy` enum (`ReportOnly`,
`RecreateOnUnsolicitedTransfer`).
- V1 `Subscription.RecoveryPolicy` property.
- V2 `Subscriptions.SubscriptionOptions.RecoveryPolicy` init property.
- Internal `IMessageAckQueue.DropPendingForSubscription(uint)` and
protected `MessageProcessor.AckQueue` accessor.

**V1 Classic —
`Libraries/Opc.Ua.Client/Subscription/Classic/Subscription.cs`:**
- Replaces the inline `Good_SubscriptionTransferred` branch with
`HandleGoodSubscriptionTransferred`. `ReportOnly` preserves the original
behaviour (stop publish timer/worker); `RecreateOnUnsolicitedTransfer`
triggers `TriggerUnsolicitedTransferRecovery`, guarded by
`m_recreateAfterTransferInProgress` for idempotency.
- Recovery drains pending acks via the new
`ClassicSubscriptionEngine.RemoveAcknowledgementsForSubscription`
**before** resetting the subscription, then runs `DeleteSubscription()`
+ `CreateAsync()` on the same session so the publish worker restarts via
`StartKeepAliveTimer`.
- Discriminator (`Session != null && !Session.Reconnecting && Created`)
defers to the reconnect pipeline if it is already running.

**V2 — `Libraries/Opc.Ua.Client/Subscription/Subscription.cs`:**
- Completes the `OnStatusChangeNotificationAsync` TODO: now always fires
`OnPublishStateChanged(mask)` so `PublishState.Transferred` actually
reaches handlers (previously the no-op override silently swallowed the
flag).
- Under `RecreateOnUnsolicitedTransfer`, calls
`AckQueue.DropPendingForSubscription(deadId)` then existing
`ResetToRecreateAsync` on a worker task — no service calls from the
dispatch path.
- V2 `SubscriptionManager` implements `DropPendingForSubscription` via
drain-and-refill on the prioritised ack channel.

**Tests — `Tests/Opc.Ua.Client.Tests/Subscription`:**
- V1 in `Classic/SubscriptionUnitTests.cs`:
`ReportOnlyPolicyDoesNotRecreateOnUnsolicitedTransferAsync`,
`RecreatePolicyRecreatesSubscriptionOnUnsolicitedTransferAsync`,
`RecreatePolicyCollapsesConcurrentNotificationsAsync`,
`RecreatePolicyDoesNotRecreateWhileSessionReconnectingAsync`.
- V2 in `SubscriptionTests.cs`:
`GoodSubscriptionTransferredRaisesPublishStateTransferredAsync`,
`GoodSubscriptionTransferredRecreatesSubscriptionWhenPolicyEnabledAsync`,
`GoodSubscriptionTransferredCollapsesConcurrentRecreatesAsync`.
- `FakeMessageAckQueue` gained a `DroppedSubscriptions` recorder so the
V2 test can assert ack pruning ran.

**Docs — `Docs/TransferSubscription.md`:**
- New "Recovering from unsolicited `Good_SubscriptionTransferred`"
section with spec context, usage examples for both engines, and the
caveat that recovery is recreate-based — **not** lossless failover; the
server-side retransmission queue and triggering relationships tied to
the invalidated subscription id are not preserved.

## Validation

| Scope | Result |
| --- | --- |
| `Libraries/Opc.Ua.Client` build (`net10.0`, `net48`) | 0 warnings, 0
errors |
| `Tests/Opc.Ua.Client.Tests` build (`net10.0`, `net472`) | 0 warnings,
0 errors |
| New tests on `net10.0` and `net472` | 7/7 pass on each |
| Full subscription unit-test suite (`SubscriptionUnitTests` +
`SubscriptionTests` + `MessageProcessorTests` +
`SubscriptionManagerTests`) | 110/110 pass on each TFM |
| Entire `Opc.Ua.Client.Tests` suite (`net10.0`) | 1268/1268 pass |
| Classic transfer integration tests
(`Tests/Opc.Ua.Subscriptions.Classic.Tests`, `net10.0`) | 20/20 pass |
| V2 transfer compliance tests (`Tests/Opc.Ua.Subscriptions.Tests`,
`net10.0`) | 9/9 pass |

> Marked as **draft** because the maintainer wanted to first confirm
whether the stack is spec-compliant before deciding which knobs to ship;
this PR documents the investigation (stack is compliant), and surfaces
an opt-in recovery policy that addresses the field symptom without
changing the default behaviour. Convert to ready-for-review once the
design intent is acknowledged.

## Related Issues

- Fixes #3540

## Checklist

- [x] I have signed the
[CLA](https://opcfoundation.org/license/cla/ContributorLicenseAgreementv1.0.pdf)
and read the
[CONTRIBUTING](https://github.com/OPCFoundation/UA-.NETStandard/blob/master/CONTRIBUTING.md)
doc.
- [x] I have added tests that prove my fix is effective or that my
feature works and increased code coverage.
- [x] I have added all necessary documentation.
- [x] I have verified that my changes do not introduce (new) build or
analyzer warnings.
- [x] I ran **all** tests locally using the **UA.slnx** solution against
at least .net **framework** and .net **10**, and all passed.
- [ ] I fixed **all** failing and flaky tests in the CI pipelines and
**all** CodeQL warnings.
- [ ] I have addressed **all** PR feedback received.

---------

Co-authored-by: marcschier <marc.schier@github.com>
2026-06-07 17:38:58 +02:00

111 lines
4.3 KiB
C#

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using System;
namespace Opc.Ua.Client.Subscriptions
{
/// <summary>
/// Subscription options
/// </summary>
public record class SubscriptionOptions
{
/// <summary>
/// The subscription is disabled
/// </summary>
public bool Disabled { get; init; }
/// <summary>
/// Set keep alive count
/// </summary>
public uint KeepAliveCount { get; init; }
/// <summary>
/// The life time of the subscription in counts of
/// publish interval.
/// LifetimeCount shall be at least 3*KeepAliveCount.
/// </summary>
public uint LifetimeCount { get; init; }
/// <summary>
/// Set desired priority of the subscription
/// </summary>
public byte Priority { get; init; }
/// <summary>
/// Set desired publishing interval
/// </summary>
public TimeSpan PublishingInterval { get; init; }
/// <summary>
/// Set desired publishing enabled
/// </summary>
public bool PublishingEnabled { get; init; }
/// <summary>
/// Set max notifications per publish
/// </summary>
public uint MaxNotificationsPerPublish { get; init; }
/// <summary>
/// Set min lifetime interval
/// </summary>
public TimeSpan MinLifetimeInterval { get; init; }
/// <summary>
/// When the V2 manager restores this subscription via
/// <see cref="ISubscriptionManager.LoadAsync"/> with
/// <c>transferSubscriptions: true</c>, request the server to
/// send the latest cached value of every monitored item as
/// part of the take-over (OPC UA Part 4 §5.13.7
/// <c>TransferSubscriptions</c>'s <c>sendInitialValues</c>
/// argument). Defaults to <c>false</c>: the server only sends
/// values that arrived after the subscription was suspended,
/// matching the post-disconnect semantics that the V2 manager
/// expects for failover-on-recreate.
/// </summary>
public bool SendInitialValuesOnTransfer { get; init; }
/// <summary>
/// Controls how the client reacts to an unsolicited
/// <c>Good_SubscriptionTransferred</c> StatusChangeNotification
/// from the server. Defaults to
/// <see cref="SubscriptionRecoveryPolicy.ReportOnly"/> which
/// matches the spec-strict behaviour (OPC UA Part 4 §5.14.7).
/// Set to
/// <see cref="SubscriptionRecoveryPolicy.RecreateOnUnsolicitedTransfer"/>
/// to automatically recreate the subscription on the same
/// session when the notification arrives without the client
/// having initiated a TransferSubscriptions request — this
/// addresses server quirks that leave subscriptions stuck.
/// </summary>
public SubscriptionRecoveryPolicy RecoveryPolicy { get; init; }
}
}