Treat zero MaxNotificationsPerPublish as unlimited (OPC 10000-4)

[marcschier]

Description

This focused change extracts the MaxNotificationsPerPublish conformance fix from #4021. It contains only Subscription.cs, SubscriptionManager.cs, and focused SubscriptionTests.

Failure

OPC UA defines zero as no notification limit. The message builder compared the queued count directly with zero, so an effective zero limit emitted no event or data-change notifications and left the queues pending. Separately, the revision logic treated a configured Server maximum of zero as a numeric cap, revising a non-zero Client request to zero and silently removing the Client's requested limit.

Fix

• Treat an effective zero limit as unlimited while draining event and data-change queues.
• When the Client requests zero, use the Server's configured maximum, which may itself remain zero/unlimited.
• When the Server maximum is zero, preserve a non-zero Client request, including uint.MaxValue.
• When the Server maximum is finite, cap zero, uint.MaxValue, and larger finite Client requests to that maximum.
• Cover the same revision matrix through both CreateSubscription and ModifySubscription.

This implements the zero-means-unlimited semantics in OPC 10000-4 v1.05.07 Sections 5.14.2.2 and 5.14.3.2. Those Service responses do not contain a revisedMaxNotificationsPerPublish field; the effective value is retained by the Server-side Subscription and its diagnostics.

Tests

• dotnet test tests\Opc.Ua.Server.Tests\Opc.Ua.Server.Tests.csproj -f net10.0 --filter "FullyQualifiedName~Opc.Ua.Server.Tests.SubscriptionTests" --no-restore: 25 passed.
• dotnet test tests\Opc.Ua.Server.Tests\Opc.Ua.Server.Tests.csproj -f net48 --filter "FullyQualifiedName~Opc.Ua.Server.Tests.SubscriptionTests" --no-restore: 25 passed.

The regression tests cover zero, finite, and uint.MaxValue Client requests against unlimited and finite Server limits for both CreateSubscription and ModifySubscription. They also verify that zero drains both event and data-change queues.

Related Issues

• Focused split from Harden security and reliability across the OPC UA stack #4021 (closed umbrella draft); no separate issue.

Checklist

• I have signed the CLA and read the CONTRIBUTING doc.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added all necessary documentation.
• I have verified that my changes do not introduce (new) build or analyzer warnings.
• 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.

[Copilot]

Pull request overview

Note

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Implements OPC UA “zero means unlimited” semantics for MaxNotificationsPerPublish, ensuring publish message construction drains notification queues when the effective limit is 0, and revising client-requested limits correctly against the server maximum.

Changes:

• Update subscription publish message construction to treat an effective 0 notification limit as unlimited.
• Fix max-notifications revision logic in SubscriptionManager to preserve non-zero client requests when the server max is 0/unlimited.
• Add regression tests for queue draining with a zero limit and for client/server limit revision behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
src/Opc.Ua.Server/Subscription/Subscription.cs	Treats server-side effective 0 as unlimited during message construction so queues drain.
src/Opc.Ua.Server/Subscription/SubscriptionManager.cs	Revises requested MaxNotificationsPerPublish using OPC UA zero/unlimited rules.
tests/Opc.Ua.Server.Tests/SubscriptionTests.cs	Adds targeted regression tests for zero/unlimited semantics and revision logic.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.74%. Comparing base (70c74e7) to head (7b0dc84).
⚠️ Report is 40 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Opc.Ua.Server/Subscription/Subscription.cs	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4047      +/-   ##
==========================================
+ Coverage   73.55%   73.74%   +0.18%     
==========================================
  Files        1345     1352       +7     
  Lines      180036   182979    +2943     
  Branches    31677    32148     +471     
==========================================
+ Hits       132427   134929    +2502     
- Misses      36911    37253     +342     
- Partials    10698    10797      +99     

Files with missing lines	Coverage Δ
.../Opc.Ua.Server/Subscription/SubscriptionManager.cs	74.58% <100.00%> (+1.28%)	⬆️
src/Opc.Ua.Server/Subscription/Subscription.cs	75.21% <85.71%> (+0.29%)	⬆️

... and 97 files with indirect coverage changes

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