Restore coherent durable Subscription snapshots (OPC 10000-4)

[marcschier]

Failure

Durable Subscription restore depended on a process-local cache, while full snapshots were written record-by-record under shared mutable keys. A restart or concurrent writer could therefore expose stale, mixed, or partially committed Subscription state.

Fix

• Write each snapshot to an immutable generation and atomically publish a protected manifest.
• Restore only the committed generation, validate record identities and completeness, and retain legacy-key fallback.
• Replace the cache only after the full snapshot is authenticated and decoded, including explicit empty snapshots.

Standard

OPC 10000-4 §6.6.2.4.4 and §6.6.2.4.5.5 require durable Subscription and retransmission state to remain coherent across redundant Server failover and restart.

Tests

• dotnet test tests\Opc.Ua.Redundancy.Server.Tests\Opc.Ua.Redundancy.Server.Tests.csproj -f net10.0 --no-restore --nologo --verbosity minimal — 542 passed

• CustomTestTarget=net48 dotnet test tests\Opc.Ua.Redundancy.Server.Tests\Opc.Ua.Redundancy.Server.Tests.csproj --no-restore --nologo --verbosity minimal -m:1 — 542 passed

• Review follow-up: SharedKeyValueSubscriptionStoreTests — 39 passed on net10.0 and 39 passed on net48.

Split

Split from #4021.

[Copilot]

Pull request overview

Note

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Improves durability/coherence of persisted Subscription snapshots by moving to immutable, generation-based records with a committed manifest, and strengthening restore-time validation to prevent mixed/partial/tampered state from being loaded.

Changes:

• Persist snapshots into per-generation immutable keys and publish a protected manifest used for restore.
• Restore only from the committed generation (or legacy keys when manifest is absent) with added identity/completeness validation.
• Expand test coverage for concurrent snapshot commits, process restart behavior, empty snapshots, tamper/corruption handling, and legacy fallback.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
tests/Opc.Ua.Redundancy.Server.Tests/Subscriptions/SharedKeyValueSubscriptionStoreTests.cs	Adds/updates tests validating generation-commit semantics, restart/legacy fallback, and tamper/corruption behavior.
src/Opc.Ua.Redundancy.Server/Subscriptions/SharedKeyValueSubscriptionStore.cs	Implements generation+manifest snapshot persistence and stricter restore-time validation; updates cache replacement behavior.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 153 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.50%. Comparing base (70c74e7) to head (d8367ff).
⚠️ Report is 21 commits behind head on master.

Files with missing lines	Patch %	Lines
...r/Subscriptions/SharedKeyValueSubscriptionStore.cs	0.00%	153 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4045      +/-   ##
==========================================
- Coverage   73.55%   73.50%   -0.05%     
==========================================
  Files        1345     1345              
  Lines      180036   180213     +177     
  Branches    31677    31702      +25     
==========================================
+ Hits       132427   132470      +43     
- Misses      36911    37031     +120     
- Partials    10698    10712      +14     

Files with missing lines	Coverage Δ
...r/Subscriptions/SharedKeyValueSubscriptionStore.cs	0.00% <0.00%> (ø)

... and 29 files with indirect coverage changes

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