fix(archive): keep checkpoint reads consistent

[yordis]

• Concurrent archive readers must never observe a truncated checkpoint while the archiver advances it.
• A complete checkpoint boundary keeps restart and catch-up validation deterministic across architectures.
• Windows checkpoint updates must avoid the open-destination overwrite failure tracked in dotnet/runtime#114230 so readers remain reliable across supported platforms.

[cursor]

PR Summary

Low Risk
Changes are confined to test-local archive storage and new XUnit coverage; no production archive path in this diff.

Overview
Archive checkpoint I/O in test LocalArchiveStorage is updated so concurrent readers never see a torn or truncated 8-byte checkpoint while the archiver advances it.

GetCheckpoint now opens the checkpoint file with FileShare.ReadWrite | FileShare.Delete so reads can continue while the file is replaced. SetCheckpoint writes the full value to a GUID-named temp file, then atomically File.Replace (or File.Move on first create) instead of overwriting in place.

New LocalArchiveStorageTests cover replacing the checkpoint with a reader handle held open and a multi-reader stress run (2,000 updates) that asserts every read stays within valid checkpoint bounds.

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Configuration used: Organization UI

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📥 Commits

Reviewing files that changed from the base of the PR and between f816350 and 21d248f.

📒 Files selected for processing (2)

• src/EventStore.Core.Tests/TransactionLog/LocalArchiveStorage.cs
• src/EventStore.Core.XUnit.Tests/Services/Archive/Storage/LocalArchiveStorageTests.cs

Walkthrough

LocalArchiveStorage now persists checkpoints through temporary-file replacement and permits compatible shared reads. A concurrent XUnit test validates that readers do not observe partial checkpoint values during repeated writes.

Changes

Checkpoint persistence

Layer / File(s)	Summary
Atomic checkpoint read and write src/EventStore.Core.Tests/TransactionLog/LocalArchiveStorage.cs	GetCheckpoint uses shared read and delete access. SetCheckpoint writes to a GUID-suffixed temporary file, replaces the checkpoint file, and cleans up the temporary file.
Concurrent checkpoint integrity test src/EventStore.Core.XUnit.Tests/Services/Archive/Storage/LocalArchiveStorageTests.cs	The test performs 10,000 checkpoint updates while multiple readers load checkpoints and verify that each value remains within the valid range.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🟡 Moderate · up to f8163

The change is intended to guarantee complete checkpoint visibility during concurrent reads, but the concurrency test may perform zero overlapping reads and therefore pass without validating that guarantee. Merge readiness requires coordinating the readers and writer or asserting that an overlapping read occurred.

Poem

A rabbit writes numbers in a neat little row
While readers hop round to see what they know
Temp files appear, then checkpoints take flight
No half-written values disturb the moonlight
“Atomic,” says Bunny, “that feels just right!”

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main change: keeping archive checkpoint reads consistent during concurrent updates.
Description check	✅ Passed	The description directly explains the concurrency issue and the required consistent checkpoint boundaries addressed by the changes.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches 💡 1

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• Create PR with unit tests
• Commit unit tests in branch yordis/fix-archive-checkpoint-publication

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Actionable comments posted: 1

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Inline comments:
In
`@src/EventStore.Core.XUnit.Tests/Services/Archive/Storage/LocalArchiveStorageTests.cs`:
- Line 35: Update the concurrent checkpoint test around SetCheckpoint to
coordinate writer and reader execution so at least one reader overlaps the write
loop before writerCompleted is set. Track successful checkpoint reads and assert
that the count is greater than zero, while preserving the existing atomicity
assertions.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Team

Run ID: 7bd3d536-15e0-4904-b34e-3681d7fdd5fc

📥 Commits

Reviewing files that changed from the base of the PR and between c77a9af and f816350.

📒 Files selected for processing (2)

• src/EventStore.Core.Tests/TransactionLog/LocalArchiveStorage.cs
• src/EventStore.Core.XUnit.Tests/Services/Archive/Storage/LocalArchiveStorageTests.cs

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Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

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