fix: use pg_lsn cast for LSN comparison in slot advance resource

[tsivaprasad]

Summary

This PR fixes an issue where ReplicationSlotAdvanceFromCTSResource compared target and current LSNs using Go’s string <= operator, which can produce incorrect results across segment boundaries (for example, "F/FFFFFFFF" is lexicographically greater than "10/00000000" even though it is numerically smaller). This could silently skip replication slot advancement during add-node workflows. The fix delegates the comparison to PostgreSQL’s native pg_lsn type for correct LSN ordering.

Changes

• Add LsnAtOrBefore(lsn1, lsn2 string) query to postgres/create_db.go using:
  SELECT @lsn1::pg_lsn <= @lsn2::pg_lsn

• Replace the Go string comparison (targetLSN <= currentLSN) in ReplicationSlotAdvanceFromCTSResource.Create() with LsnAtOrBefore() to ensure correct LSN ordering semantics.

PLAT-627

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

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Run ID: ecce51e9-5725-4652-b1af-30098ca7c2ec

📥 Commits

Reviewing files that changed from the base of the PR and between 00fd2d7 and b538cdb.

📒 Files selected for processing (2)

• server/internal/database/replication_slot_advance_from_cts_resource.go
• server/internal/postgres/create_db.go

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📝 Walkthrough

Walkthrough

This PR replaces Go string-based LSN comparison with a PostgreSQL-native comparison. A new LsnAtOrBefore query helper is introduced to safely compare LSN values using pg_lsn type casting. The replication slot advancement logic is updated to use this helper instead of direct string ordering.

Changes

LSN Comparison and Replication Slot Advancement

Layer / File(s)	Summary
PostgreSQL LSN comparison helper server/internal/postgres/create_db.go	New exported LsnAtOrBefore(lsn1, lsn2 string) Query[bool] function that performs LSN comparison in SQL by casting both inputs to PostgreSQL pg_lsn type.
Replication slot advancement with database LSN comparison server/internal/database/replication_slot_advance_from_cts_resource.go	ReplicationSlotAdvanceFromCTSResource.Create now uses postgres.LsnAtOrBefore(targetLSN, currentLSN) to determine whether advancement is necessary, replacing prior in-code string comparison and adding early return when slot position already satisfies the target.

Poem

A rabbit hops through slots so grand,
Comparing LSNs across the land—
No more string tricks, we trust the source,
PostgreSQL guides our faithful course! 🐰✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely describes the main fix: using PostgreSQL's pg_lsn type for correct LSN comparison instead of Go string comparison, which is the core issue addressed in this PR.
Description check	✅ Passed	The description comprehensively covers the required template sections: Summary explains the bug, Changes lists both modifications, Testing section is present, Checklist is included, and the issue is linked via PLAT-627.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
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.

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✨ Finishing Touches

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• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch PLAT-627-lsn-string-comparison-in-replication-slot-advance-from-cts-resource-causes-silent-slot-advance-skip

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