fix(replica): upload only missing L0 gap files

[corylanou]

Summary

During L0 gap healing, upload only TXIDs absent from the remote L0 listing. Existing files above the gap are left untouched while the replica position still catches up.

This PR stacks on #1155 and must remain based on issue-1151-failed-upload-breaks-compaction until that PR merges.

Problem

The #1155 recovery path recalculates the replica position just before an interior L0 gap. The sync loop then uploads the entire suffix even though calcPos already listed the files that exist above the gap.

The regression test demonstrated the behavior before this fix: deleting TXID 3 from a remote 1-4 listing caused WriteLTXFile calls for both TXIDs 3 and 4. Only TXID 3 was missing.

Solution

Return the sorted remote L0 listing from calcPos and retain it for the catch-up sync. The upload loop advances past TXIDs covered by that listing and calls WriteLTXFile only for missing TXIDs. The listing remains available across upload-limited sync batches and is cleared after catch-up or an error.

Behavior Change

Gap-heal action	Before	After
Missing L0 TXID	Uploaded	Uploaded
Existing L0 TXID above gap	Re-uploaded	Skipped
Final replica position	Catches up	Catches up

Scope

In scope:

• Reuse the remote L0 listing during gap heal
• Preserve existing gap recovery and compaction semantics
• Assert that only the missing L0 TXID is written

Not in scope:

• Dual-writer lineage detection
• Conditional backend writes

Test Plan

• GOTOOLCHAIN=go1.25.13 go test -race -run "^(TestReplica_InvalidatePos_HealsL0Gap|TestDB_Compact/L0GapHealsViaReplica)$" -count=1 .
• GOTOOLCHAIN=go1.25.13 go test -race ./...
• GOTOOLCHAIN=go1.25.13 pre-commit run --all-files
• TestLTXBehavior: low-volume, high-volume, and burst-volume profiles passed all applicable assertions and restore integrity checks
• GOTOOLCHAIN=go1.25.13 go test -tags "integration,soak" -run "^TestLTXBehavior_NoExcessiveSnapshots$" -short -v -timeout 5m ./tests/integration/

Related

• Fixes replica: gap heal re-uploads the entire suffix, overwriting existing remote L0 files #1457
• Stacks on fix(compactor): stop compaction at TXID gap to prevent repeated failures #1155

[github-actions]

PR Build Metrics

⚠️ Attention needed — Go toolchain outdated (1.25.13 → 1.25.14)

Check	Status	Summary
Binary size	✅	37.17 MB (+4.0 KB / +0.01%)
Dependencies	✅	No changes
Vulnerabilities	✅	None detected
Go toolchain	⚠️	1.25.13 → 1.25.14 available
Module graph	✅	1230 edges (0)

Binary Size

	Size	Change
Base (ffe640d)	37.17 MB
PR (566c142)	37.17 MB	+4.0 KB (+0.01%)

Dependency Changes

No dependency changes.

govulncheck Output

=== Symbol Results ===

No vulnerabilities found.

Your code is affected by 0 vulnerabilities.
This scan also found 1 vulnerability in packages you import and 3
vulnerabilities in modules you require, but your code doesn't appear to call
these vulnerabilities.
Use '-show verbose' for more details.

Build Info

Metric	Value
Build time	43s
Go version	go1.25.13
Commit	566c142

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_{🤖 Updated on each push.}