[ktsaou]

Stacked on #16. Merge order: #13, #14, #15, #16, then this. Completes SOW-0137 (.agents/sow/done/SOW-0137-20260726-rust-writer-residual-array-open-performance.md).

Decisions for review (all agent-chosen; internal only, no byte or API change)

• C1, taken: the global ENTRY-array append opens the tail array mutably once to read its capacity and write the slot, instead of a read-only open followed by a mutable open.
• C2, taken: the regular DATA-array link keeps the tail capacity the chain walk already read, instead of reopening the tail.
• C3, taken (after review): a new entry array gets slot 0 written at allocation, instead of being reopened. The first rejection was wrong: every new DATA object allocates an array, so mixed workloads hit this 34,157 times per 20,000 rows. Entry-array HMACs cover only the object header, so sealing is unchanged.
• Not taken: regular-format DATA links still walk the array chain from the head. That is systemd's own regular-format algorithm, and removing it would need a new in-memory tail cache.

Mutable opens run the same type, size and bounds checks as read-only ones. Publication order is unchanged.

Measurement

Callgrind instructions at 20,000 rows, before → C1+C2 → C1+C2+C3:

Format	Workload	Before	C1+C2	+C3
compact	netflow	2119.1M	2114.3M (−0.2%)	2113.1M (−0.06%)
compact	mixed	3060.1M	3054.8M (−0.2%)	3041.5M (−0.44%)
regular	netflow	3980.7M	3777.0M (−5.1%)	3775.7M (−0.03%)
regular	mixed	3940.6M	3788.4M (−3.9%)	3774.9M (−0.36%)

Release timing, baseline vs C1+C2+C3, regular format, 20 ABBA pairs per workload at host load 36-41: mixed +3.7% median (faster in 19/20 pairs), netflow +12.1% (19/20). The host was loaded, so timing is secondary evidence; an earlier -1.4% mixed run is not reproduced but remains unexplained.

Validation

• Output is byte-identical to the baseline in all four configurations, with and without C3.
• Stock journalctl --verify passes all four outputs.
• cargo test --workspace passes.
• Live matrix (go, rust and stock readers × regular, zstd, compact, sealed): 8/8.

---

Summary by cubic

Removes redundant offset-array opens in the Rust writer so regular-format writes execute 3.9-5.1% fewer instructions with byte-identical output. Completes SOW-0137.

• The ENTRY-array append now opens the tail array mutably once instead of read-only then mutable.
• The regular DATA-array link keeps the tail capacity the chain walk already read instead of reopening it.
• Newly allocated entry arrays write slot 0 through the allocation guard, removing the reopen that followed each allocation.
• Mutable opens run the same type, size, and bounds checks as read-only ones; publication order is unchanged.
• Output is byte-identical in all four benchmark configurations, journalctl --verify passes, and the live reader matrix passes 8/8.

^{Written for commit 9dfa6e1. Summary will update on new commits.}

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[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

View in Codacy

🟢 Metrics -13 complexity · -3 duplication

Metric	Results
Complexity	-13
Duplication	-3

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AI Reviewer: first review requested successfully. AI can make mistakes. Always validate suggestions.

_{TIP This summary will be updated as you push new changes.}

[codacy-production]

Pull Request Overview

✅ No blocking issues noticed

No blocking implementation findings were identified. The changed paths lack automated tests covering reuse, growth, validation, and reader interoperability; these should be added to protect the refactoring.

About this PR

• Please add automated tests covering the changed ENTRY/DATA append paths, growth and linking behavior, malformed-input validation, and reader interoperability.

Test suggestions

• Append an ENTRY entry when the tail has capacity, verifying the slot and header updates.
• Append a regular DATA entry when the tail has capacity, using the capacity discovered during traversal.
• Exercise ENTRY and DATA array growth paths and verify correct linking and tail metadata.
• Reject malformed, wrong-type, undersized, or out-of-bounds arrays with the same validation behavior.
• Verify byte-identical output and interoperability with journal readers.

Prompt proposal for missing tests

Consider implementing these tests if applicable:
1. Append an ENTRY entry when the tail has capacity, verifying the slot and header updates.
2. Append a regular DATA entry when the tail has capacity, using the capacity discovered during traversal.
3. Exercise ENTRY and DATA array growth paths and verify correct linking and tail metadata.
4. Reject malformed, wrong-type, undersized, or out-of-bounds arrays with the same validation behavior.
5. Verify byte-identical output and interoperability with journal readers.

_{TIP Improve review quality by adding custom instructions
TIP How was this review? Give us feedback}

[cubic-dev-ai]

All reported issues were addressed across 3 files

Architecture diagram

sequenceDiagram
    participant W as Writer
    participant JF as JournalFile
    participant OA as OffsetArray

    Note over W,OA: ENTRY Array Append Flow (C1)
    W->>JF: entry_array_tail_entries()
    JF-->>W: tail_entries count
    W->>JF: offset_array_mut(tail_offset)
    JF->>OA: validate & open mutable
    OA-->>JF: capacity
    JF-->>W: tail_guard with capacity
    alt Room in tail array
        W->>OA: set(tail_entries, entry_offset)
        W->>JF: journal_header_mut()
        JF->>JF: update tail_entry_array_offset & n_entries
    else Tail array full
        W->>JF: drop(tail_guard)
        W->>JF: grow_entry_array_tail()
    end

    Note over W,OA: Regular DATA Array Link Flow (C2)
    W->>JF: append_data_entry_array_link()
    W->>JF: walk array chain from head
    JF->>OA: offset_array_ref (per array in chain)
    OA-->>JF: next_offset & capacity
    JF-->>W: tail_offset & tail_capacity (from walk)
    alt Entries fit in tail
        W->>JF: offset_array_mut(tail_offset)
        JF->>OA: validate & open mutable
        W->>OA: set slot with entry link
    else Tail full
        W->>JF: allocate & grow new array
        JF-->>W: new tail offset
        W->>JF: offset_array_mut(new_tail)
        W->>OA: set slot 0
    end

    Note over W,OA: Validation Remains (all paths)
    JF->>JF: type/size/arena checks before mutation

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