Add optional parallel compression steps

[rohan-patnaik]

Summary

• add optional parallel execution for compression heavy steps via ZENPDF_COMPRESS_PARALLELISM
• keep default single-threaded behavior while allowing faster wall-clock when enabled
• document new setting in tool techniques and env example

Testing

• Not run (not requested)

[coderabbitai]

📝 Walkthrough

Walkthrough

This pull request introduces parallel execution capabilities for PDF optimization steps in the worker. A new environment variable ZENPDF_COMPRESS_PARALLELISM controls concurrent task execution. The main worker code refactors optimization workflows into modular task functions that execute concurrently when parallelism > 1, with sequential execution as the default behavior.

Changes

Cohort / File(s)	Summary
Configuration & Documentation apps/worker/.env.example, docs/TOOL_TECHNIQUES.md	Added ZENPDF_COMPRESS_PARALLELISM environment variable with default value of 1, and documented its purpose as controlling optional concurrent execution of heavy optimization steps.
Core Optimization Logic apps/worker/zenpdf_worker/tools.py	Refactored PDF optimization workflow to support parallel execution using ThreadPoolExecutor; extracted step recording into _make_step_entry function; modularized image optimization, pdfsizeopt, and ghostscript workflows into dedicated task functions returning structured outcomes; added conditional orchestration to run tasks concurrently when parallelism > 1; consolidated temporary file tracking across parallel tasks; expanded ghostscript workflow with probe, full run, and conditional retry phases.

Sequence Diagram(s)

sequenceDiagram
    participant Coord as Orchestrator
    participant ImgOpt as Image<br/>Optimization
    participant PdfOpt as pdfsizeopt<br/>Task
    participant GS as Ghostscript<br/>Task
    participant TempMgr as Temp File<br/>Manager

    rect rgba(100, 150, 200, 0.5)
    Note over Coord: Parallelism > 1 Flow
    Coord->>Coord: Evaluate task thresholds
    par Concurrent Execution
        Coord->>ImgOpt: Execute async
        ImgOpt->>ImgOpt: Optimize images
        ImgOpt-->>Coord: Return (steps, temps, candidates)
    and
        Coord->>PdfOpt: Execute async
        PdfOpt->>PdfOpt: Run pdfsizeopt
        PdfOpt-->>Coord: Return (steps, temps, candidates)
    and
        Coord->>GS: Execute async
        GS->>GS: Probe + Full run + Retry
        GS-->>Coord: Return (steps, temps, candidates)
    end
    end

    rect rgba(150, 200, 100, 0.5)
    Note over Coord: Post-Execution
    Coord->>Coord: Aggregate results<br/>from all tasks
    Coord->>TempMgr: Collect & deduplicate<br/>temp files
    TempMgr->>TempMgr: Remove artifacts
    Coord-->>Coord: Return optimized PDF
    end

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Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Poem

🐰 Hopping through code with parallel grace,
Threads now dance at a faster pace,
Image and PDF tasks intertwine,
Optimization steps compress just fine,
Faster PDFs hop off to the race! 🚀

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 7.69% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Add optional parallel compression steps' directly and accurately summarizes the main change: introducing optional parallelization for compression operations via the ZENPDF_COMPRESS_PARALLELISM environment variable.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch codex/compress-parallelism

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[rohan-patnaik]

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