investigate: STL/heap usage reduction for ETL compatibility (#174)

[vtz]

Summary

Adds docs/investigations/stl-etl-reduction.md: a quantitative audit of heap-allocating STL usage in include/ and src/ (excluding tests/examples) for #174.

Key findings (high level)

• std::vector<uint8_t> is central to the message / serialization / SD / TP / RPC / events data path (~202 matching lines across 37 files). Bounded ETL replacement needs an explicit max payload policy (MTU-shaped ~1400, single-buffer TCP-ish up to 64 KiB, TP default 1 MiB in TpConfig, UDP datagram cap ~64 KiB; 32-bit length field allows larger logical sizes only if the stack and product allow it).
• std::unordered_map appears in 12 files; many maps are already effectively bounded (TP concurrency, session IDs, event IDs). Diagnostic enum→string maps are easy constexpr wins.
• std::function is used in public callbacks and in RTOS/ThreadX/Zephyr thread_impl via heap-allocated std::function—the platform path is a high-value target for etl::delegate or fn-pointer context.
• Message pooling on RTOS does not remove vector payload heap on POSIX or in the object model itself.
• Recommendation: ETL as optional (SOMEIP_USE_ETL-style) plus type aliases / capacity macros; not a required dependency for multi-platform builds.

Deliverable

Full tables, file/line references, categorization (easy / moderate / hard / not recommended), suggested follow-up tickets, and optional-vs-required ETL assessment are in the report.

Made with Cursor

Summary by CodeRabbit

• Documentation
  • Added an in-depth investigation report analyzing memory allocation patterns and container usage throughout the codebase. The report provides an inventory-based methodology, detailed findings organized by subsystem, identified optimization opportunities with priority classification, documentation of existing configuration constraints, and recommended implementation strategies with prioritized follow-up suggestions.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 946b8683-0252-4805-bb82-7f9f25ff369a

📥 Commits

Reviewing files that changed from the base of the PR and between 0f8b7a8 and 8f082ab.

📒 Files selected for processing (1)

• docs/investigations/stl-etl-reduction.md

📝 Walkthrough

Walkthrough

A comprehensive investigative documentation file analyzing STL container usage patterns and heap allocations throughout the codebase, providing an inventory-oriented methodology, categorized findings, and recommendations for potential Embedded Template Library (ETL) optimization.

Changes

Cohort / File(s)

Summary

STL/Heap Investigation Documentation docs/investigations/stl-etl-reduction.md

Added 292-line investigation document cataloging std::vector, std::string, std::unordered_map, smart pointers, std::function, std::queue, and raw heap allocations. Includes methodology, categorized findings by subsystem (core payload, serialization, transport, TP, SD, RPC/events), feasibility assessment ("easy wins," "moderate effort," "hard/not feasible"), and recommended approach with follow-up ticket suggestions.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

Possibly related issues

• Investigate reducing heap/STL usage in favor of ETL-compatible patterns #174 — This PR fulfills the investigation objectives by delivering a comprehensive inventory and analysis of STL/heap usage patterns with feasibility recommendations and prioritized follow-up actions.

Poem

🐰 Whiskers twitching through the code so deep,
We've mapped where vectors dance and heaps do leap!
STL's scattered—time to trim the fat,
A roadmap written, now here's where we're at! 🎯

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically summarizes the main change: an investigation document on reducing STL/heap usage for ETL compatibility, directly corresponding to the added investigation file.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch investigate/174-stl-etl-reduction

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[coderabbitai]

Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@docs/investigations/stl-etl-reduction.md`:
- Line 1: Remove the trailing whitespace that the pre-commit hook fixed in the
document titled "# Investigation: Reducing heap / STL usage for ETL
compatibility": open that file, strip any trailing spaces at the ends of lines
(or run the project's pre-commit hooks locally), save the file, and commit the
change so CI no longer fails.
- Line 30: Update the phrasing in the markdown: replace "very large" with
"arbitrarily large" or "unbounded" in the sentence that describes the on-wire
Length field, and change "consumers that" to "consumers who" when referring to
developers/organizations; the sentence also references
TpConfig::max_message_size and max_segment_size (in include/tp/tp_types.h) —
keep those symbols unchanged while applying the wording edits for improved
style.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: dfe1378f-f235-440a-afca-06498866291c

📥 Commits

Reviewing files that changed from the base of the PR and between 4b76d18 and 0f8b7a8.

📒 Files selected for processing (1)

• docs/investigations/stl-etl-reduction.md