feat: size the message cap to this device's records

[DavidCozens]

Pull request

What this stage adds

SOLIDSYSLOG_MAX_MESSAGE_SIZE drops from the library's default of 2048 to 256.
RFC 5424 section 6.1 says a receiver should accept 2048; over UDP, RFC 5426
section 3.2 guarantees only 480. This device's records are far shorter than
either, so 256 stays well inside every guarantee and anything longer is truncated
rather than dropped.

Flash +6,032 B (unchanged)
RAM +1,956 B (+1,520)
Log stack +680 B (+664)

The cost is all RAM, and not where it looks. Setting the cap made the log task
overflow its stack outright:

[device] FATAL: stack overflow in task log

The record is built on the stack of whichever task calls Log, and the log seam
was still at the FreeRTOS minimum it inherited from the baseline. Measured at 800
bytes used, so the seam grows to 2 KiB by the same two-fold rule everything else
here uses. That is 1,536 bytes of stack allocation against a net rise of 1,520:
the smaller cap and address pool give 16 back.

Worth knowing, and not yet explained: the previous stage formatted and sent the
same record on the same 512-byte stack and reported 136 bytes used, with no
overflow and the record arriving intact. Introducing the tunables file is what
moved the formatter onto the caller's stack. The failure was loud rather than
silent — configCHECK_FOR_STACK_OVERFLOW is 2, with a hook — which is the only
reason this is a paragraph and not a corruption bug.

Files

 CMakeLists.txt                     |  4 ++++
 README.md                          | 13 ++++++++-----
 app/AppConfig.h                    |  9 +++++----
 app/config/solid_syslog_tunables.h | 18 ++++++++++++++++++
 measurements/message-cap.csv       | 13 +++++++++++++
 measurements/stages.tsv            |  1 +
 run-report.md                      | 26 +++++++++++++-------------
 7 files changed, 62 insertions(+), 22 deletions(-)

Checklist

• The diff touches build infra (1 file(s)) — see Files above; deliberate for this stage.
• measurements/message-cap.csv committed.
• Row added to measurements/stages.tsv.
• README regenerated: python3 scripts/gen-cost-table.py.
• ./run.sh green (build + QEMU + baseline self-check) — run-report.md committed.

---

Context for review: 7 of 21 sequential single-commit PRs replaying the integration on top of the Baseline root. Each lands green and reviewed before the next is built on it, because a change to an early commit would force every commit above it to be re-run and re-measured.

Summary by CodeRabbit

• New Features

  • Added device-specific logging configuration with a 256-byte message limit and support for one address.
  • Added documentation for the new message-cap stage and updated resource usage figures.

• Performance

  • Increased the logging task’s reserved stack capacity to improve runtime reliability.

• Documentation

  • Updated run reports, measurements, and self-check results to reflect the message-cap configuration.

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 2332bb03-41aa-44ec-af51-239ba8c14cd9

📥 Commits

Reviewing files that changed from the base of the PR and between 719742e and b63fd21.

⛔ Files ignored due to path filters (2)

• measurements/message-cap.csv is excluded by !**/*.csv
• measurements/stages.tsv is excluded by !**/*.tsv

📒 Files selected for processing (5)

• CMakeLists.txt
• README.md
• app/AppConfig.h
• app/config/solid_syslog_tunables.h
• run-report.md

📝 Walkthrough

Walkthrough

The PR adds device-specific SolidSyslog tunables for a 256-byte message cap and one address, wires them through CMake, increases the logging task stack allocation, and updates README and run-report values for the message-cap configuration.

Changes

Message Cap Configuration

Layer / File(s)	Summary
SolidSyslog tunables and build wiring app/config/solid_syslog_tunables.h, CMakeLists.txt	Defines the 256-byte message limit and single-address pool, then configures CMake to use the new tunables header.
Logging task stack sizing app/AppConfig.h	Changes LOG_TASK_STACK_WORDS to four times configMINIMAL_STACK_SIZE and updates its comment.
Message-cap documentation and measurements README.md, run-report.md	Documents truncation and revised costs, and updates the run label, memory metrics, ELF size values, timestamp, and message-cap self-check references.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• cososo-ltd/solid-syslog-example#10: Adds related SolidSyslog build-time tunable integration.
• cososo-ltd/solid-syslog-example#18: Changes the same SOLIDSYSLOG_MAX_MESSAGE_SIZE tunable.
• cososo-ltd/solid-syslog-example#26: Changes the same logging task stack-size macro.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly reflects the main change: reducing the message cap for this device's records.
Description check	✅ Passed	All required sections are present and the checklist is mostly filled, though one item is intentionally contradicted by the build/config changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch stage/message-cap

---

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@README.md`:
- Around line 19-24: Update the RAM-cost prose in the README to match the
generated authoritative measurement of RAM +1,956 B, or explicitly clarify that
the existing 1,536-byte figure is only a subcomponent of the total. Keep the
generated cost-table block unchanged.

🪄 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: defaults

Review profile: CHILL

Plan: Pro

Run ID: df3f93c7-8986-4ee3-afd0-601f06986daf

📥 Commits

Reviewing files that changed from the base of the PR and between 47835a5 and 719742e.

⛔ Files ignored due to path filters (2)

• measurements/message-cap.csv is excluded by !**/*.csv
• measurements/stages.tsv is excluded by !**/*.tsv

📒 Files selected for processing (5)

• CMakeLists.txt
• README.md
• app/AppConfig.h
• app/config/solid_syslog_tunables.h
• run-report.md