pmdalmsensors, qa/1480: handle sensor read failures gracefully

[natoscott]

When sensors -j is called for a specific chip (e.g. NVMe) and the device returns EAGAIN, the empty or invalid output causes json.loads() to raise an exception, crashing the fetch callback. Catch JSON parse errors during fetch and preserve the last known sensor values instead.

At init time (no chip filter), re-raise the exception so the PMDA fails loudly rather than silently starting with zero metrics.

Also harden the QA test: ensure the awk sum is always numeric so _within_tolerance does not receive empty arguments.

Relates: #2665

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Run ID: 4f25ba9e-4143-4b4c-bd4d-41cef6043f9d

📥 Commits

Reviewing files that changed from the base of the PR and between 32cb4b1 and 5b519c5.

📒 Files selected for processing (2)

• qa/1480
• src/pmdas/lmsensors/pmdalmsensors.python

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Improved PMDA value aggregation so accumulated totals are consistently reported as numeric values, including zero.
  • Improved sensor data handling by separating output decoding from JSON parsing.
  • Chip-specific parsing failures now fail gracefully without interrupting sensor collection, while broader parsing errors continue to be reported.

Walkthrough

The PR updates PMDA numeric aggregation and changes lmsensors JSON parsing so chip-specific failures return silently while top-level failures propagate.

Changes

PMDA aggregation

Layer / File(s)	Summary
Normalize aggregated output qa/1480	The PMDA aggregation expression outputs sum + 0 instead of sum.

lmsensors parsing

Layer / File(s)	Summary
Handle lmsensors JSON parsing errors src/pmdas/lmsensors/pmdalmsensors.python	lmsensors_get decodes subprocess output before JSON parsing. Chip-specific JSON failures return without raising, while top-level failures are re-raised.

Possibly related PRs

• performancecopilot/pcp#2665: This PR also modifies lmsensors_get and relates to chip-specific JSON error handling.

Poem

A rabbit checks the sensor stream,
And sums each value bright and clean.
Bad chip data slips away,
While top-level errors choose to stay.
Zero joins the final score—
Hop, hop, safer than before!

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the affected PMDA and the main change: graceful handling of sensor read failures.
Description check	✅ Passed	The description accurately explains fetch-time error handling, initialization behavior, QA hardening, and the related issue.
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.

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