libpcp_web: fix double-free in pmwebapi_labelsetdup

[natoscott]

pmwebapi_labelsetdup() performs a shallow struct copy (*dup = *lp) and then selectively deep-copies json and labels — but only when nlabels > 0. Two problems:

1. When nlabels <= 0 (or json is NULL), the dup shares the json pointer with the source. When pmDiscoverInvokeLabelsCallBacks frees the source labelset after invoking callbacks, the stored duplicate retains the dangling pointer. The next label discovery cycle frees the old duplicate, double-freeing the json allocation.

2. The compound flag and hash pointer were never handled — if set, the dup shares the opaque hash with the source, again leading to a double-free via pmFreeLabelSets.

Fix both by clearing compound/hash immediately after the struct copy and, for the nlabels <= 0 / NULL-json case, NULLing json, jsonlen and labels before returning. This matches what __pmDupLabelSets in libpcp already does correctly for the same cases.

Resolves a pmproxy SIGABRT (double-free) in archive discovery:
pmFreeLabelSets ← pmSeriesDiscoverLabels ← pmDiscoverInvokeLabelsCallBacks

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

Fixes: #2686

[coderabbitai]

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Reviewing files that changed from the base of the PR and between 26d4ca5 and c7ed4fc.

📒 Files selected for processing (1)

• src/libpcp_web/src/util.c

🚧 Files skipped from review as they are similar to previous changes (1)

• src/libpcp_web/src/util.c

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Improved handling when duplicating label sets with no labels or missing data.
  • Ensured duplicated label sets start with clean state information, preventing stale metadata from being carried over.
  • Preserved existing duplication behavior for valid, populated label sets.
  • Increased reliability when processing label-based metadata and JSON responses.
  • Empty or invalid inputs now produce a properly initialized empty label set.

Walkthrough

pmwebapi_labelsetdup now resets duplicate state and safely handles nonpositive label counts or null JSON pointers. Valid inputs continue to duplicate the JSON data and label array.

Changes

Label set duplication

Layer / File(s)	Summary
Duplicate label set initialization src/libpcp_web/src/util.c	pmwebapi_labelsetdup clears hash and compound. It returns an initialized empty label set for empty or missing JSON data. Valid inputs retain JSON and label-array duplication.

Possibly related PRs

• performancecopilot/pcp#2659: Changes duplicate label set handling and cleanup.

Poem

A rabbit reset each label in the set,
Cleared stale state and left no debt.
With empty JSON, an empty set stayed,
While valid copies safely hopped away.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: fixing a double-free in pmwebapi_labelsetdup.
Description check	✅ Passed	The description explains the double-free cause, the pointer ownership fix, and the affected pmproxy crash.
Linked Issues check	✅ Passed	The changes address issue #2686 by preventing the pmproxy double-free crash during archive discovery.
Out of Scope Changes check	✅ Passed	The changes are limited to labelset duplication state and pointer handling required to fix issue #2686.

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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
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Inline comments:
In `@src/libpcp_web/src/util.c`:
- Around line 885-890: Reset dup->nlabels to 0 in the empty-duplicate branch of
the label duplication function before returning dup, alongside clearing json,
jsonlen, and labels. This must cover both lp->nlabels <= 0 and lp->json == NULL
cases so the returned duplicate cannot appear populated when labels is NULL.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

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

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository YAML (base), Repository UI (inherited), Organization UI (inherited)

Review profile: CHILL

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Run ID: bcecb745-92eb-4948-88cd-6d7a38a58af4

📥 Commits

Reviewing files that changed from the base of the PR and between 918d12b and 26d4ca5.

📒 Files selected for processing (1)

• src/libpcp_web/src/util.c