inhibit: preserve source-only matches in equal-label index

[sueun-dev]

Pull Request Checklist

Please check all the applicable boxes.

• Please list all open issue(s) discussed with maintainers related to this change
  • None.
• Is this a new Receiver integration?
  • I have already tried to use the Webhook Receiver Integration and 3rd party integrations before adding this new Receiver Integration
• Is this a bugfix?
  • I have added tests that can reproduce the bug which pass with this bugfix applied
• Is this a new feature?
  • I have added tests that test the new feature's functionality
• Does this change affect performance?
  • I have provided benchmarks comparison that shows performance is improved or is not degraded
  • I have added new benchmarks if required or requested by maintainers
• Is this a breaking change?
  • My changes do not break the existing cluster messages
  • My changes do not break the existing api
• I have added/updated the required documentation (not needed; this restores the documented self-inhibition behavior)
• I have signed-off my commits
• I will follow best practices for contributing to this project

Which user-facing changes does this PR introduce?

[BUGFIX] Inhibition: Fix a case where a source-only alert could be ignored when another source alert with the same equal labels also matched the target side.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 34f2300f-eee9-4cfb-b77c-de5e4bc4e90e

📥 Commits

Reviewing files that changed from the base of the PR and between 55d0d79 and 3738527.

📒 Files selected for processing (2)

• inhibit/inhibit.go
• inhibit/inhibit_test.go

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📝 Walkthrough

Walkthrough

hasEqual now falls back to the alert cache when an indexed source alert also matches target matchers. Tests cover source-only inhibition with a newer two-sided matching alert.

Changes

Inhibition source lookup

Layer / File(s)	Summary
Cache fallback and regression coverage inhibit/inhibit.go, inhibit/inhibit_test.go	hasEqual scans uncanceled cached alerts for an eligible equal-label source alert when the indexed alert is excluded. Tests configure target matchers and verify the source-only inhibition case.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 37385

This localized bugfix preserves source-only inhibition matches without introducing an actionable merge-blocking risk; it is merge-ready after normal checks and review.

Sequence Diagram(s)

sequenceDiagram
  participant hasEqual
  participant IndexedSourceAlerts
  participant AlertCache
  hasEqual->>IndexedSourceAlerts: Find equal-label source alert
  IndexedSourceAlerts-->>hasEqual: Return indexed alert
  hasEqual->>AlertCache: Scan uncanceled alerts when two-sided match is excluded
  AlertCache-->>hasEqual: Return eligible source fingerprint or no match

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the inhibit area and the primary fix for preserving source-only matches.
Description check	✅ Passed	The description covers the checklist, bugfix tests, compatibility, sign-off, best practices, and user-facing release notes.
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 unit tests (beta)

• Create PR with unit tests

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[SoloJacobs]

@siavashs I believe this is a regression introduced in #4607 . Could you have a look?