fix(daemon): recover stale service PID files safely

[wizzomafizzo]

Summary

• Recover a confirmed foreign/reused PID during service Start, under the existing start gate, without signaling that process or changing PID-file format.
• Keep unknown/inaccessible process identity fail-closed. Stop and Restart still refuse unrelated PIDs.
• Distinguish cached service executables and shell script arguments from unrelated processes merely mentioning a service file; retain deleted-executable identity across binary replacement.
• Cover stale-PID recovery with competing starts, healthy-service idempotence, unrelated-process survival and unknown identity.

Ref ZaparooProject/Main_MiSTer#24. This is the Core-owned cleanup companion to Main's watchdog identity check; Core should ship before relying on automatic PID-reuse recovery.

Validated with task test, task lint, task deadlock, task cross-lint:all and task vulncheck. Original code failed the recovery regression. Vulnerability scan reports no called vulnerable code; it notes uncalled dependency advisories. No device service, deployment or release changes. Work isolated from the owner's active checkout.

Summary by CodeRabbit

• Bug Fixes
  • Daemon startup now recovers from stale PID files that reference unrelated running processes.
  • Unrelated processes are no longer signaled during PID-file recovery.
  • Process identity checks are stricter, improving reliability when validating service processes.
  • Repeated or concurrent starts continue to maintain a single service process.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: fca89297-245d-4ca2-aa3c-75340286b7ac

📥 Commits

Reviewing files that changed from the base of the PR and between d346a2e and 3cf1dfa.

📒 Files selected for processing (2)

• pkg/service/daemon/daemon.go
• pkg/service/daemon/daemon_test.go

Included review availability: Your plan provides up to 2 included reviews per hour; 1 remains after this review.

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📝 Walkthrough

Walkthrough

The daemon now uses typed PID mismatch errors and explicit process identity checks. Startup removes a confirmed stale PID file without signaling the unrelated process. Tests cover concurrent recovery, shell identity validation, and unavailable process identities.

Changes

Daemon PID handling

Layer / File(s)	Summary
Process identity validation pkg/service/daemon/daemon.go, pkg/service/daemon/daemon_test.go	Process identity checks now return errors, handle deleted executables, and restrict shell-backed matches to valid service interpreter arguments. Tests cover rejected arguments and unavailable identities.
Stale PID recovery during startup pkg/service/daemon/daemon.go, pkg/service/daemon/daemon_test.go	Start removes an unchanged PID file after confirming that its PID belongs to another process. Concurrent starts publish one service PID and do not signal the unrelated process.

Priority: ⬇️ Low

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

Merge Risk: ⚪ Minimal · up to 3cf1d

Daemon startup now safely replaces confirmed stale PID files without signaling unrelated processes, while preserving fail-closed handling for unknown identities. The covered recovery and idempotence behavior is ready to merge.

Sequence Diagram(s)

sequenceDiagram
  participant Start
  participant Running
  participant serviceProcessIdentity
  participant PIDFile
  Start->>Running: Check recorded PID
  Running->>serviceProcessIdentity: Identify recorded PID
  serviceProcessIdentity-->>Running: Confirm foreign process
  Running-->>Start: Return servicePIDMismatchError
  Start->>PIDFile: Re-read and remove unchanged stale PID
  Start-->>Start: Launch service

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: safely recovering stale service PID files during daemon startup.
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.

• Fix all pre-merge checks with AI

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

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[codecov]

Codecov Report

❌ Patch coverage is 78.78788% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/service/daemon/daemon.go	78.78%	7 Missing ⚠️

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