Harden monit-access-helper.sh cgroupv2 mount point detection

[gberche-orange]

Restrict the inspection of /proc/self/mounts to cgroupv2 device (1st column) in addition to existing cgroup fstype (column 3).

Also fail fast in case of multiple detected mount points.

Fix #585

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 4d710f04-7760-4289-8d44-90db9bf4fd9b

📥 Commits

Reviewing files that changed from the base of the PR and between 1e4a114 and 50cc4d1.

📒 Files selected for processing (1)

• stemcell_builder/stages/bosh_monit/assets/monit-access-helper.sh

Walkthrough

This PR hardens the cgroup v2 mount resolution in the monit access helper script. The permit_monit_access function now validates cgroup2 filesystem mounts more strictly by matching on both the mount source and filesystem type fields from /proc/self/mounts, counts matching mounts, and verifies that the 0:: cgroup path exists. This prevents the script from failing when multiple cgroup2 filesystems are mounted in the system, such as when container orchestration tools like Cilium mount additional cgroup2 filesystems alongside the system's unified hierarchy.

Suggested reviewers

• aramprice
• mariash
• rkoster

🚥 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%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly and clearly summarizes the main change: hardening cgroupv2 mount point detection in monit-access-helper.sh.
Description check	✅ Passed	The description includes the required merge-forward strategy instructions and explains the specific changes (matching device column and fail-fast on multiple mounts), and references the linked issue #585.
Linked Issues check	✅ Passed	The PR addresses all coding requirements from issue #585: matching both device column (1st) and cgroup2 fs type (3rd column) and failing fast when multiple mount points are detected.
Out of Scope Changes check	✅ Passed	All changes are scoped to hardening the cgroupv2 mount point detection logic as required by issue #585; no unrelated changes are present.

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