[rjcloudsigma]

GitHub Code Quality reports 17 open findings on master. This change addresses every finding while preserving the Python 2.7 / Python 3.6+ API contract and existing runtime behavior.

Findings	Review and change
#22, #23, #24 — file not always closed	The existing helper already closed descriptors if flag setup failed, but obscured ownership from the analyzer. Keep descriptor acquisition and failure cleanup in the same scope, with an explicit ownership return from _open_at. Add three regression tests covering root, native/procfs open and duplicate-descriptor cleanup; verify the root remains usable after duplicate setup failure.
#19, #20, #21 — empty except	Explain the intentional numeric/sequence encoding fallbacks and best-effort destructor cleanup. Preserve exception behavior.
#13, #14, #15, #16, #17, #18 — mixed returns	Return None explicitly for skipped descriptor writes and unsuccessful tree/group/VM lookups. Preserve existing results.
#25, #26 — imprecise assertions	Use precise boolean identity assertions while still invoking ==. Using assertNotEqual here could bypass the custom equality method on Python 2.
#27, #28 — unused imports	Remove unused DictOrFlagFile and BaseFileInterface imports.
#12 — implicit string concatenation in a list	Name the intentionally concatenated child program before passing it to Popen; retain exactly the same program and stdin gate.

Validation: all 137 tests pass locally on Python 3.12; source and universal-wheel builds pass strict twine check; git diff --check passes. Independent code review found no actionable defects. All 16 GitHub checks pass, including Python 2.7 and 3.6–3.15, distribution installation, Linux cgroup integration, CodeQL, and Code Quality. Passing CI, passing Code Quality analysis.

No finding has been dismissed or suppressed. No version bump or release is included.