Preserve argv through systemd scope wrappers

[schickling-assistant]

Problem

On Linux, st2 launches managed tasks through systemd-run --user --scope. systemd-run expands dollar-bearing arguments by default, so caller-owned argv is not opaque: a literal $HOME:${UNSET}:$$ changes before it reaches the task.

Goal

Preserve every task program and argument byte-for-byte. Retain scope isolation, PTY/I/O inheritance, exit propagation, outliving-child membership, and collection when systemd supports opaque argv.

Decisions

Use systemd’s native --expand-environment=no before the command separator on systemd 254 or newer. Older systemd versions degrade to the existing detached launch with a warning instead of passing an unsupported option or changing caller-owned argv. Do not add shell quoting, st2-specific escaping, or deterministic scope names. Detached platforms, including macOS, retain the existing direct program/argv pass-through.

Verification

• nix develop --command cargo test --lib isolate::tests::wrap_ -- --nocapture — 2 passed.
• Linux systemd 260 E2E preserved $HOME, ${UNSET}, and $$; PTY status, descriptors, exit status, outliving-child membership, collection, and cleanup also passed.
• New decision and experiment records pass strict VRS shape checks.
• Version-capability tests reject systemd 249 and 252, accept 254 and 257, and reject malformed version output.
• Rebased onto current main.

Pre-flip deviations

• Full clippy stops at the unchanged manual_is_multiple_of warning in crates/st2-resource-protocol/src/lib.rs:410. The same package-level command fails identically on pristine current main.
• Full tests: branch 684 passed, 10 failed, 1 ignored. Pristine current main fails the same 10 macOS path/resync tests (677 passed, 10 failed, 1 ignored).

Complexity

No new abstraction or dependency. One wrapper option and mode-injected unit tests.

Concerns

--expand-environment=no requires systemd 254 or newer. Older Linux installations launch in degraded detached mode without cgroup isolation and emit a warning. macOS is unaffected because it already uses direct pass-through.

Friction & bottlenecks

The repository-wide clippy/test gates have reproducible pristine-main deviations listed above. Focused behavior tests and live Linux E2E evidence are green.

Follow-ups

None for argv transparency.

References

• Stack: Preserve argv through systemd scope wrappers #455 → Expose observed task resource targets #461
• VRS decision: docs/vrs/.decisions/0016-systemd-scope-wrappers-disable-environment-expansion.md

Posted on behalf of @schickling

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agent_persona	generalist
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[schickling]

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Review	Status	Commit	Review trigger
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