Launch /bin/sh as interactive login shell from init

[dburkart]

Closes #900

Summary

• After the existing hello fork+exec+wait smoke tests, init now launches /bin/sh as an interactive login shell with a proper argv (["sh"]) and environment (HOME=/, PATH=/bin:/usr/bin, TERM=vt100, PS1=vibix$ ).
• If the shell exits (user types exit), init respawns it so the system stays usable.
• The init: launching /bin/sh smoke marker (already in SMOKE_MARKERS) fires before each launch attempt.

Test plan

• cargo xtask build passes
• cargo fmt --all -- --check passes
• cargo xtask smoke — smoke test environment has no KVM; CI will validate all markers including init: launching /bin/sh

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 33412e2d-3b61-44b0-b523-608b1d2ac3a2

📥 Commits

Reviewing files that changed from the base of the PR and between 94ba305 and 418ecd2.

📒 Files selected for processing (2)

• base/init/src/main.rs
• tests/fixtures/ext2_image.sha256

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📝 Walkthrough

Walkthrough

The init binary's /bin/sh launcher is reworked from a one-shot smoke test into a respawn loop that continuously launches an interactive shell with proper environment variables (HOME, PATH, TERM, PS1) and diagnostic logging. The test fixture hash is updated to reflect the changed init binary.

Changes

Init Shell Respawn Loop

Layer / File(s)	Summary
Diagnostic Constants base/init/src/main.rs	New SH_RESPAWN_MSG diagnostic added to log shell respawn events.
Core Respawn Implementation base/init/src/main.rs	Replaces single /bin/sh launch with a loop: fork creates child process, exec runs /bin/sh with minimal argv/envp (HOME, PATH, TERM, PS1 environment variables), parent calls wait4 to reap child, then logs respawn event and loops to relaunch shell on exit.
Test Fixture Update tests/fixtures/ext2_image.sha256	SHA256 hash updated to reflect the modified init binary containing respawn logic.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related issues

• #895: This PR implements the core epic objective to run an interactive /bin/sh from init with respawning behavior and proper environment setup.
• #900: This PR directly addresses the work items in this issue—fork+exec /bin/sh with minimal environment, respawn on exit, and diagnostic logging for init validation.

Possibly related PRs

• dburkart/vibix#892: Implements the /bin/sh shell runtime and command execution entrypoints that receive input from the interactive session launched by this PR's init respawn loop.

Poem

🐰 Shell respawns in loops sublime,
Init forks and execs in time,
Each exit births a new shell run,
Interactive vibes, oh what fun!
HOME and PATH set just right,
Userspace shell shines so bright! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: launching /bin/sh as an interactive login shell from init, which is the primary objective of the PR.
Description check	✅ Passed	The description is directly related to the changeset, detailing how init now launches /bin/sh with proper arguments, environment variables, and respawn behavior as outlined in issue #900.
Linked Issues check	✅ Passed	The PR successfully implements all coding requirements from issue #900: fork+exec /bin/sh without -c flag, provides minimal environment (HOME=/, PATH, TERM, PS1), handles stdin/stdout, implements respawn logic, and includes the smoke marker.
Out of Scope Changes check	✅ Passed	All changes are scoped to the interactive /bin/sh launch requirement. The modifications to init/src/main.rs implement the respawn loop and environment setup, while the test fixture hash update reflects the expected system behavior changes.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch m900-interactive-sh

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