fix(interpreter): bound nested bash/sh child shells

[chaliy]

Requested by Михайло · project thread

What changed

Nested bash/sh child shells are bounded: each one counts against max_function_depth, plus a dedicated cap of 8 nested child shells. Past the cap the shell fails with maximum nesting depth exceeded (exit 2).

Why

A script that re-runs itself through sh crashed the whole host process with a stack overflow. In a multi-tenant process that aborts every tenant (new TM-DOS-125). Found while bounding recursive $(MAKE) for the upcoming make builtin.

Before

$ bashkit -c 'echo "sh s.sh" > s.sh; sh s.sh; echo rc=$?'
thread 'main' has overflowed its stack
fatal runtime error: stack overflow, aborting

After

$ bashkit -c 'echo "sh s.sh" > s.sh; sh s.sh; echo rc=$?'
sh: maximum nesting depth exceeded
rc=2

Risk

• Low. Real scripts rarely nest shells more than a few levels deep. The cap of 8 was measured: about 15 bare sh levels fit a 2 MiB debug-build stack, and a level costs several function calls. Recorded as L-PROC-004.
• Forked background jobs and pipeline stages inherit the depth, because they are polled on the caller's stack.

Checklist

• Failing test first (recursive_child_shell_is_bounded aborted the test binary before the fix)
• cargo fmt, cargo clippy --all-targets -D warnings
• bashkit lib + integration tests pass locally
• Threat model (knowledge + public), limitations, log updated

https://claude.ai/code/session_019aFikmptPc91Fj4N2iDXQA

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	bashkit	d42ae92	Commit Preview URL Branch Preview URL	Oct 07 2026, 09:25 AM