workflow-fix: bootstrap_pod.sh python shim deadlocks uv interpreter discovery on pod venv rebuilds (uv.lock branch change)

Provenance

workflow_fix_target: scripts/bootstrap_pod.sh Surfaced by the experimenter on pod-2225-fu1 (task #2225 fu1 round, 2026-08-13) as a formal workflow-fix-candidate block; auto-filed by the #2225 follow-up-round orchestrator per .claude/rules/workflow-fix-on-bug.md. Runbook persisted to experimenter agent memory @ commit ca87126778.

Symptom

The bootstrap-installed /usr/local/bin/python shim (exec uv run python) recursively invokes uv during uv sync's interpreter discovery; the nested uv run blocks on the project lock the parent uv sync holds — a silent futex deadlock with stacked get_interpreter_info probes (~1 every 5 min, zero output). Fires on ANY pod whose checked-out branch changes uv.lock after bootstrap (observed pod-2225-fu1, 2026-08-13: branch checkout changed uv.lock → implicit sync → deadlock).

Adjacent traps from the same incident (document alongside the fix): MooseFS errno-116 (stale file handle) hits FRESH /workspace venvs too, with serialized installs at ~1.3 MB/s (multi-hour ETA); the working recovery is building the venv on the overlay disk (/root/eps-venv) and symlinking .venv to it — uv run resolves the symlink; flash-attn must be re-installed after any rebuild (bootstrap installs it outside the lock).

Proposed fix (candidate's sketch, confidence: high)

• Make the shim non-recursive for uv's probes: exec the resolved venv python binary directly, or short-circuit when the parent is uv (e.g. UV_INTERNAL set); AND/OR
• Export UV_PYTHON=/usr/bin/python3.11 system-wide in bootstrap so interpreter discovery never executes the shim.
• Consider defaulting pod venvs onto the overlay disk (/root/eps-venv + .venv symlink) in bootstrap to dodge the MooseFS errno-116 + throughput trap for all future rebuilds, and re-adding flash-attn post-rebuild.
• Add a .claude/rules/gotchas.md entry for the three stacked traps (the failure-lesson block marked gotcha_candidate: yes).

Acceptance

• On a pod whose branch changes uv.lock post-bootstrap, uv run python -c "import torch" triggers a sync that completes without deadlock.
• Shim behavior for interactive python unchanged.
• Gotchas entry landed; bootstrap change smoke-tested on a fresh provision.