Runaway CPU investigation for python3.13: unknown userspace loop at _PyEval_EvalFrameDefault

Prompt

## Plan Pass

You are planning a fixer patch before any edits happen.

Read the evidence bundle at `./evidence.json`. The prepared workspace is `./workspace` and it was acquired via `debian-source`. The original pre-edit snapshot is available at `./source` if you need to inspect it. For interpreter processes, plan from the script/application entrypoint evidence first and include the runtime only as a second investigation target unless the evidence proves a runtime bug.

Validation expectation: try the project-level build/test entrypoint from the workspace root before reporting only a focused leaf compile. Detected candidate(s): `./configure && make`. If the project-level command fails because dependencies or generated files are missing, include the exact command and failure reason in `## Validation`, then run the narrowest relevant compile/test that is still reproducible from a clean checkout.

Upstream-style expectation: before planning or editing, check for contribution/style docs (`CONTRIBUTING`, `HACKING`, `README-hacking`, `README.md`, `docs/`, `dev-docs/`) and scan the touched subsystem for local helpers. If the project has wrappers for file IO, path-relative IO, process spawning, memory allocation, logging, locking, or platform compatibility, prefer those wrappers over generic libc/std APIs. For public upstream review, prepare the final patch against the upstream default integration branch, usually `master` or `main`; use installed or stable source trees for reproduction/backport validation, not as the default merge-request base unless the contribution docs explicitly say to target a stable branch. Do not invent a reproducer or user-visible failure that is not in the evidence bundle; if the evidence is profiler-only or indirect, describe it as a targeted mitigation or stop with a diagnosis instead of presenting a speculative patch as a confirmed bug fix. In the plan and final validation, name any such helper, convention, branch-base choice, or evidence limit you found, or say that no relevant local helper was found. Treat this as a `python3.13` upstream patch, not just a Debian-local workaround.

Codex subagent/fan-out expectation: for any non-trivial investigation, do not keep all checking in the lead context. Proactively spawn bounded leaf subagents for independent source inspection, duplicate/upstream search, implementation sanity checks, and verification while the lead owns integration. Use the newest available mini model for read-only scouts, and use `gpt-5.3-codex-spark` aggressively for tiny coding, test, and verification leaves even when the lead model is `gpt-5.5`. Pass model names explicitly when spawning. Keep subagents as leaves: tell them not to spawn their own agents, not to run recursive `codex review`, not to perform public/external/destructive actions, and not to duplicate the same live-state operation. Merge their evidence into the final patch/report instead of merely saying a subagent looked. Inspect the relevant code, nearby callers, project contribution docs, and local helper/compat APIs, but do not edit files in this pass.

Return a short markdown plan with these exact sections:

## Problem
## Evidence Confidence
## Proposed Subject
## Patch Plan
## Risks
## Validation

Classify `## Evidence Confidence` as exactly one of `reproduced`, `observed`, or `inferred`. Use `inferred` only for a no-patch diagnosis/report plan unless you can name the extra evidence you will collect before editing; inferred source patches are blocked by Fixer because they are not pull-request-ready. For `observed` source-patch plans, plan to say in the final `## Issue Connection` that the failure was observed by Fixer and not independently reproduced. If the plan touches authentication, authorization, credentials, cryptography, sandboxing, permissions, timing/throttling behavior, or PostgreSQL database-core behavior such as dynamic library resolution, extension loading, transaction semantics, locking, planner/executor behavior, catalog changes, or storage changes, treat it as high-risk: do not plan a source patch unless validation can reproduce the behavior and analyze the semantic impact; otherwise plan a no-patch diagnosis/report. The plan must explain how the proposed code change addresses the observed issue evidence, call out any prior Fixer patch that should be improved or replaced, reject awkward control flow such as avoidable `goto` if there is a cleaner bounded alternative, name any local helper APIs or maintainer conventions the patch should follow, and keep the intended maintainer-facing explanation clear enough that someone unfamiliar with the local complaint wording can still follow the fix. In `## Validation`, name the reproducible configure/build/test entrypoint you will try from the workspace root before any focused leaf compile or smoke check, and include one bounded independent reproduction attempt for the collected failure signal when it is safe and cheap. Do not plan to claim `reproduced` unless that reproduction command or test can actually show the failure.

Response

## Plan Pass



## Workflow Note

model: gpt-5.5

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