---
model: claude-opus-4-7[1m]
service: claude
session: subint-phase-b-hang-classification
timestamp: 2026-04-20T19:27:39Z
git_ref: HEAD (pre-commit; on branch `subint_spawner_backend`)
scope: docs
substantive: true
raw_file: 20260420T192739Z_5e8cd8b2_prompt_io.raw.md
---

## Prompt

Collab between user (`goodboy`) and `claude` to
disambiguate two distinct hang modes hit during
Phase B subint-spawn-backend bringup (issue #379).
The user ran the failing suites, captured `strace`
evidence on hung pytest pids, and set the framing:

> "These cannot be the same bug. Different fix
> paths. Write them up separately or we'll keep
> conflating them."

Follow-on asks:
- Cross-link each doc from its reproducer test so
  a future triager lands on the analysis without
  needing to grep `ai/conc-anal/`.
- On `test_stale_entry_is_deleted`: arm
  `dump_on_hang(seconds=20, path=...)` so a
  regression captures a stack dump (pytest stderr
  capture otherwise eats `faulthandler` output).
- Keep `test_stale_entry_is_deleted` un-`skip`ped
  so the dump file remains inspectable.

Defer the actual fix for the class-2 hang
(Ctrl-C-able, tractor-side bug) to a later commit
per the incremental-commits preference.

## Response summary

Wrote two new `ai/conc-anal/` classification docs
and cross-linked them from the reproducer tests:

1. **`subint_sigint_starvation_issue.md`** —
   class 1: abandoned-legacy-subint thread +
   shared GIL starves main trio loop →
   signal-wakeup-fd pipe fills → SIGINT silently
   dropped (`write() = EAGAIN`). Pytest process
   un-Ctrl-C-able. Structurally a CPython limit;
   blocked on `msgspec` PEP 684 support
   (jcrist/msgspec#563). Reproducer:
   `test_stale_entry_is_deleted[subint]`.

2. **`subint_cancel_delivery_hang_issue.md`** —
   class 2: parent-side trio task parks on an
   orphaned IPC channel after subint teardown;
   no clean EOF delivered to waiting receiver.
   Ctrl-C-able (main trio loop iterating fine).
   OUR bug to fix. Candidate fix: explicit
   parent-side channel abort in `subint_proc`'s
   hard-kill teardown. Reproducer:
   `test_subint_non_checkpointing_child`.

Test-side cross-links:
- `tests/discovery/test_registrar.py`:
  `test_stale_entry_is_deleted` → `trio.run(main)`
  wrapped in `dump_on_hang(seconds=20,
  path=<per-method-tmp>)`; long inline comment
  summarizes `strace` evidence + root-cause chain
  and points at both docs.
- `tests/test_subint_cancellation.py`:
  `test_subint_non_checkpointing_child` docstring
  extended with "KNOWN ISSUE (Ctrl-C-able hang)"
  section pointing at the class-2 doc + noting
  the class-1 doc is NOT what this test hits.

## Files changed

- `ai/conc-anal/subint_sigint_starvation_issue.md`
  — new, 205 LOC
- `ai/conc-anal/subint_cancel_delivery_hang_issue.md`
  — new, 161 LOC
- `tests/discovery/test_registrar.py` — +52/-1
  (arm `dump_on_hang`, inline-comment cross-link)
- `tests/test_subint_cancellation.py` — +26
  (docstring "KNOWN ISSUE" block)

## Human edits

Substantive collab — prose was jointly iterated:

- User framed the two-doc split, set the
  classification criteria (Ctrl-C-able vs not),
  and provided the `strace` evidence.
- User decided to keep `test_stale_entry_is_deleted`
  un-`skip`ped (my initial suggestion was
  `pytestmark.skipif(spawn_backend=='subint')`).
- User chose the candidate fix ordering for
  class 2 and marked "explicit parent-side channel
  abort" as the surgical preferred fix.
- User picked the file naming convention
  (`subint_<hang-shape>_issue.md`) over my initial
  `hang_class_{1,2}.md`.
- Assistant drafted the prose, aggregated prior-
  session root-cause findings from Phase B.2/B.3
  bringup, and wrote the test-side cross-linking
  comments.

No further mechanical edits expected before
commit; user may still rewrap via
`scripts/rewrap.py` if preferred.