[None][fix] mpi_session: guard MpiPoolSession.shutdown against partial init

[lowsfer]

Summary

MpiPoolSession.shutdown() reads self.n_workers (in a log line) and self._wait_shutdown unguarded, but it is reachable on a session that never completed __init__:

• from __del__ → shutdown_abort() → shutdown() on a partially-constructed object, and
• from the released-session path, e.g. tests/unittest/executor/test_proxy_fast_death.py::test_pool_session_shutdown_never_blocks_after_release, which builds the object with MpiPoolSession.__new__(MpiPoolSession).

In those states the attributes are absent and shutdown() raises AttributeError: 'MpiPoolSession' object has no attribute 'n_workers' (then ... '_wait_shutdown'). The _pool_dead check immediately above already uses getattr(self, '_pool_dead', False) defensively for exactly this reason.

Fix

Guard both accesses with getattr(...), matching the existing _pool_dead style, so a released or partially-constructed session shuts down cleanly instead of raising.

Why main is currently red

This is a cross-PR merge skew, each half green on its own:

• the unguarded self.n_workers log line was added by [None][test] disagg startup: detect and fail fast on worker OOM/crash #16456 (2026-07-16), and
• the __new__-based test that trips it was added by [TRTLLM-13409][fix] make proxy shutdown non-blocking when the engine is dead #16312 (2026-07-20).

Neither failed in isolation; the combination on main fails deterministically. main has been red on test_pool_session_shutdown_never_blocks_after_release since #16312 landed, and the fix is a small robustness guard.

Test

test_proxy_fast_death.py now passes in full (23 passed, was 1 failed):

python -m pytest tests/unittest/executor/test_proxy_fast_death.py -q
# 23 passed

(No behavior change for fully-initialized sessions: n_workers / _wait_shutdown are always set by __init__; the guards only affect objects that bypass it.)

🤖 Generated with Claude Code

Summary by CodeRabbit

• Documentation

  • Improved formatting and organization of API documentation.
  • Clarified session initialization and external communication examples.

• Bug Fixes

  • Improved shutdown logging when the worker count is unavailable.
  • Preserved existing worker cleanup and shutdown behavior.

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• tensorrt_llm/llmapi/mpi_session.py

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📝 Walkthrough

Walkthrough

Changes

MPI session updates

Layer / File(s)	Summary
Docstrings and shutdown logging tensorrt_llm/llmapi/mpi_session.py	Docstrings were reformatted, and shutdown logging now uses a fallback worker count without changing pool teardown behavior.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Suggested reviewers: cascade812

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise, follows the required ticket/type format, and matches the main fix in the PR.
Description check	✅ Passed	The PR description covers the bug, fix, regression context, and test results, so it is mostly complete.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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