[None][test] Make the session-reuse pool seam isinstance-transparent

[sunnyqgg]

Summary

• The session-reuse layer ([None][test] Optimize LLM test startup overhead #15777) replaces the MpiPoolSession attribute of three library modules with a factory function at test time. Any library code doing isinstance(x, MpiPoolSession) against a patched module then raises TypeError: isinstance() arg 2 must be a type.
• [https://nvbugs/6435642][fix] detect killed MPI executor workers #16338 added exactly such a check in proxy.py (killed-worker detection). The combination broke every bare LLM() creation on post-7/15 bases: ~8000 test failures across 15+ builds and 12+ PRs in two days, including post-merge main (e.g. build 48117 / PR [None][fix] avoid attention workspace resize during CUDA graph capture #16319 as an innocent victim). [https://nvbugs/6435642][fix] handle session reuse worker registration #16444 worked around it consumer-side with an exclusion-based match.
• This PR removes the hazard class at its source: the seam now holds a real class whose metaclass routes construction to the reuse factory (__call__) and instance/subclass checks to the real MpiPoolSession (__instancecheck__/__subclasscheck__). Both usage patterns work, including isinstance consumers added in the future — no more whack-a-mole.
• Follow-up option: _register_worker_processes in proxy.py can revert its exclusion-based match back to the natural isinstance(self.mpi_session, MpiPoolSession) form.

Changes

• tests/test_common/session_reuse.py: install an isinstance-transparent metaclass shim at the seams instead of a bare function.
• tests/unittest/llmapi/test_session_reuse.py: shim unit test (construction routing + instance/subclass transparency) and an end-to-end regression test against the real patched tensorrt_llm.executor.proxy seam (raises TypeError on pre-fix code).

Test plan

• tests/unittest/llmapi/test_session_reuse.py — 23 passed (21 existing + 2 new) with the session-reuse plugin active
• Pre-merge CI

Summary by CodeRabbit

• Bug Fixes

  • Improved session reuse compatibility with Python type checks.
  • Prevented errors when validating pooled sessions with isinstance or issubclass.

• Tests

  • Added coverage for transparent session construction and type-checking behavior.
  • Added end-to-end validation for the session reuse integration.

[coderabbitai]

📝 Walkthrough

Walkthrough

The session reuse test seam now replaces MpiPoolSession with a type-preserving shim that forwards construction to factories while keeping isinstance and issubclass checks valid. Tests cover both the shim directly and the patched proxy seam.

Changes

Session reuse seam

Layer / File(s)	Summary
Transparent MpiPoolSession shim tests/test_common/session_reuse.py	Adds a metaclass-based shim that routes construction to the selected factory and delegates type checks to the real session class; seam installation now uses the shim.
Shim behavior validation tests/unittest/llmapi/test_session_reuse.py	Adds unit and integration checks for factory construction and isinstance/issubclass compatibility after patching the proxy seam.

Estimated code review effort: 3 (Moderate) | ~15–30 minutes

Possibly related PRs

• NVIDIA/TensorRT-LLM#16411: Addresses the same MpiPoolSession isinstance compatibility issue in proxy.py.
• NVIDIA/TensorRT-LLM#16419: Changes proxy logic to avoid isinstance checks after MpiPoolSession rebinding.
• NVIDIA/TensorRT-LLM#16497: Uses the real MpiPoolSession class for runtime type checks under monkey-patching.

Suggested reviewers: lori-ren

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is clear, concise, and directly reflects the main change to make the session-reuse pool seam isinstance-transparent.
Description check	✅ Passed	The description covers the issue, the solution, and testing, though it doesn't follow the template headings exactly.
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

🧹 Nitpick comments (1)

tests/test_common/session_reuse.py (1)

206-234: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Add precise annotations to the new functions.

• tests/test_common/session_reuse.py#L206-L234: annotate the shim factory and each metaclass hook.
• tests/unittest/llmapi/test_session_reuse.py#L300-L317: add -> None.
• tests/unittest/llmapi/test_session_reuse.py#L320-L329: annotate reuse_cache precisely and add -> None.

As per coding guidelines, “Annotate every function.”

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@tests/test_common/session_reuse.py` around lines 206 - 234, The new shim
functions lack required annotations. In tests/test_common/session_reuse.py lines
206-234, annotate _isinstance_transparent_shim and each _SeamMeta hook with
precise parameter and return types; in
tests/unittest/llmapi/test_session_reuse.py lines 300-317, add -> None; and in
lines 320-329, precisely annotate reuse_cache and add -> None.

Source: Coding guidelines

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@tests/test_common/session_reuse.py`:
- Around line 206-234: The new shim functions lack required annotations. In
tests/test_common/session_reuse.py lines 206-234, annotate
_isinstance_transparent_shim and each _SeamMeta hook with precise parameter and
return types; in tests/unittest/llmapi/test_session_reuse.py lines 300-317, add
-> None; and in lines 320-329, precisely annotate reuse_cache and add -> None.

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 1228fed and 0ba6d84.

📒 Files selected for processing (2)

• tests/test_common/session_reuse.py
• tests/unittest/llmapi/test_session_reuse.py

[sunnyqgg]

Closing: this change has been folded into #15908 (cherry-picked as-is, then generalized — the shim now lives in the shared _session_utils and is applied at BOTH the session-reuse and session-prefetch seams). Tracking continues there.