Fix/test plugin registration stub

[kahlun]

Summary

Fixes a pre-existing bug in the shared, session-scoped _stub_training_engine_runtimes
fixture in tests/test_plugin_registration.py. This fixture backs the registration
tests for every vendor in this repo (TPU, MLU, MetaX, Iluvatar, FlagOS, XPU, ...),
not just XPU — it was already broken for all of them against current verl-core, it
just hadn't been exercised yet (see Motivation). Fix the issue #20

Motivation

While retesting the XPU plugin against a newer verl-core checkout, pytest tests/test_plugin_registration.py failed at fixture setup with:

ImportError: cannot import name 'FSDPTurboEngineWithLMHead' from 'verl.workers.engine.fsdp' (unknown location)
../verl/verl/workers/engine/__init__.py:17: ImportError

Root cause: verl-core's verl/workers/engine/__init__.py does
from .fsdp import FSDPEngine, FSDPEngineWithLMHead, FSDPTurboEngineWithLMHead.
This fixture replaces verl.workers.engine.fsdp in sys.modules with a bare
stand-in module so the full registration suite can run without a real FSDP/Megatron
runtime — but the stand-in predates FSDPTurboEngineWithLMHead, which verl-core
added upstream on 2026-08-17 (d4701e4e, "add fsdpturbo backend engine support"),
and was never updated to include it.

This is not XPU-specific and not something introduced by this repo's own history:
this repo's CI has not hit it yet only because pyproject.toml pins verl>=0.7.0,
and the latest PyPI release stayed at 0.9.0 (2026-08-14, predates the new class)
until 0.9.1 shipped on 2026-09-20. Any CI run against 0.9.1 or a current
verl-core main will hit this same error regardless of which vendor's test runs
first — confirmed by reproducing it with an unmodified verl-core checkout and the
unmodified fixture on main.

A second, related bug surfaced once the missing symbol above was patched: the
fixture built its _StubEngine class and only assigned it to the mocked module
attributes after entering mock.patch.dict(sys.modules, ...). In that window,
verl-core's own internal mindspeed wiring re-imports verl.workers.engine, which
runs a live @EngineRegistry.register(...) decorator that asserts
issubclass(engine_class, BaseEngine) at class-definition time. It observed the
pre-assignment placeholder instead of _StubEngine and failed with
AssertionError at verl/workers/engine/base.py:376.

Changes

• tests/test_plugin_registration.py — _stub_training_engine_runtimes fixture:
  • Add the missing FSDPTurboEngineWithLMHead stub to the mocked
    verl.workers.engine.fsdp module, matching the real module's __all__.
  • Reorder setup so BaseEngine is imported for real and _StubEngine is defined
    and assigned to every mocked module attribute before mock.patch.dict(sys.modules, ...)
    takes effect, instead of after. Removes the placeholder/real-stub split that let
    verl-core's own internal imports observe a non-BaseEngine object mid-setup.

No new test cases are added and no XPU-specific code is touched — this is scoped
entirely to the shared fixture (1 file changed, 28 insertions, 22 deletions).

Testing

• pytest tests/ -v passes — 62 passed, 6 warnings on real B60 hardware.
  Before this fix: 1 error (ImportError above). After stubbing the missing
  symbol alone (partial fix): 1 error (AssertionError above). Both resolved
  together by this PR.
• Manually verified on target hardware (if applicable) — N/A, test-infra-only
  change; no engine or runtime behavior is touched.

Acceptance Baseline (for new hardware adaptation PRs)

N/A — this PR does not add or modify a hardware platform/engine. It fixes a bug in
shared test infrastructure used by every vendor in this repo.

Checklist

• Code follows the project's style and passes pre-commit checks
• [] Documentation updated (if applicable) — N/A, no user-facing behavior change
• No secrets or credentials included

[Copilot]

Copilot review overview

🟡 Changes recommended

Move the BaseEngine import under the environment patch to preserve runtime isolation.

Get a fresh assessment by requesting another Copilot review.

Review effort: Lite
Findings: 1

Open (1)

• Import BaseEngine after disabling external plugins · New

What changed in this PR

This PR updates the shared plugin-registration fixture for newer verl-core versions.

Changes:

• Adds the missing FSDPTurboEngineWithLMHead stub.
• Initializes stub classes before patching runtime modules.

File	Summary
tests/​test_plugin_registration.py	Updates engine stubs, but BaseEngine is imported before the environment patch, potentially loading hardware plugins and defeating fixture isolation.

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