Share one Kit app across test files instead of booting it per file

[mataylor-nvidia]

Draft. The last commit adds two temporary CI jobs whose only purpose is to measure whether this is worth continuing. They are meant to be deleted before merge. See Probe below.

Problem

Kit-dependence is currently a property of importing a test file. 156 test modules construct AppLauncher at module scope, so Isaac Sim boots during pytest collection:

simulation_app = AppLauncher(headless=True).app

Nothing declares that dependency, so tools/conftest.py compensates by running every test file in its own subprocess. Kit startup is therefore paid once per file, and there is no way to ask "which tests actually need Kit?" without importing them.

Change

launch_kit() (isaaclab/test/launch.py) replaces the module-scope AppLauncher. It is idempotent: the first module in a process boots Kit, later modules get the running app. It stays at module scope because a test module's own imports (pxr, omni, ...) run at collection, before any fixture could help.

kit / kit_cameras / kitless markers let a file declare its launch configuration, so files that can share a process can be grouped without importing them. kit_solo opts a file out.

test_kit_marker_contract.py keeps the markers from drifting by checking, via AST, that each file's declaration matches what it does at module scope. It is AST-based rather than text-based because several kit-free files mention AppLauncher only in a docstring saying they do not use it — a grep flags those, an AST walk does not.

conftest.py is unchanged.

Scope

source/isaaclab/test/sim is migrated as the pilot: 27 files map to kit, 4 to kit_cameras. The other ~125 files across other packages are untouched and keep working exactly as before — an unmarked file is treated as legacy and still gets its own process. _ENFORCED_ROOTS in the guard is empty, so no file is yet required to carry a marker; it grows per package as migration proceeds.

The transform is applied by tools/codemods/kit_launch_migration.py, added because those ~125 files remain. It edits line ranges in place rather than round-tripping through ast.unparse, which would discard comments and isort directives, and preserves each launch call's position so Kit-dependent imports below it still run after Kit starts.

Ordering constraint

kit and kit_cameras files must not share a process in that order. A camera-enabled app can serve tests that do not need cameras, but cameras cannot be enabled after startup, so launch_kit() raises rather than handing back an app that would silently fail to render.

Relatedly, pytest -m deselects tests but still imports every collected module, so marker filtering alone cannot keep a kit_cameras file out of a kit run. Both probe jobs list their files explicitly for that reason.

Probe (temporary)

kit-reuse-probe-per-file and kit-reuse-probe-batched run the same 30 files from source/isaaclab/test/sim and differ only in how many Kit apps they boot — 30 versus 1. The gap between the two job durations is what reuse is worth per 30 files, and it decides whether migrating the remaining files is justified. Files in TESTS_TO_SKIP are excluded from both sides so the two jobs cover the same tests.

To let a job bypass the per-file orchestrator, run-package-tests gains a test-path input defaulting to "tools" — the previously hard-coded value — so every existing caller is unaffected.

Note that migrating files does not by itself speed up CI, because tools/conftest.py still gives each file its own process. Teaching the runner to batch same-profile files is separate work, deliberately not in this PR.

Incidental fix

The guard found a real bug on its first run: test_operational_space.py assigned pytestmark twice, and the second assignment discarded arm_ci, so that file had been excluded from the ARM CI lane. Merged into a single list.

Testing

• test_kit_marker_contract.py passes (~1.7s, no Kit). Verified it has teeth by planting five deliberate violations — wrong marker vs. AppLauncher, kitless importing omni, unit calling launch_kit, kit_cameras without cameras=True, two profile markers at once — and confirming each rule fires.
• uv run isaaclab -f clean.
• The migrated files themselves have not been run locally; no Isaac Sim on the authoring machine. CI is the first execution, which is part of what this draft is for.

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