feat(runtimes): Add XGBoost runtime(KEP-2598)

[Krishna-kg732]

What this PR does

Implements the XGBoost runtime plugin for Kubeflow Trainer V2, as proposed in KEP-2598. This plugin enables distributed XGBoost training using Rabit/Collective coordination by automatically injecting DMLC environment variables into trainer containers.

Changes

New Files

• pkg/runtime/framework/plugins/xgboost/xgboost.go — Plugin implementing EnforceMLPolicyPlugin and CustomValidationPlugin. Injects DMLC_TRACKER_URI, DMLC_TRACKER_PORT, DMLC_TASK_ID, DMLC_NUM_WORKER env vars and auto-derives numWorkersPerNode from GPU resources (1 worker per GPU, or 1 per node for CPU).
• pkg/runtime/framework/plugins/xgboost/xgboost_test.go — Unit tests covering EnforceMLPolicy (nil guards, single/multi-node CPU, GPU resources, numNodes override) and Validate (reserved DMLC_* env name rejection).

Modified Files

• pkg/apis/trainer/v1alpha1/trainingruntime_types.go — Added XGBoostMLPolicySource struct, XGBoost field to MLPolicySource, and updated CEL mutual exclusion validation rule.
• pkg/constants/constants.go — Added XGBoost/Rabit constants and XGBoostReservedEnvNames set.
• pkg/runtime/framework/plugins/registry.go — Registered the XGBoost plugin.
• pkg/runtime/framework/plugins/plainml/plainml.go — Added XGBoost to the PlainML fallback guard.
• pkg/runtime/framework/core/framework_test.go — Updated TestNew to include XGBoost in expected plugin lists.
• pkg/util/testing/wrapper.go — Added XGBoostPolicy() test helper.

How was this tested?

• go test ./pkg/runtime/framework/plugins/xgboost/... ✅ (9 test cases)
• go test ./pkg/runtime/framework/core/ -run TestNew ✅
• go test ./pkg/runtime/framework/plugins/... ✅ (all plugins pass)

TODO (follow-up PRs)

• Add E2E tests
• Add ClusterTrainingRuntime YAML manifests
• Add example notebook

/kind feature
/area runtime

[google-oss-prow]

@Krishna-kg732: The label(s) area/runtime cannot be applied, because the repository doesn't have them.

Details

In response to this:

What this PR does

Adds the XGBoost runtime plugin scaffold to the Trainer V2 framework. This is the foundational PR for KEP-2598: XGBoost Runtime — it introduces the plugin structure and API types without the full implementation, which will follow in a subsequent PR.

Changes

New Files

• pkg/runtime/framework/plugins/xgboost/xgboost.go — Plugin scaffold implementing EnforceMLPolicyPlugin with a stub EnforceMLPolicy (Rabit env injection TODO)

Modified Files

• pkg/apis/trainer/v1alpha1/trainingruntime_types.go — Added XGBoostMLPolicySource struct and XGBoost field to MLPolicySource
• pkg/constants/constants.go — Added XGBoost/Rabit constants (DMLC_TRACKER_URI, DMLC_TRACKER_PORT, DMLC_TASK_ID, DMLC_NUM_WORKER) and reserved env set
• pkg/runtime/framework/plugins/registry.go — Registered the XGBoost plugin
• pkg/runtime/framework/plugins/plainml/plainml.go — Added XGBoost to the PlainML fallback guard

What's NOT in this PR (intentionally)

• EnforceMLPolicy implementation (Rabit env var injection) — will be in a follow-up PR
• Unit tests and E2E tests — will accompany the implementation PR
• ClusterTrainingRuntime YAML manifests

How was this tested?

• go build ./pkg/runtime/framework/plugins/... ✅
• go vet ./pkg/runtime/framework/plugins/xgboost/... ✅

/kind feature
/area runtime

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[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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Details

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[github-actions]

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[Copilot]

Pull request overview

Adds an initial XGBoost runtime plugin scaffold to the Trainer V2 runtime framework (per KEP-2598), along with the API wiring and constants needed to support a future Rabit env var injection implementation.

Changes:

• Introduces an xgboost runtime plugin scaffold implementing EnforceMLPolicyPlugin (stubbed behavior for now).
• Extends the TrainingRuntime API (MLPolicySource) with an xgboost policy source and updates the “only one policy” validation rule.
• Adds XGBoost/Rabit-related env var constants and registers the plugin in the runtime plugin registry (and updates PlainML fallback guard).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pkg/runtime/framework/plugins/xgboost/xgboost.go	New XGBoost plugin scaffold (EnforceMLPolicy stub + plugin name/factory).
pkg/runtime/framework/plugins/registry.go	Registers the XGBoost plugin in the plugin factory registry.
pkg/runtime/framework/plugins/plainml/plainml.go	Ensures PlainML no-ops when XGBoost (and JAX) ML policy sources are configured.
pkg/constants/constants.go	Adds Rabit/XGBoost env var constants + reserved env name set.
pkg/apis/trainer/v1alpha1/trainingruntime_types.go	Adds XGBoostMLPolicySource + MLPolicySource.XGBoost, and updates ML policy exclusivity validation.

[coveralls]

Pull Request Test Coverage Report for Build 22064651978

Details

• 77 of 84 (91.67%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.2%) to 57.176%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/runtime/framework/plugins/registry.go	0	1	0.0%
pkg/runtime/framework/plugins/xgboost/xgboost.go	72	78	92.31%

Totals
Change from base Build 22051165353:	1.2%
Covered Lines:	1466
Relevant Lines:	2564

---

💛 - Coveralls