Fix dual residual computation in final solution output

[yuwenchen95]

Description

get_lp_stats() reported a spurious, large l2_dual_residual for QPs and SOCPs solved via the barrier path, even at a fully optimal solution. The final-solution dual residual computed in iteration_data_t::to_solution() (cpp/src/barrier/barrier.cu) omitted the Q*x quadratic term, computing only ‖Aᵀy + z − c‖ instead of ‖Aᵀy + z − c − Qx‖.

Adds the missing Q*x term to the reported dual residual, mirroring the existing correct pattern used internally.

Also adds a regression test (qp_diagonal_only_dual_residual) that reuses the diagonal-Hessian QP problem from the linked issue and asserts l2_dual_residual is near zero at the known optimum.

Issue

Closes #1598

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: a8158909-d027-454b-8a0a-715f6390ee2a

📥 Commits

Reviewing files that changed from the base of the PR and between 07bd5ea and 83578c0.

📒 Files selected for processing (1)

• cpp/tests/qp/unit_tests/lp_parser_solve_test.cu

🚧 Files skipped from review as they are similar to previous changes (1)

• cpp/tests/qp/unit_tests/lp_parser_solve_test.cu

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📝 Walkthrough

Walkthrough

The barrier solution calculation now includes Q*x in the dual residual for quadratic objectives. A regression test solves a diagonal QP and validates optimal termination, objective value, and near-zero dual residual.

Changes

Quadratic dual residual

Layer / File(s)	Summary
Residual calculation and QP regression cpp/src/barrier/barrier.cu, cpp/tests/qp/unit_tests/lp_parser_solve_test.cu	The solution residual calculation subtracts Q*x when the quadratic matrix is present. The regression test checks optimal termination, objective -32.0, and a near-zero l2_dual_residual.

Estimated code review effort: 2 (Simple) | ~10 minutes

Mergeability Score: ⚪ Minimal · up to 83578

This localized fix corrects the reported dual-residual calculation and adds a regression test; no actionable merge-blocking risk remains beyond normal checks and review.

Suggested reviewers: rg20, ramakrishnap-nv, chris-maes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: correcting dual residual computation in final solution output.
Description check	✅ Passed	The description accurately explains the missing Q*x term and the regression test added for issue #1598.
Linked Issues check	✅ Passed	The PR addresses the dual residual defect and adds the requested regression coverage for the linked issue [#1598].
Out of Scope Changes check	✅ Passed	The code changes are limited to dual residual computation and its regression test, with no unrelated changes identified.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[github-actions]

CI Test Summary

✅ All 31 test job(s) passed.

[chris-maes]

LGTM. Thanks!

[chris-maes]

/merge