test: clean up gRPC server process groups

[mlubin]

Makes the gRPC integration test's ServerProcess helper clean up reliably. Each spawned cuopt_grpc_server is now put in its own process group (setpgid), and teardown signals the whole group, so the GPU worker child can no longer be orphaned when a test ends or a server is restarted.

stop() now returns success/failure and is idempotent, the process group is verified before it's ever used as a kill target, and teardown paths assert the group is gone. Adds ServerProcessLifecycleTests covering idle stop, idempotent stop, and crash-during-solve cleanup.

Test-only change; no library or API changes.

The change is motivated by CI failures like https://github.com/NVIDIA/cuopt/actions/runs/28380812204/job/84090251116?pr=1481. The Codex-diagnosed issue here is:

1. ErrorRecoveryTests.ClientHandlesServerCrashDuringSolve starts a 120-second MIP in a forked gRPC worker.
2. ServerProcess::stop() signals only the server parent.
3. After five seconds it SIGKILLs that parent, but not the worker forked in spawn_worker().
4. The test reports success while the orphan worker continues using the GPU.
5. INCUMBENT_CALLBACK_TEST starts next and competes with that worker for VRAM

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[mlubin]

/ok to test df1c0de

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

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📥 Commits

Reviewing files that changed from the base of the PR and between fd1e773 and d4ec23e.

📒 Files selected for processing (1)

• cpp/tests/linear_programming/grpc/grpc_integration_test.cpp

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📝 Walkthrough

Walkthrough

Refactors ServerProcess in the gRPC integration test file to manage a process group, add group shutdown helpers, and make stop() return bool. Fixture teardown and restart paths now assert successful shutdowns.

Changes

ServerProcess process-group lifecycle refactor

Layer / File(s)	Summary
ServerProcess struct, start(), and pgid_ member cpp/tests/linear_programming/grpc/grpc_integration_test.cpp	Adds the new includes, introduces pgid_ and destructor cleanup reporting, and hardens start() with reuse checks, binary-path validation, fork handling, and child process-group setup.
stop(), shutdown helpers, and wait_for_ready crash detection cpp/tests/linear_programming/grpc/grpc_integration_test.cpp	Replaces stop() with process-group SIGTERM and optional SIGKILL shutdown, adds lifecycle reset and group reaping helpers, and changes readiness failure detection to check for server death with WNOHANG.
Fixture TearDown stop() assertions cpp/tests/linear_programming/grpc/grpc_integration_test.cpp	Updates teardown and restart paths across the gRPC integration fixtures to assert successful stop() calls before cleanup or restart.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 4.00% 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 is concise and accurately summarizes the main test-only change to gRPC server process-group cleanup.
Description check	✅ Passed	The description clearly matches the changeset by explaining process-group cleanup, idempotent stop behavior, and the related lifecycle tests.
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]

Actionable comments posted: 1

🤖 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.

Inline comments:
In `@cpp/tests/linear_programming/grpc/grpc_integration_test.cpp`:
- Around line 162-181: The stop() logic in grpc_integration_test.cpp only sends
SIGKILL inside the branch where reap_parent() fails, so a lingering GPU worker
can survive when the server parent exits promptly. Update stop() to escalate to
kill(-pgid_, SIGKILL) whenever wait_for_group_exit() shows the process group has
not exited, not just when reap_parent() returns false, and keep the cleanup
around clear_lifecycle_state() only after the full group is confirmed gone.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: f3c4bb1d-39d0-4334-81f9-b3f48630ebc2

📥 Commits

Reviewing files that changed from the base of the PR and between 2ca56e7 and df1c0de.

📒 Files selected for processing (1)

• cpp/tests/linear_programming/grpc/grpc_integration_test.cpp

[mlubin]

/ok to test fd1e773

[github-actions]

CI Test Summary

✅ All 31 test job(s) passed.

[mlubin]

/ok to test d4ec23e

[mlubin]

/merge