Fix race condition in add_external_solutions_to_population

[akifcorduk]

B&B preemption will be set right after the solution is set but if we don't acquire the mutex before the early return check in population, the writes to solutions_in_external_queue_ is not visible to heuristic thread. Early return then finishes the heuristics and returns no solution.

Summary by CodeRabbit

• Bug Fixes
  • Fixed a race condition so external solutions are always processed safely, improving thread safety and stability when handling concurrent incoming solutions.

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📥 Commits

Reviewing files that changed from the base of the PR and between a8ad1a9 and df78b71.

📒 Files selected for processing (1)

• cpp/src/mip_heuristics/diversity/population.cu

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

• cpp/src/mip_heuristics/diversity/population.cu

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📝 Walkthrough

Walkthrough

Remove early-exit guard in external-solution addition; the function now always calls get_external_solutions() (which acquires solution_mutex) and proceeds to add solutions, with an added comment clarifying mutex acquisition to avoid race conditions.

Changes

Cohort / File(s)	Summary
External solution queue handling cpp/src/mip_heuristics/diversity/population.cu	Removed early return based on solutions_in_external_queue_; always invokes get_external_solutions() (which now performs mutex acquisition) and proceeds to add external solutions. Added comment about mutex necessity to prevent races.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The pull request title 'Fix race condition in add_external_solutions_to_population' directly and accurately describes the main change in the changeset, which removes an early exit guard to fix a race condition.

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[aliceb-nv]

Super good catch Akif, thanks! Just one remark regarding a possible race condition

[aliceb-nv]

I think get_external_solutions() is called directly by diversity_manager, we may still need the lock in certain cases

[akifcorduk]

oh good catch. then let's keep it as it is before but skip the early return check.

[akifcorduk]

/merge