From a8ad1a931416bac9f60df88c108547fc5483ab5f Mon Sep 17 00:00:00 2001
From: akifcorduk <akifcorduk@gmail.com>
Date: Thu, 26 Feb 2026 23:51:37 -0800
Subject: [PATCH 1/2] fix race condition in
 add_external_solutions_to_population

---
 cpp/src/mip_heuristics/diversity/population.cu | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/cpp/src/mip_heuristics/diversity/population.cu b/cpp/src/mip_heuristics/diversity/population.cu
index c2138c91cd..601fe232f0 100644
--- a/cpp/src/mip_heuristics/diversity/population.cu
+++ b/cpp/src/mip_heuristics/diversity/population.cu
@@ -179,10 +179,17 @@ void population_t<i_t, f_t>::add_external_solution(const std::vector<f_t>& solut
 template <typename i_t, typename f_t>
 void population_t<i_t, f_t>::add_external_solutions_to_population()
 {
-  // early exit to avoid taking the population lock
-  if (!solutions_in_external_queue_.load()) { return; }
-
-  auto new_sol_vector = get_external_solutions();
+  std::vector<solution_t<i_t, f_t>> new_sol_vector;
+  {
+    // do the mutex here, otherwise it is a race condition.
+    // preemption will be set right after the solutions_in_external_queue_ is set in BB
+    // but if we don't acquire the mutex here, the writes to solutions_in_external_queue_
+    // is not visible. Early return then finishes the heuristics and returns no solution.
+    std::lock_guard<std::mutex> lock(solution_mutex);
+    // early exit to avoid the function(even though it will do nothing)
+    if (!solutions_in_external_queue_.load()) { return; }
+    new_sol_vector = get_external_solutions();
+  }
   add_solutions_from_vec(std::move(new_sol_vector));
 }
 
@@ -198,7 +205,6 @@ void population_t<i_t, f_t>::preempt_heuristic_solver()
 template <typename i_t, typename f_t>
 std::vector<solution_t<i_t, f_t>> population_t<i_t, f_t>::get_external_solutions()
 {
-  std::lock_guard<std::mutex> lock(solution_mutex);
   std::vector<solution_t<i_t, f_t>> return_vector;
   i_t counter                     = 0;
   f_t new_best_feasible_objective = best_feasible_objective;

From df78b7146c81326c66fef00478c4b99c1e8414f2 Mon Sep 17 00:00:00 2001
From: akifcorduk <akifcorduk@gmail.com>
Date: Fri, 27 Feb 2026 02:48:27 -0800
Subject: [PATCH 2/2] revert mutex and only remove the early return check

---
 cpp/src/mip_heuristics/diversity/population.cu | 14 +++-----------
 1 file changed, 3 insertions(+), 11 deletions(-)

diff --git a/cpp/src/mip_heuristics/diversity/population.cu b/cpp/src/mip_heuristics/diversity/population.cu
index 601fe232f0..bca87223d9 100644
--- a/cpp/src/mip_heuristics/diversity/population.cu
+++ b/cpp/src/mip_heuristics/diversity/population.cu
@@ -179,17 +179,8 @@ void population_t<i_t, f_t>::add_external_solution(const std::vector<f_t>& solut
 template <typename i_t, typename f_t>
 void population_t<i_t, f_t>::add_external_solutions_to_population()
 {
-  std::vector<solution_t<i_t, f_t>> new_sol_vector;
-  {
-    // do the mutex here, otherwise it is a race condition.
-    // preemption will be set right after the solutions_in_external_queue_ is set in BB
-    // but if we don't acquire the mutex here, the writes to solutions_in_external_queue_
-    // is not visible. Early return then finishes the heuristics and returns no solution.
-    std::lock_guard<std::mutex> lock(solution_mutex);
-    // early exit to avoid the function(even though it will do nothing)
-    if (!solutions_in_external_queue_.load()) { return; }
-    new_sol_vector = get_external_solutions();
-  }
+  // don't do early exit checks here. mutex needs to be acquired to prevent race conditions
+  auto new_sol_vector = get_external_solutions();
   add_solutions_from_vec(std::move(new_sol_vector));
 }
 
@@ -205,6 +196,7 @@ void population_t<i_t, f_t>::preempt_heuristic_solver()
 template <typename i_t, typename f_t>
 std::vector<solution_t<i_t, f_t>> population_t<i_t, f_t>::get_external_solutions()
 {
+  std::lock_guard<std::mutex> lock(solution_mutex);
   std::vector<solution_t<i_t, f_t>> return_vector;
   i_t counter                     = 0;
   f_t new_best_feasible_objective = best_feasible_objective;