Fix race conditions in froot and Solver::timeExceeded

Static variables should have been declared thread-local, but haven't.

* Fixes race conditions in froot, which could have resulted in incorrect simulation results for models with events/heavisides/piecewise, for multi-threaded simulations.

* Fixes race conditions for the max-time check, which could have resulted in incorrect termination of simulations in case of multi-threaded simulations in combination with a time limit.

src/solver.cpp

@@ -1035,7 +1035,7 @@ void Solver::setMaxTime(double maxtime) {
 void Solver::startTimer() const { simulation_timer_.reset(); }
 
 bool Solver::timeExceeded(int interval) const {
-    static int eval_counter = 0;
+    thread_local static int eval_counter = 0;
 
     // 0 means infinite time
     if (maxtime_.count() == 0)

src/solver_cvodes.cpp

@@ -1126,7 +1126,8 @@ static int froot(realtype t, N_Vector x, realtype* root, void* user_data) {
     if (model->ne != model->ne_solver) {
         // temporary buffer to store all root function values, not only the ones
         // tracked by the solver
-        static std::vector<realtype> root_buffer(model->ne, 0.0);
+        thread_local static std::vector<realtype> root_buffer(model->ne, 0.0);
+        root_buffer.resize(model->ne);
         model->froot(t, x, root_buffer);
         std::copy_n(root_buffer.begin(), model->ne_solver, root);
     } else {