Mutex warning when using optimization_problem_->Solve

[gaschler]

PoseGraph doesn't hold a mutex when calling OptimizationProblem::Solve, there is a comment https://github.com/googlecartographer/cartographer/blob/master/cartographer/mapping/internal/3d/pose_graph_3d.cc#L600.

Maybe it should at least hold a smaller mutex that guards the input to the optimization problem.

cartographer/cartographer/mapping/internal/3d/pose_graph_3d.cc:604:32:
warning: passing variable 'constraints_' by reference requires holding mutex 'mutex_' [-Wthread-safety-reference]
  optimization_problem_->Solve(constraints_, frozen_trajectories_,
                               ^
cartographer/cartographer/mapping/internal/3d/pose_graph_3d.cc:604:46:
warning: passing variable 'frozen_trajectories_' by reference requires holding mutex 'mutex_' [-Wthread-safety-reference]
  optimization_problem_->Solve(constraints_, frozen_trajectories_,
                                             ^
cartographer/cartographer/mapping/internal/3d/pose_graph_3d.cc:605:32:
warning: passing variable 'landmark_nodes_' by reference requires holding mutex 'mutex_' [-Wthread-safety-reference]
                               landmark_nodes_);

[ojura]

Looks like the mutex annotation for these three should be removed from the header?

https://github.com/googlecartographer/cartographer/blob/c46fe073b48dd1e33d38c285163fbda8fca4fefa/cartographer/mapping/internal/3d/pose_graph_3d.h#L264

[gaschler]

Removing the annotation would only hide the issue. If we want one thread (main) to dispatch work items while a background thread is supposed to optimize, then the proper solution is to have more fine-grained mutexes and not just one for PoseGraph, I think.