Set/get particle properties via the script interface

[RudolfWeeber]

Objective

• to handle particel access completely via the scrip interface and eliminate the particle-related mpi callbacks
• The non-scriptinterface mpi callbacks are in teh way of having more than one espressomd.System object per script
• They are also the last "client" of the current particle memory layout which we may wish to replace for performance reasons

Background

• the current ParticleHandle Python object is already backed by the script interface. However, the script object only lives on the head node (defined via __so_creation_policy__ in the class definition in particle_data.py)
• the script interface implementaiton src/scriptinterface/particle_data/ParticleHandle.hpp/cpp) then uses manually written mpi calls to send/get ghe particle information from a remote mpi node. The goal is to eliminate these callbacks and let the script interface do all the communication.

Steps

• Switch the creation policy of the ParticleHandle script object ot global (src/python/espressomd/particle_data.py). this means that all commands (parameter set/get, method calls) on the object will run on all mpi nodes

• To ease the transition, guad all exisitng setters and getters in src/scriptinterface/particle_data/ParticleHandle.cpp to only run mpi rank 0. Before each call, e.g., the v setters and getters defined in the constructor via add_parameters, and in the method calls in do_call_method()

  • use m_comm.rank() to determine the mpi rank
  • if it is !=0 return None

  After this is done, the particle access from Python should work unchanged, eventhough the creation policy is now global.

• If it is not yet present, copyin the scr/script_interface/communicaiton.hpp file from the Walberla branch (on github.com/RudolfWeeber/espresso). This contains necessary communicaiton helpers

• Now the setterws/getters and method calls can be refactored step by step

  • Getters (defined in the constructor via add_parameters()

    • Declare a result variable as a boost::optional, e.g., boost::optional<Utils::Vector3d> for the velocity
    • Get the particle information via cell_structure.get_local_particle()
    • If it is a nullptr (the particle is not local on this node), set the result to an empty optional {}.
    • If the particle is a ghost (i.e., the specific mpi node has only a ghost but not the original particle), set the result to an empty optional as well (determine via Particle::is_ghost()
    • Otherwise, the current node has the particle and can return the info. Set the result to an optional with the respective particle property, e.g. {p.v() (see src/core/Particle.hpp)
    • Use reduce_optional() from scr/script_interface/communicaiton.hpp to make the result available on the head node, where python runs, and make sure that only one mpi rank provided an answer.
    • Return teh result. Casting to a Variant should wokr automatically for all standrad cases
    • Test. If it works, consider putting the entire code in a function template (use decl_type to find out the return type of the particle property in a generic fashion)`

To be continued for setters and meethod calls