Whenever an application thorn wants to apply a boundary condition (i.e. not using the hard-coded boundary conditions from thorn Boundary), it is necessary to identify which points on the local component correspond to physical (as opposed to symmetry, inter-process or refinement boundaries). This functionality is available in KrancNumericalTools/GenericFD. The function is GenericFD_GetBoundaryInfo in https://github.com/ianhinder/Kranc/blob/master/Auxiliary/Cactus/KrancNumericalTools/GenericFD/src/GenericFD.c
The function takes various Cactus variables as input and outputs arrays indicating the nature of each boundary: bounding box and whether the face is a symmetry, physical or interprocessor boundary (including refinement boundaries).
There is also a function GenericFD_LoopOverBoundary which splits the domain into the 26 different regions (6 faces, 12 edges and 8 corners) and calls a function (passed by pointer) on each region that corresponds to a physical boundary. Input parameters to the function identify the boundary normal, face, direction, etc. This calls GenericFD_GetBoundaryInfo to identify this information. This functionality is obviously core to implementing boundary conditions.
I propose that this functionality be included in Cactus. Thorn Boundary would be a logical place, since GenericFD_GetBoundaryInfo calls aliased functions usually provided by CoordBase or multipatch thorns, so I don't think this belongs in the flesh. The function-pointer interface might not be the right one - perhaps we could refactor the code so that there was a macro similar to the flesh CCTK_LOOP macros. The user would write
where args gave names for the local variables indicating the normal direction, face number etc. This macro would evaluate whatever was inside it on each of the physical boundaries with appropriate values for the arguments.