How to use a XDMF mesh in `initialize`?

[BenjaminRodenberg]

The API function initialize only accepts a coupling_subdomain:

fenics-adapter/fenicsprecice/fenicsprecice.py

Line 267 in a64f1d6

def initialize(self, coupling_subdomain, read_function_space=None, write_object=None, fixed_boundary=None):

A coupling_subdomain has to be defined symbolically by inheriting from the fenics.SubDomain (see here). For cases of practical relevance the geometry is, however, not defined in terms of such high-level objects (RectangleMesh, SubDomain) but via a mesh (e.g. in XDMF format) and potentially some flags/filtering to define the part of the mesh that belongs to the coupling boundary.

From the provided tutorials it is unclear how one can use an XDMF mesh with the FEniCS adapter. I see two possible solutions:

1. FEniCS offers a way to convert an XDMF (or other) mesh to its internal datastructures such that we can create a SubDomain to define the coupling boundary using the XDMF mesh. In this case no changes are required for the adapter and we can use the existing interface of initialize. The adapter could potentially offer a helper function to create a Mesh + SubDomain from XDMF + flags.
2. FEniCS does not offer the functionality from 1. We need to extend the interface to something like initialize(self, xdmf, flags, ...).

Refer to https://precice.discourse.group/t/issues-with-reading-solidmesh-with-fenics-provided-mesh-is-empty/2311 for the discussion that triggered opening this issue.

[precice-bot]

This issue has been mentioned on preCICE Forum on Discourse. There might be relevant details there:

https://precice.discourse.group/t/issues-with-reading-solidmesh-with-fenics-provided-mesh-is-empty/2311/4