[Libmesh-devel] [BUG] Boundary Mesh -- some progress

[John P. <pet...@cf...>]

Hi,

I think I've found the problem, but I haven't fixed it yet...

The sync() function uses the Elem::build_side() method to construct
an element which is later placed into the boundary mesh.  The
problem is that the build_side() method doesn't return an actual
Elem, but a proxy which forwards all subsequent calls to its
"parent" element, including resetting the node pointers, which
messes up the original mesh and eventually results in the seg-fault
you see.

The build_side() optimization is typically a very good thing: it
does not dynamically create new storage for an element when you
don't really need it.  Unfortunately in this case we DO need it,
but there is no Elem::expensive_build_side() (there used to be
before we optimized it out with the current version.)

Elem::side() won't work either: it returns a "minimal" side i.e.
a Quad4/8/9 all return an Edge2.  I noticed that all of the build_side()
functions still have the old code, but it is commented out. One
fix would be to add an extra argument to build_side which by
default would return the lightweight proxy, but could be used to
get a non-proxy, full-ordered side instead.

What do the developers think?
John





Martin L,b|(Bthi writes:
 > Hi
 > 
 > Back to the BoundaryMesh problems again. Before I embark on trying to
 > fix the bug described below, I wonder what the state is in the
 > discussion. Ben once wrote that BoundaryMesh has to go (and I totally
 > agree), but we need a replacement.
 > 
 > Specifically I would like to create a mesh of all boundary elements
 > (2D for a 3D mesh), and solve an equation system on that mesh, that
 > takes data from the 3D problem solution vector. So obviously one needs
 > to share at least node ids, or references to the nodes. What would be
 > the best approach now, and in the future?
 > 
 > The bug I talked about above is easily reproduced:
 > 
 > Add the following lines to any example (I used ex3). Libmesh segfaults
 > on the second system.solve(). 
 > 
 >     for (unsigned int i=0; i<3; i++)
 >       {
 >         system.solve();
 >         BoundaryMesh boundary_mesh (mesh.mesh_dimension()-1);
 >         mesh.boundary_info->sync(boundary_mesh, false);
 >         GMVIO(boundary_mesh).write("boundary.gmv");
 >       }
 > 
 > 
 > Thanks, Martin