Hi,

i looked deeper into Blender:

Here is a python script that makes Blender crash
(on both Linux and Windows systems):

based on Blender 2.31a sources

-------------------------------------------------------
--

import Blender
from Blender import *

scene = Scene.GetCurrent()

mat = Material.New("mat") # this line is important

mesh = NMesh.New("mesh")
mesh.verts = [ NMesh.Vert(0,0,0), NMesh.Vert(1,0,0),
NMesh.Vert(1,0,1) ]
mesh.faces = [ NMesh.Face(mesh.verts) ]
mesh.addMaterial(mat)

obj1 = Object.New("Mesh", "obj1")
obj1.link(mesh)
scene.link(obj1)
mesh.update(1)

mesh2 = NMesh.GetRaw(mesh.name) # or simply mesh2 =
mesh
obj2 = Object.New("Mesh", "obj2")
obj2.link(mesh2) # <--- THIS link() corrupts the
mesh!!!
scene.link(obj2)

-------------------------------------------------------
--

The problem is the obj2.link(mesh2). If the mesh is
already linked
to another object, the link will corrupt the mesh's
material setting.

First another question: what is obj2.link(mesh)
supposed to do excatly?
It seems it does the same as obj1.shareFrom(obj2).
BTW: it would be helpful if there is a way to get the
linked object(s) from
a mesh (or other data a object can link to).


Ok, back to the crash. I assume the above script is
valid code, means it should
do what it is supposed to do. And that is to link to
objects to the same mesh.

The problem here is the material setting. At the
second link() Blender runs into
following code in NMesh.c

Mesh *Mesh_FromPyObject (PyObject *pyobj, Object *ob)
{
if (pyobj->ob_type == &NMesh_Type) {
Mesh *mesh;
BPy_NMesh *nmesh = (BPy_NMesh *)pyobj;

if (nmesh->mesh) {
mesh = nmesh->mesh;

*** unlinks the materials from <mesh> (in
EXPP_unlink_mesh())
*** and frees the mesh->mat array (in
unlink_existingMeshData())
*** but does NOT set mesh->totcol to 0!
unlink_existingMeshData(mesh);
*** sets mesh->mat = NULL, but mesh->totcol is still
<> 0
convert_NMeshToMesh(mesh, nmesh);
}
else {
nmesh->mesh = Mesh_fromNMesh(nmesh);
mesh = nmesh->mesh;
}

....
}

and this code means:
static int convert_NMeshToMesh (Mesh *mesh, BPy_NMesh
*nmesh)
{
...

/*@ material assignment moved to PutRaw */

...
}

Have i do a PutRaw() after a obj.link(mesh)??? I
assume not


After Blender has run through this code, Blender does
not crash immediately
but e.g. if you open the OOPS Schematic View. In this
case, Blender wants
to draw the link from the mesh to the material and
iterates over the
mesh->mat array, *but* it is not checked if mesh->mat
is <> 0:

in oops.c:
void add_mesh_oopslinks(Mesh *me, Oops *oops, short
flag)
{
int a;

if(flag & OOPS_MA) {
for(a=0; a<me->totcol; a++) {
*******************
**** crash because me->mat == 0 but me->totcol > 0
*******************
if(me->mat[a]) {
add_oopslink("ma", oops, ID_MA, me->mat+a, 0.0, (float)
(0.5*OOPSY));
}
}
}
....
}



Sorry, i'm not familiar enough with the blender python
code to supply a patch
immediatelly, but it should help the blender python
devs to fix the problem


I add this post to the bug tracker too.

-Bala Gi

##########################################

ascotan supplied following fix:

The problem is that when the NMesh is converted to a
mesh datablock for linking to the object the mesh->mat
pointer is left NULL and materials are not added.
To fix the problem add the lines:

f(nmesh->materials){
mesh->mat = EXPP_newMaterialList_fromPyList (nmesh-
>materials);
}

at the end of the function:
convert_NMeshToMesh() in NMesh.c