I finally got a chance to debug this, here is some code which causes the threading to fail:
from panda3d.core import *
loadPrcFileData('', 'threading-model Cull/Draw')
from direct.directbase.DirectStart import *
base.wireframeOn()
res_x, res_y = 16, 9
# writers
vdata = GeomVertexData('vdata', GeomVertexFormat.getV3t2(), Geom.UHDynamic)
vtxwriter = GeomVertexWriter(vdata, 'vertex')
texwriter = GeomVertexWriter(vdata, 'texcoord')
# verts
for y in range(res_y):
for x in range(res_x):
vtxwriter.addData3f(x, -y, 0)
vtxwriter.addData3f(x+1, -y, 0)
vtxwriter.addData3f(x+1, -y-1, 0)
vtxwriter.addData3f(x, -y-1, 0)
for i in range(4):
texwriter.addData2f(0, 0)
# polys
prim = GeomTriangles(Geom.UHStatic)
for i in range(res_x * res_y):
prim.addVertices(i*4+2, i*4+1, i*4)
prim.addVertices(i*4+3, i*4+2, i*4)
prim.closePrimitive()
# model
geom = Geom(vdata)
geom.addPrimitive(prim)
node = GeomNode('geom')
node.addGeom(geom)
model = render.attachNewNode(node)
run()