extrude() doesn't extrude?

I would like to extrude a simple surface, but the following fails:

import gmsh

gmsh.initialize()

p0 = gmsh.model.geo.addPoint(0.0, 0.0, 0.0)
p1 = gmsh.model.geo.addPoint(1.0, 0.0, 0.0)
p2 = gmsh.model.geo.addPoint(0.0, 1.0, 0.0)

l0 = gmsh.model.geo.addLine(p0, p1)
l1 = gmsh.model.geo.addLine(p1, p2)
l2 = gmsh.model.geo.addLine(p2, p0)

cl = gmsh.model.geo.addCurveLoop([l0, l1, l2])
s = gmsh.model.geo.addPlaneSurface([cl])

out_dim_tags = gmsh.model.geo.extrude([s], 0.0, 0.0, 1.0)
print(out_dim_tags)

gmsh.model.geo.synchronize()
gmsh.model.mesh.generate(3)

msh_filename = "out.msh"
gmsh.write(msh_filename)

The out_dim_tags are empty, indicating that something goes wrong. Any hints?

(I'd also love to get the "end" surface tags and side surface tags.)