Surface calculation with radius field produces zero-sized surfaces

[brittonsmith]

I'm listing this as a separate issue from Issue #1406 because I think they are unrelated. To be honest, I cannot tell if this is a real issue or not.

Bug report

When making surfaces using the radius field, I find that the surfaces almost always have zero size even for rather large data containers. When constructing surfaces with other fields, like "density", "x", etc, this does not seem to be the case.

Code for reproduction

In this script, we make a sphere with size 0.5 and 0.4 using the IsolatedGalaxy dataset. Both spheres have about 3.6 million cells. The sphere
http://paste.yt-project.org/show/7153/

import yt

ds = yt.load("IsolatedGalaxy/galaxy0030/galaxy0030")

sp = ds.sphere(ds.domain_center, 0.5)
print ("sphere size:", sp["density"].size)
units = sp["radius"].units
surf = ds.surface(sp, "radius", sp.radius.to(units))
print ("surface size:", surf["density"].size)

sp = ds.sphere(ds.domain_center, 0.4)
print ("sphere size:", sp["density"].size)
units = sp["radius"].units
surf = ds.surface(sp, "radius", sp.radius.to(units))
print ("surface size:", surf["density"].size)
surf = ds.surface(sp, "radius", sp["radius"].max())
print ("surface size:", surf["density"].size)

Actual outcome

yt : [INFO     ] 2017-05-19 07:43:52,125 Parameters: current_time              = 0.0060000200028298
yt : [INFO     ] 2017-05-19 07:43:52,125 Parameters: domain_dimensions         = [32 32 32]
yt : [INFO     ] 2017-05-19 07:43:52,125 Parameters: domain_left_edge          = [ 0.  0.  0.]
yt : [INFO     ] 2017-05-19 07:43:52,126 Parameters: domain_right_edge         = [ 1.  1.  1.]
yt : [INFO     ] 2017-05-19 07:43:52,126 Parameters: cosmological_simulation   = 0.0
Parsing Hierarchy : 100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 173/173 [00:00<00:00, 37610.25it/s]
yt : [INFO     ] 2017-05-19 07:43:52,145 Gathering a field list (this may take a moment.)
sphere size: 3628948
yt : [INFO     ] 2017-05-19 07:43:54,153 Extracting (sampling: ('gas', 'density'))
surface size: 917
sphere size: 3620292
yt : [INFO     ] 2017-05-19 07:44:00,449 Extracting (sampling: ('gas', 'density'))
surface size: 0
yt : [INFO     ] 2017-05-19 07:44:05,949 Extracting (sampling: ('gas', 'density'))
surface size: 0

Even when using the maximum value of the radius field, the surface is empty. This seems wrong, but that's all I can say.

[brittonsmith]

I think a better way to demonstrate this to set the surface value to be half of the radius, or some other value between the minimum and maximum. When I do that, I always get an empty surface.