domain.coba_field in {"FLAGS", ...}

You are using ' and " inconsistently here it seems. I'm sometimes inconsistent with that myself though...

missing .

Can be const and moved down to where the variable is actually used (to reduce the scope of the variable).

Now there is VOLUME_INTERPOLATION_LINEAR and VOLUME_INTERP_LINEAR. Either the names should become more specific or become unified.

Hm, it might be that we don't want to use the term coba in the Python API, at least I've never seen it. Internally, coba stands for ColorBand, while in the Python API we use ColorRamp. Maybe @Campbell Barton (campbellbarton) has an opinion on that.

Should probably be CLOSEST instead of RAW here as well.

This coba_field should be updated too. Probably an oversight during the last renaming commit.

In my view, a switch should always have a default label. For cases where it wouldn't make much sense, you can do something like this:

default:
    /* Should never get here. */
    assert(false);

Missing default label.

I think this comment is not needed anymore.

Missing default label.

Missing default label.

Missing default label. Probably best to NULL the pointers in this case.

missing .

This function does not seem to be needed and the default one can be used.

rna_Fluid_cobafield_set can be set to NULL after removing the function and in order to use the default setter.

Uh, my bad, this is GLSL! Using assert() is not possible here and will give a crash. Since all 6 cases are covered in the % 6 switch, it should be fine to not have a default block here after all.

So you can reverse this change.