Prevent mask from being incorrectly used

`Remapping.Remapper` performs spectral interpolation. When interpolating, every
point in a given element is used to calculate the target value. As a result, the
result is incorrect if one or more of the nodes in an element are masked.

This commit prevents users from accidentally obtaining subtly incorrect results
by calling the Remapper on a space that is not supported.

Note that if none of the nodes in an element are masked, the interpolation
routine would be fine. A more refined implementation of this check could take
this into consideration.

Author: Sbozzolo

src/Remapping/distributed_remapping.jl

@@ -308,6 +308,19 @@ function _Remapper(
     ArrayType = ClimaComms.array_type(space)
     horizontal_topology = Spaces.topology(space)
     horizontal_mesh = horizontal_topology.mesh
+    quad = Spaces.quadrature_style(space)
+
+    space_has_mask = !(Spaces.get_mask(space) isa DataLayouts.NoMask)
+    element_has_more_than_one_node = Quadratures.degrees_of_freedom(quad) > 1
+
+    # Spectral remapping with a mask makes sense only if there is only one node
+    # in the element (the code will go through, but the results will be
+    # incorrect)
+    if space_has_mask && element_has_more_than_one_node
+        error(
+            "Remapping does not support masks, unless each element contains exactly one nodal point",
+        )
+    end
 
     is_1d = typeof(horizontal_topology) <: Topologies.IntervalTopology
 
@@ -339,7 +352,6 @@ function _Remapper(
     ξs_split = Tuple([ξ[i] for ξ in ξs_combined] for i in 1:num_hdims)
 
     # Compute the interpolation matrices
-    quad = Spaces.quadrature_style(space)
     quad_points, _ = Quadratures.quadrature_points(FT, quad)
     Nq = Quadratures.degrees_of_freedom(quad)
 

test/Remapping/distributed_remapping.jl

@@ -16,7 +16,8 @@ import ClimaCore:
     Quadratures,
     Topologies,
     Remapping,
-    Hypsography
+    Hypsography,
+    CommonSpaces
 using ClimaComms
 ClimaComms.@import_required_backends
 const context = ClimaComms.context()
@@ -925,3 +926,18 @@ end
         [Geometry.XPoint(x) for x in range(-500.0, 500.0, length = 180)],
     )
 end
+
+@testset "Mask" begin
+    @test_throws ErrorException Remapping.Remapper(
+        CommonSpaces.ExtrudedCubedSphereSpace(;
+            z_elem = 10,
+            z_min = 0,
+            z_max = 1,
+            radius = 10,
+            h_elem = 10,
+            n_quad_points = 4,
+            staggering = CommonSpaces.CellCenter(),
+            enable_mask = true,
+        ),
+    )
+end