Fix StopIteration when computing mass of a compound containing an empty compound (#2078)

cadquery/occ_impl/shapes.py

@@ -825,20 +825,15 @@ def _mass_calc_function(obj: Shape) -> Any:
         # special handling of compounds - first non-compound child is assumed to define the type of the operation
         if type_ == ta.TopAbs_COMPOUND:
 
-            # if the compound is not empty check its children
-            if obj:
-                # first child
-                child = next(iter(obj))
-
-                # if compound, go deeper
-                while child.ShapeType() == "Compound":
-                    child = next(iter(child))
-
-                type_ = shapetype(child.wrapped)
-
-            # if the compound is empty assume it was meant to be a solid
-            else:
-                type_ = ta.TopAbs_SOLID
+            # descend to the first non-compound child, if any; the sentinel
+            # guards against an empty compound at the top level or nested inside
+            child = next(iter(obj), None)
+            while child is not None and child.ShapeType() == "Compound":
+                child = next(iter(child), None)
+
+            # an empty or empty-nested compound has no child to inspect;
+            # assume it was meant to be a solid (consistent with an empty compound)
+            type_ = ta.TopAbs_SOLID if child is None else shapetype(child.wrapped)
 
         # get the function based on dimensionality of the object
         return shape_properties_LUT[type_]

tests/test_shapes.py

@@ -459,6 +459,17 @@ def test_special():
     assert cs[0].Volume() == approx(3 ** 3)
 
 
+def test_nested_empty_compound_mass():
+
+    # a bare empty compound already resolves to zero mass
+    assert compound().Volume() == approx(0)
+
+    # a compound whose first child is an empty compound must not crash and
+    # must stay consistent with the bare empty compound (see issue #2078)
+    assert compound(compound()).Volume() == approx(0)
+    assert compound(compound()).Area() == approx(0)
+
+
 def test_center():
 
     v = vertex(1, 1, 1)