Correctly handle the case of explicit zero mass in xml_native_writer.

PiperOrigin-RevId: 450901455
Change-Id: I77aaf2a214ac05ed1ed1e6ffe473bf98f2fea8ef

src/xml/xml_native_writer.cc

@@ -290,7 +290,8 @@ void mjXWriter::OneGeom(XMLElement* elem, mjCGeom* pgeom, mjCDef* def) {
   WriteAttr(elem, "gap", 1, &pgeom->gap, &def->geom.gap);
   WriteAttr(elem, "gap", 1, &pgeom->gap, &def->geom.gap);
   if (mjuu_defined(pgeom->_mass)) {
-    WriteAttr(elem, "mass", 1, &pgeom->mass, &def->geom.mass);
+    double mass = pgeom->GetVolume() * def->geom.density;
+    WriteAttr(elem, "mass", 1, &pgeom->mass, &mass);
   } else {
     WriteAttr(elem, "density", 1, &pgeom->density, &def->geom.density);
   }

test/xml/xml_native_writer_test.cc

@@ -214,6 +214,23 @@ TEST_F(XMLWriterTest, WritesMass) {
   mj_deleteModel(model);
 }
 
+TEST_F(XMLWriterTest, ZeroMass) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <worldbody>
+      <body>
+        <geom type="box" size=".05 .05 .05" mass="0"/>
+      </body>
+    </worldbody>
+  </mujoco>
+  )";
+  mjModel* model = LoadModelFromString(xml);
+  std::string saved_xml = SaveAndReadXml(model);
+  EXPECT_THAT(saved_xml, Not(HasSubstr("density")));
+  EXPECT_THAT(saved_xml, HasSubstr("mass=\"0\""));
+  mj_deleteModel(model);
+}
+
 TEST_F(XMLWriterTest, OverwritesDensity) {
   static constexpr char xml[] = R"(
   <mujoco>