Geoms of type mesh with zero density don't trigger the check for valid mesh volume.

PiperOrigin-RevId: 629416817
Change-Id: I23eaa0d61a703988a4eb96a3cad09e562384035e

src/user/user_objects.cc

@@ -2344,6 +2344,7 @@ void mjCGeom::Compile(void) {
 
   // compute geom mass and inertia
   if (inferinertia) {
+    // mass is defined
     if (mjuu_defined(mass)) {
       if (mass==0) {
         mass_ = 0;
@@ -2353,11 +2354,19 @@ void mjCGeom::Compile(void) {
         density = mass / GetVolume();
         SetInertia();
       }
-    } else {
-      mass_ = density * GetVolume();
-      SetInertia();
     }
 
+    // mass is not defined
+    else {
+      if (density == 0) {
+        mass_ = 0;
+      } else {
+        mass_ = density * GetVolume();
+        SetInertia();
+      }
+    }
+
+
     // check for negative values
     if (mass_<0 || inertia[0]<0 || inertia[1]<0 || inertia[2]<0 || density<0)
       throw mjCError(this, "mass, inertia or density are negative in geom");

test/user/user_objects_test.cc

@@ -755,6 +755,32 @@ TEST_F(MjCGeomTest, NanSize) {
   EXPECT_THAT(error.data(), HasSubstr("line 5"));
 }
 
+TEST_F(MjCGeomTest, BadMeshZeroMassDensityDoesntError) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <asset>
+      <mesh name="bad_mesh"
+        vertex="0 0 0  1 0 0  0 1 0  0 0 1"
+        face="0 2 1" />
+    </asset>
+    <worldbody>
+      <body>
+        <geom type="mesh" mesh="bad_mesh" mass="0"/>
+      </body>
+      <body>
+        <geom type="mesh" mesh="bad_mesh" density="0"/>
+      </body>
+    </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  mjModel* model = LoadModelFromString(xml, error.data(), error.size());
+  ASSERT_THAT(model, NotNull()) << error.data();
+  EXPECT_EQ(model->body_mass[1], 0);
+  EXPECT_EQ(model->body_mass[2], 0);
+  mj_deleteModel(model);
+}
+
 // ------------- test height fields --------------------------------------------
 
 using MjCHFieldTest = MujocoTest;