Make sure we never apply the ArticulationRoot API more than once.

PiperOrigin-RevId: 779185190
Change-Id: If441d0e2ba24191883839977b46864db2b4b8204

src/experimental/usd/plugins/mjcf/mujoco_to_usd.cc

@@ -18,6 +18,7 @@
 #include <cstddef>
 #include <string>
 #include <unordered_map>
+#include <unordered_set>
 #include <utility>
 #include <vector>
 
@@ -231,6 +232,8 @@ class ModelWriter {
   std::vector<pxr::SdfPath> joint_paths_;
   // Mapping from mesh names to Mesh prim path.
   std::unordered_map<std::string, pxr::SdfPath> mesh_paths_;
+  // Set of body ids that have had the articulation root API applied.
+  std::unordered_set<int> articulation_roots_;
   // Whether to write physics data.
   bool write_physics_ = false;
 
@@ -1998,9 +2001,13 @@ class ModelWriter {
     // then we need to apply the articulation root API.
     if (parent_id != kWorldIndex) {
       int parent_parent_id = mjs_getId(mjs_getParent(parent->element)->element);
-      if (parent_parent_id == kWorldIndex) {
+      // We guard against applying the API more than once, which can happen when
+      // there are multiple children.
+      if (parent_parent_id == kWorldIndex &&
+          articulation_roots_.find(parent_id) == articulation_roots_.end()) {
         ApplyApiSchema(data_, parent_path,
                        pxr::UsdPhysicsTokens->PhysicsArticulationRootAPI);
+        articulation_roots_.insert(parent_id);
       }
     }
 

test/experimental/usd/plugins/mjcf/mjcf_file_format_test.cc

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include <algorithm>
 #include <string>
 #include <vector>
 
@@ -41,6 +42,7 @@
 #include <pxr/usd/sdf/assetPath.h>
 #include <pxr/usd/sdf/declareHandles.h>
 #include <pxr/usd/sdf/fileFormat.h>
+#include <pxr/usd/sdf/listOp.h>
 #include <pxr/usd/sdf/path.h>
 #include <pxr/usd/sdf/schema.h>
 #include <pxr/usd/usd/common.h>
@@ -49,6 +51,7 @@
 #include <pxr/usd/usd/primRange.h>  // IWYU pragma: keep, used for TraverseAll
 #include <pxr/usd/usd/stage.h>
 #include <pxr/usd/usd/timeCode.h>
+#include <pxr/usd/usd/tokens.h>
 #include <pxr/usd/usdGeom/capsule.h>
 #include <pxr/usd/usdGeom/cube.h>
 #include <pxr/usd/usdGeom/cylinder.h>
@@ -1131,6 +1134,45 @@ TEST_F(MjcfSdfFileFormatPluginTest, TestPhysicsToggleSdfFormatArg) {
                           pxr::UsdPhysicsRigidBodyAPI);
 }
 
+TEST_F(MjcfSdfFileFormatPluginTest, TestArticulationRootAppliedOnce) {
+  static constexpr char kXml[] = R"(
+    <mujoco model="physics_test">
+      <worldbody>
+        <body name="parent" pos="0 0 0">
+          <geom name="parent_geom" type="sphere" size="1"/>
+          <body name="child_1" pos="1 0 0">
+            <geom name="child_1_geom" type="sphere" size="1"/>
+          </body>
+          <body name="child_2" pos="2 0 0">
+            <geom name="child_2_geom" type="sphere" size="1"/>
+          </body>
+        </body>
+      </worldbody>
+    </mujoco>
+  )";
+
+  pxr::SdfFileFormat::FileFormatArguments args;
+  args["usdMjcfToggleUsdPhysics"] = "true";
+  pxr::SdfLayerRefPtr layer = LoadLayer(kXml, args);
+
+  // This test is particular in the sense that the authoring mistake, which is
+  // made on the SdfLayer level, would disappear when we access the COMPOSED
+  // stage because duplicates are removed. So we need to check the SdfLayer
+  // directly to see the problem.
+  auto primSpec = layer->GetPrimAtPath(pxr::SdfPath("/physics_test/parent"));
+  EXPECT_TRUE(primSpec);
+
+  pxr::VtValue apiSchemasValue = primSpec->GetInfo(pxr::UsdTokens->apiSchemas);
+  const pxr::SdfTokenListOp& listOp =
+      apiSchemasValue.UncheckedGet<pxr::SdfTokenListOp>();
+  const pxr::SdfTokenListOp::ItemVector& prependedItems =
+      listOp.GetPrependedItems();
+
+  int count = std::count(prependedItems.begin(), prependedItems.end(),
+                         pxr::UsdPhysicsTokens->PhysicsArticulationRootAPI);
+  EXPECT_EQ(count, 1);
+}
+
 TEST_F(MjcfSdfFileFormatPluginTest, TestPhysicsRigidBody) {
   static constexpr char kXml[] = R"(
     <mujoco model="physics_test">