Make mju_getXMLDependencies read XMLs through the resource provider.

The function called tinyxml2's LoadFile directly, which only works on
the OS file system. Reading through mju_openResource lets it work
against any registered backend (VFS, HTTP, github:, ...).

test/xml/xml_utils_test.cc

@@ -13,11 +13,14 @@
 // limitations under the License.
 
 
+#include <map>
 #include <set>
 #include <string>
 #include <vector>
 
+#include <gmock/gmock.h>
 #include <gtest/gtest.h>
+#include <mujoco/mjplugin.h>
 #include <mujoco/mujoco.h>
 #include "test/fixture.h"
 
@@ -42,5 +45,64 @@ TEST_F(MujocoTest, GetXMLDependenciesTest) {
                                                 kModelPaths.end()};
   EXPECT_EQ(dependency_set, expected_dependency_set);
 }
+
+// Custom resource provider that serves XML strings from an in-memory map,
+// used to exercise the non-filesystem code path.
+namespace memxml {
+static const std::map<std::string, std::string>* g_files = nullptr;
+
+int Open(mjResource* resource) {
+  return g_files && g_files->count(resource->name) ? 1 : 0;
+}
+int Read(mjResource* resource, const void** buffer) {
+  auto it = g_files->find(resource->name);
+  if (it == g_files->end()) return -1;
+  *buffer = it->second.data();
+  return static_cast<int>(it->second.size());
+}
+void Close(mjResource* resource) {}
+}  // namespace memxml
+
+// Verifies mju_getXMLDependencies works against a non-filesystem resource
+// provider (the case the WASM/HTTP build relies on).
+TEST_F(MujocoTest, GetXMLDependenciesViaResourceProvider) {
+  const std::map<std::string, std::string> files = {
+      {"memxml:/scene.xml",
+       "<mujoco>\n"
+       "  <include file=\"child.xml\"/>\n"
+       "  <compiler meshdir=\"meshes\"/>\n"
+       "  <asset>\n"
+       "    <mesh name=\"m\" file=\"m.obj\"/>\n"
+       "  </asset>\n"
+       "</mujoco>"},
+      {"memxml:/child.xml",
+       "<mujoco>\n"
+       "  <asset>\n"
+       "    <texture name=\"t\" type=\"2d\" file=\"t.png\"/>\n"
+       "  </asset>\n"
+       "</mujoco>"},
+  };
+  memxml::g_files = &files;
+
+  mjpResourceProvider provider = {
+      .prefix = "memxml",
+      .open = memxml::Open,
+      .read = memxml::Read,
+      .close = memxml::Close,
+  };
+  ASSERT_GT(mjp_registerResourceProvider(&provider), 0);
+
+  mjStringVec dependencies;
+  mju_getXMLDependencies("memxml:/scene.xml", &dependencies);
+  std::set<std::string> dep_set(dependencies.begin(), dependencies.end());
+
+  EXPECT_THAT(dep_set, testing::UnorderedElementsAre(
+      "memxml:/scene.xml",
+      "memxml:/child.xml",
+      "memxml:/meshes/m.obj",
+      "memxml:/t.png"));
+
+  memxml::g_files = nullptr;
+}
 }  // namespace
 }  // namespace mujoco