Move GlobalModel class and functionality into its own file.

We'll need some of this functionality for tests.

PiperOrigin-RevId: 853652149
Change-Id: Ib924d886b73784a0f370285e0b82b2ed833e3e77

src/xml/xml_global.cc

@@ -0,0 +1,77 @@
+// Copyright 2026 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "xml/xml_global.h"
+
+#include <mutex>
+#include <string>
+#include <type_traits>
+#include <mujoco/mujoco.h>
+#include "xml/xml.h"
+#include "xml/xml_util.h"
+
+namespace {
+
+// global user model class
+class GlobalModel {
+ public:
+  // deletes current model and takes ownership of model
+  void Set(mjSpec* spec = nullptr);
+
+  // writes XML to string
+  std::string ToXML(const mjModel* m, char* error, int error_sz);
+
+ private:
+  // using raw pointers as GlobalModel needs to be trivially destructible
+  std::mutex* mutex_ = new std::mutex();
+  mjSpec* spec_ = nullptr;
+};
+
+std::string GlobalModel::ToXML(const mjModel* m, char* error,
+                                              int error_sz) {
+  std::lock_guard<std::mutex> lock(*mutex_);
+  if (!spec_) {
+    mjCopyError(error, "No XML model loaded", error_sz);
+    return "";
+  }
+  return WriteXML(m, spec_, error, error_sz);
+}
+
+void GlobalModel::Set(mjSpec* spec) {
+  std::lock_guard<std::mutex> lock(*mutex_);
+  if (spec_ != nullptr) {
+    mj_deleteSpec(spec_);
+  }
+  spec_ = spec;
+}
+
+
+// returns a single instance of the global model
+GlobalModel& GetGlobalModel() {
+  static GlobalModel global_model;
+
+  // global variables must be trivially destructible
+  static_assert(std::is_trivially_destructible_v<decltype(global_model)>);
+  return global_model;
+}
+
+}  // namespace
+
+void SetGlobalXmlSpec(mjSpec* spec) {
+  GetGlobalModel().Set(spec);
+}
+
+std::string GetGlobalXmlSpec(const mjModel* m, char* error, int error_sz) {
+  return GetGlobalModel().ToXML(m, error, error_sz);
+}