Add mj_copyBack for copying real-valued arrays from mjModel back to mjSpec.

Also add `SaveAndReadXML` function to test fixture using `mjSpec` as input.

PiperOrigin-RevId: 765393821
Change-Id: Ic257addd2fc89678fc11b226c168c077626cc51e

src/user/user_api.cc

@@ -375,6 +375,14 @@ int mjs_setDeepCopy(mjSpec* s, int deepcopy) {
 
 
 
+// copy real-valued arrays from model to spec, returns 1 on success
+int mj_copyBack(mjSpec* s, const mjModel* m) {
+  mjCModel* model = static_cast<mjCModel*>(s->element);
+  return model->CopyBack(m);
+}
+
+
+
 // delete object, return 0 on success
 int mjs_delete(mjsElement* element) {
   mjCModel* model;

src/user/user_api.h

@@ -66,6 +66,9 @@ MJAPI int mjs_activatePlugin(mjSpec* s, const char* name);
 // Turn deep copy on or off attach. Returns 0 on success.
 MJAPI int mjs_setDeepCopy(mjSpec* s, int deepcopy);
 
+// Copy real-valued arrays from model to spec, returns 1 on success.
+MJAPI int mj_copyBack(mjSpec* s, const mjModel* m);
+
 
 //---------------------------------- Attachment ----------------------------------------------------
 

src/user/user_model.cc

@@ -4759,6 +4759,11 @@ bool mjCModel::CopyBack(const mjModel* m) {
     return false;
   }
 
+  if (spec.element->signature != m->signature) {
+    errInfo = mjCError(0, "incompatible signatures in CopyBack");
+    return false;
+  }
+
   // option and visual
   option = m->opt;
   visual = m->vis;

src/xml/xml.cc

@@ -395,7 +395,7 @@ mjSpec* ParseSpecFromString(std::string_view xml, const mjVFS* vfs, char* error,
 }
 
 // Main writer function - calls mjXWrite
-std::string WriteXML(const mjModel* m, const mjSpec* spec, char* error, int nerror) {
+std::string WriteXML(const mjModel* m, mjSpec* spec, char* error, int nerror) {
   LocaleOverride locale_override;
 
   // check for empty model

src/xml/xml.h

@@ -30,6 +30,6 @@ mjSpec* ParseSpecFromString(std::string_view xml, const mjVFS* vfs = nullptr,
                             char* error = nullptr, int nerror = 0);
 
 // Main writer function
-std::string WriteXML(const mjModel* m, const mjSpec* spec, char* error, int nerror);
+std::string WriteXML(const mjModel* m, mjSpec* spec, char* error, int nerror);
 
 #endif  // MUJOCO_SRC_XML_XML_H_

src/xml/xml_api.cc

@@ -230,7 +230,8 @@ mjSpec* mj_parseXMLString(const char* xml, const mjVFS* vfs, char* error, int er
 
 // save spec to XML file, return 0 on success, -1 otherwise
 int mj_saveXML(const mjSpec* s, const char* filename, char* error, int error_sz) {
-  std::string result = WriteXML(NULL, s, error, error_sz);
+  // cast to mjSpec since WriteXML can in principle perform mj_copyBack (not here)
+  std::string result = WriteXML(NULL, (mjSpec*)s, error, error_sz);
   if (result.empty()) {
     return -1;
   }
@@ -247,7 +248,7 @@ int mj_saveXML(const mjSpec* s, const char* filename, char* error, int error_sz)
 // save spec to XML string, return 0 on success, -1 on failure
 // if length of the output buffer is too small, returns the required size
 int mj_saveXMLString(const mjSpec* s, char* xml, int xml_sz, char* error, int error_sz) {
-  std::string result = WriteXML(NULL, s, error, error_sz);
+  std::string result = WriteXML(NULL, (mjSpec*)s, error, error_sz);
   if (result.empty()) {
     return -1;
   } else if (result.size() >= xml_sz) {

src/xml/xml_base.cc

@@ -44,8 +44,8 @@ mjXBase::mjXBase() {
 
 
 // set model field
-void mjXBase::SetModel(const mjSpec* _model, const mjModel* m) {
-  spec = (mjSpec*)_model;
+void mjXBase::SetModel(mjSpec* _model, const mjModel* m) {
+  spec = _model;
 }
 
 

src/xml/xml_base.h

@@ -92,7 +92,7 @@ class mjXBase : public mjXUtil {
   };
 
   // set the model allocated externally
-  virtual void SetModel(const mjSpec*, const mjModel* = nullptr);
+  virtual void SetModel(mjSpec*, const mjModel* = nullptr);
 
   // read alternative orientation specification
   static int ReadAlternative(tinyxml2::XMLElement* elem, mjsOrientation& alt);

src/xml/xml_native_writer.cc

@@ -885,12 +885,12 @@ mjXWriter::mjXWriter(void) {
 
 
 // cast model
-void mjXWriter::SetModel(const mjSpec* _spec, const mjModel* m) {
+void mjXWriter::SetModel(mjSpec* _spec, const mjModel* m) {
   if (_spec) {
     model = static_cast<mjCModel*>(_spec->element);
   }
   if (m) {
-    model->CopyBack(m);
+    mj_copyBack(&model->spec, m);
   }
 }
 

src/xml/xml_native_writer.h

@@ -29,7 +29,7 @@ class mjXWriter : public mjXBase {
  public:
   mjXWriter();                                        // constructor
   virtual ~mjXWriter() = default;                     // destructor
-  void SetModel(const mjSpec* _spec, const mjModel* m = nullptr);
+  void SetModel(mjSpec* _spec, const mjModel* m = nullptr);
 
   // write XML document to string
   std::string Write(char *error, std::size_t error_sz);