Do not delete attached specs after compilation.

Also, copy them when copying the spec.

PiperOrigin-RevId: 688998464
Change-Id: I7229fd8032eafede582194ccf73ab4160fe06768

src/user/user_model.cc

@@ -388,6 +388,9 @@ mjCModel& mjCModel::operator+=(const mjCModel& other) {
     }
     CopyList(numerics_, other.numerics_);
     CopyList(texts_, other.texts_);
+    for (const auto* s : other.specs_) {
+      specs_.push_back(mj_copySpec(s));
+    }
   }
   CopyList(flexes_, other.flexes_);
   CopyList(pairs_, other.pairs_);
@@ -3648,12 +3651,6 @@ mjModel* mjCModel::Compile(const mjVFS* vfs, mjModel** m) {
     return nullptr;
   }
 
-  // destroy attached specs
-  for (auto spec : specs_) {
-    mj_deleteSpec(spec);
-  }
-  specs_.clear();
-
   // restore error handler, mark as compiled, return mjModel
   _mjPRIVATE__set_tls_error_fn(save_error);
   _mjPRIVATE__set_tls_warning_fn(save_warning);

src/xml/xml.cc

@@ -387,11 +387,11 @@ mjSpec* ParseXML(const char* filename, const mjVFS* vfs,
   return spec;
 }
 
-mjSpec* ParseSpecFromString(std::string_view xml, char* error, int nerror) {
+mjSpec* ParseSpecFromString(std::string_view xml, const mjVFS* vfs, char* error, int nerror) {
   RegisterResourceProvider();
   std::string xml2 = {xml.begin(), xml.end()};
   std::string str = "LoadModelFromString:" +  xml2;
-  return ParseXML(str.c_str(), nullptr, error, nerror);
+  return ParseXML(str.c_str(), vfs, error, nerror);
 }
 
 // Main writer function - calls mjXWrite

src/xml/xml.h

@@ -26,7 +26,8 @@ mjSpec* ParseXML(const char* filename, const mjVFS* vfs, char* error, int nerror
 
 // Returns a newly-allocated mjSpec, loaded from the contents of xml.
 // On failure returns nullptr and populates the error array if present.
-mjSpec* ParseSpecFromString(std::string_view xml, char* error = nullptr, int nerror = 0);
+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);

src/xml/xml_api.cc

@@ -221,7 +221,7 @@ mjSpec* mj_parseXML(const char* filename, const mjVFS* vfs, char* error, int err
 
 // parse spec from string
 mjSpec* mj_parseXMLString(const char* xml, const mjVFS* vfs, char* error, int error_sz) {
-  return ParseSpecFromString(xml, error, error_sz);
+  return ParseSpecFromString(xml, vfs, error, error_sz);
 }