Do not delete attached specs after compilation.

Also, copy them when copying the spec.

PiperOrigin-RevId: 688998464
Change-Id: I7229fd8032eafede582194ccf73ab4160fe06768

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);
 }