Use mj_parseXMLString instead of a test-specific LoadFromModelString.

This removes the need for a custom mjResourceProvider, but does require
reimplementing part of mj_loadXML in the fixture.

PiperOrigin-RevId: 854170820
Change-Id: Ib455cc5cdb5138213eb7e996dc32ac156f22a859

src/xml/xml_global.h

@@ -20,7 +20,7 @@
 
 #include <mujoco/mujoco.h>
 
-void SetGlobalXmlSpec(mjSpec* spec = nullptr);
+MJAPI void SetGlobalXmlSpec(mjSpec* spec = nullptr);
 
 std::string GetGlobalXmlSpec(const mjModel* m, char* error, int error_sz);
 

test/fixture.cc

@@ -41,6 +41,7 @@
 #include <mujoco/mjmodel.h>
 #include <mujoco/mjxmacro.h>
 #include <mujoco/mujoco.h>
+#include "src/xml/xml_global.h"
 
 namespace mujoco {
 namespace {
@@ -85,26 +86,27 @@ const std::string GetModelPath(std::string_view path) {  // NOLINT
 
 mjModel* LoadModelFromString(std::string_view xml, char* error,
                              int error_size, mjVFS* vfs) {
-  // register string resource provider if not registered before
-  if (mjp_getResourceProvider("LoadModelFromString:") == nullptr) {
-    mjpResourceProvider resourceProvider;
-    mjp_defaultResourceProvider(&resourceProvider);
-    resourceProvider.prefix = "LoadModelFromString";
-    resourceProvider.open = +[](mjResource* resource) {
-      resource->data = &(resource->name[strlen("LoadModelFromString:")]);
-      return 1;
-    };
-    resourceProvider.read =
-        +[](mjResource* resource, const void** buffer) {
-          *buffer = resource->data;
-          return (int) strlen((const char*) resource->data);
-        };
-    resourceProvider.close = +[](mjResource* resource) {};
-    mjp_registerResourceProvider(&resourceProvider);
+  if (error) {
+    error[0] = '\0';
   }
-  std::string xml2 = {xml.begin(), xml.end()};
-  std::string str = "LoadModelFromString:" +  xml2;
-  return mj_loadXML(str.c_str(), vfs, error, error_size);
+
+  // This duplicates the logic in mj_loadXML, but allows us to use a string
+  // directly rather than having to write the contents to a file. Most
+  // importantly, we "save" the spec to global storage so that subsequent calls
+  // to mj_saveLastXML will be done using the parsed mjSpec.
+  mjSpec* spec = mj_parseXMLString(xml.data(), vfs, error, error_size);
+
+  mjModel* model = nullptr;
+  if (spec) {
+    model = mj_compile(spec, vfs);
+
+    if (error && (!model || mjs_isWarning(spec))) {
+      strncpy(error, mjs_getError(spec), error_size);
+      error[error_size - 1] = '\0';
+    }
+  }
+  SetGlobalXmlSpec(spec);
+  return model;
 }
 
 static void AssertModelNotNull(mjModel* model,