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

test/fixture.cc

@@ -130,9 +130,7 @@ std::string GetFileContents(const char* path) {
   return sstream.str();
 }
 
-std::string SaveAndReadXml(const mjModel* model) {
-  EXPECT_THAT(model, testing::NotNull());
-
+std::string SaveAndReadXmlImpl(const mjModel* model, const mjSpec* spec) {
   constexpr int kMaxPathLen = 1024;
   std::string path_template =
       std::filesystem::temp_directory_path().append("tmp.XXXXXX").string();
@@ -148,7 +146,11 @@ std::string SaveAndReadXml(const mjModel* model) {
   EXPECT_NE(_mktemp_s(filepath), EINVAL);
 #endif
 
-  mj_saveLastXML(filepath, model, nullptr, 0);
+  if (spec) {
+    mj_saveXML(spec, filepath, nullptr, 0);
+  } else if (model) {
+    mj_saveLastXML(filepath, model, nullptr, 0);
+  }
   std::string contents = GetFileContents(filepath);
 
 #if defined(_POSIX_VERSION) && _POSIX_VERSION >= 200112L
@@ -159,6 +161,16 @@ std::string SaveAndReadXml(const mjModel* model) {
   return contents;
 }
 
+std::string SaveAndReadXml(const mjModel* model) {
+  EXPECT_THAT(model, testing::NotNull());
+  return SaveAndReadXmlImpl(model, nullptr);
+}
+
+std::string SaveAndReadXml(const mjSpec* spec) {
+  EXPECT_THAT(spec, testing::NotNull());
+  return SaveAndReadXmlImpl(nullptr, spec);
+}
+
 std::vector<mjtNum> GetCtrlNoise(const mjModel* m, int nsteps,
                                  mjtNum ctrlnoise) {
   std::vector<mjtNum> ctrl;

test/fixture.h

@@ -102,6 +102,9 @@ mjModel* LoadModelFromPath(const char* model_path);
 // Returns a string loaded from first saving the model given an input.
 std::string SaveAndReadXml(const mjModel* model);
 
+// Returns a string loaded from first saving the spec given an input.
+std::string SaveAndReadXml(const mjSpec* spec);
+
 // Adds control noise.
 std::vector<mjtNum> GetCtrlNoise(const mjModel* m, int nsteps,
                                  mjtNum ctrlnoise = 0.01);

test/xml/xml_native_writer_test.cc

@@ -1498,6 +1498,65 @@ TEST_F(DecompilerTest, SavesStatistics) {
   mj_deleteModel(model);
 }
 
+TEST_F(DecompilerTest, SaveAndReadXml) {
+  static constexpr char xml1[] = R"(
+  <mujoco>
+    <worldbody>
+      <geom size="1"/>
+      <geom size="2"/>
+    </worldbody>
+  </mujoco>
+  )";
+  static constexpr char xml2[] = R"(
+  <mujoco>
+    <worldbody>
+      <geom size="1"/>
+      <geom size="2"/>
+      <geom size="3"/>
+    </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  mjModel* m1 = LoadModelFromString(xml1, error.data(), error.size());
+  ASSERT_THAT(m1, NotNull()) << error.data();
+  m1->geom_size[0] = 10;
+  m1->geom_size[3] = 20;
+  std::string saved_xml = SaveAndReadXml(m1);
+  EXPECT_THAT(saved_xml, HasSubstr("geom size=\"10\""));
+  EXPECT_THAT(saved_xml, HasSubstr("geom size=\"20\""));
+
+  // parse the mjSpec, save it and read it back
+  mjSpec* spec = mj_parseXMLString(xml2, nullptr, error.data(), error.size());
+  EXPECT_THAT(spec, NotNull()) << error.data();
+  mjModel* m2 = mj_compile(spec, nullptr);
+  std::string saved_xml1 = SaveAndReadXml(spec);
+  EXPECT_THAT(saved_xml1, HasSubstr("geom size=\"1\""));
+  EXPECT_THAT(saved_xml1, HasSubstr("geom size=\"2\""));
+  EXPECT_THAT(saved_xml1, HasSubstr("geom size=\"3\""));
+
+  // modify the mjModel, save it and read it back
+  m2->geom_size[0] = .1;
+  m2->geom_size[3] = .2;
+  m2->geom_size[6] = .3;
+  EXPECT_EQ(mj_copyBack(spec, m1), 0);
+  EXPECT_THAT(mjs_getError(spec), HasSubstr("CopyBack"));
+  EXPECT_EQ(mj_copyBack(spec, m2), 1);
+  std::string saved_xml2 = SaveAndReadXml(spec);
+  EXPECT_THAT(saved_xml2, HasSubstr("geom size=\"0.1\""));
+  EXPECT_THAT(saved_xml2, HasSubstr("geom size=\"0.2\""));
+  EXPECT_THAT(saved_xml2, HasSubstr("geom size=\"0.3\""));
+
+  // check that using mjModel as argument writes in the wrong mjSpec
+  std::string saved_xml3 = SaveAndReadXml(m2);
+  EXPECT_THAT(saved_xml3, Not(HasSubstr("geom size=\"0.1\"")));
+  EXPECT_THAT(saved_xml3, Not(HasSubstr("geom size=\"0.2\"")));
+  EXPECT_THAT(saved_xml3, Not(HasSubstr("geom size=\"0.3\"")));
+
+  mj_deleteSpec(spec);
+  mj_deleteModel(m1);
+  mj_deleteModel(m2);
+}
+
 TEST_F(DecompilerTest, DoesntSaveInferredStatistics) {
   static constexpr char xml[] = R"(
   <mujoco>