Copybara import of the project:

--
a5221e754c88580d103bfc7bfaf1ed32927ca499 by omar rayyan <olr7742@nyu.edu>:

Fix .mjz loading in simulate

COPYBARA_INTEGRATE_REVIEW=https://github.com/google-deepmind/mujoco/pull/3384 from omarrayyann:fix-mjz-simulate a5221e754c88580d103bfc7bfaf1ed32927ca499
PiperOrigin-RevId: 944573851
Change-Id: Ifba77ee9f9b97a7d8c0b0a47ac82d390a273672b

doc/changelog.rst

@@ -17,6 +17,9 @@ General
 - The :el:`attach` element now supports self-attachment (attaching elements of the current model to itself) by omitting
   the :at:`model` attribute. It also supports attaching a frame via the new :at:`frame` attribute, which is mutually
   exclusive with :at:`body`.
+- Fixed loading of ``.mjz`` archives in :ref:`simulate<saSimulate>`: the archive was unmounted before model compilation,
+  so assets contained in it failed to load. Failures in the ``mjz`` decoder now emit a warning with the underlying
+  error instead of the generic "could not decode content" message.
 
 .. admonition:: Breaking API changes
    :class: attention

simulate/main.cc

@@ -241,13 +241,21 @@ mjModel* LoadModel(const char* file, mj::Simulate& sim) {
   } else if (extension == ".xml") {
     mnew = mj_loadXML(filename, nullptr, loadError, kErrorLength);
   } else {
-    mjSpec* spec = mj_parse(filename, nullptr, nullptr, loadError, kErrorLength);
+    mjVFS vfs;
+    mj_defaultVFS(&vfs);
+    mjSpec* spec = mj_parse(filename, nullptr, &vfs, loadError, kErrorLength);
     if (!spec) {
-      mju::strcpy_arr(loadError, "could not parse model");
+      if (!loadError[0]) {
+        mju::strcpy_arr(loadError, "could not parse model");
+      }
     } else {
-      mnew = mj_compile(spec, nullptr);
+      mnew = mj_compile(spec, &vfs);
+      if (!mnew) {
+        mju::strcpy_arr(loadError, mjs_getError(spec));
+      }
       mj_deleteSpec(spec);
     }
+    mj_deleteVFS(&vfs);
   }
 
   // remove trailing newline character from loadError

src/xml/mjz/mjz_decoder.cc

@@ -233,9 +233,14 @@ mjPLUGIN_LIB_INIT(mjz_decoder) {
     if (size <= 0) {
       return nullptr;
     }
-    char error[1024];
-    return ParseZipBuffer(buffer, size, resource->name, const_cast<mjVFS*>(vfs),
-                          error, sizeof(error));
+    char error[1024] = "";
+    mjSpec* spec = ParseZipBuffer(buffer, size, resource->name,
+                                  const_cast<mjVFS*>(vfs), error,
+                                  sizeof(error));
+    if (!spec && error[0]) {
+      mju_warning("%s", error);
+    }
+    return spec;
   };
   mjp_registerDecoder(&decoder);
 }