Modify mj_PLUGIN_LIB_INIT to support multiple plugins in the same compilation unit.

Previously on MSVC we used DllMain to register plugins, you cannot have multiple definitions of DllMain in a single unit so you would get errors if you tried to register two plugins. This modifies the implementation to insert a function pointer into the C runtime initialization instead.

PiperOrigin-RevId: 896612678
Change-Id: I07732147b955d741c836acff6da986db0e9b9eff

doc/APIreference/APIglobals.rst

@@ -691,17 +691,13 @@ mjPLUGIN_LIB_INIT
 
 .. code-block:: C
 
-   #define mjPLUGIN_LIB_INIT                                                                 \
-     static void _mjplugin_dllmain(void);                                                    \
-     mjEXTERNC int __stdcall mjDLLMAIN(void* hinst, unsigned long reason, void* reserved) {  \
-       if (reason == 1) {                                                                    \
-         _mjplugin_dllmain();                                                                \
-       }                                                                                     \
-       return 1;                                                                             \
-     }                                                                                       \
-     static void _mjplugin_dllmain(void)
+   #define mjPLUGIN_LIB_INIT(n)                                      \
+        static void _mj_init_##n(void) __attribute__((constructor)); \
+        static void _mj_init_##n(void)
 
-Register a plugin as a dynamic library. See :ref:`plugin registration<exRegistration>` for more details.
+Register a plugin before `main()` is called. This macro takes a unique identifier `n` as an argument that is used to avoid
+name collisions between different plugin initialization functions. See :ref:`plugin registration<exRegistration>` for
+more details.
 
 
 .. _tyXMacro:

doc/changelog.rst

@@ -44,6 +44,10 @@ General
    - The ``vertcollide`` field in :ref:`mjsFlex` has been removed. It is no longer required since
      :doc:`MuJoCo Warp <mjwarp/index>` supports native flex collisions.
 
+   - :ref:`mjPLUGIN_LIB_INIT` macro now requires a name argument to avoid initialization function name collisions.
+     When building with MSVC, we now use the C runtime initialization section to initialize plugins instead of
+     ``DllMain``. See :ref:`plugin registration<exRegistration>` for more details.
+
    - The :ref:`mjtWarning` enum value ``mjWARN_VGEOMFULL`` is removed. Exhaustion of visual geoms is now handled
      internally by the :ref:`mjvScene`.
 

doc/programming/extension.rst

@@ -230,12 +230,11 @@ troubleshoot issues with a model) can be statically linked into the application.
 :ref:`mjpPlugin` struct in the ``main`` function, then passing it to :ref:`mjp_registerPlugin` to be registered with
 MuJoCo.
 
-Generally, reusable plugins are expected to be packaged as dynamic libraries. A dynamic library containing one or more
-MuJoCo plugins should make sure that all plugins are registered when the library is loaded. In GCC-compatible compilers,
-this can be achieved by calling :ref:`mjp_registerPlugin` in a function that is declared with
-``__attribute__((constructor))``, while in MSVC this can be done in a DLL entry point (canonically known as
-``DllMain``). MuJoCo provides a convenience macro :ref:`mjPLUGIN_LIB_INIT` that expands to either of these
-constructs depending on the compiler used.
+Generally, reusable plugins are expected to be packaged as libraries and should be registered when the library is
+loaded. In GCC-compatible compilers, this can be achieved by calling :ref:`mjp_registerPlugin` in a function that is
+declared with ``__attribute__((constructor))``, while in MSVC this can be done by injecting code into the C runtime
+initialization. MuJoCo provides a convenience macro :ref:`mjPLUGIN_LIB_INIT` that expands to either of these constructs
+depending on the compiler used.
 
 Users of plugins that are delivered as dynamic libraries as described above can load the library using the function
 :ref:`mj_loadPluginLibrary`. This is the preferred way to load dynamic libraries containing MuJoCo plugins (rather than,