Refactor thread pool implementation.

- Make mjTask non-opaque and remove C++ Task class.
- Make C++ thread pool a subclass of a skeletal mjThreadPool C struct.
- Make the mjTask status enum more consistent with the rest of MuJoCo.
- Change mju_threadPoolEnqueue to take just the mjTask. Users must now prepare the mjTask by assigning the function pointer and argument into the struct.
- Rename files in thread/ to be more consistent with the rest of MuJoCo.
- Run threading tests in CMake.
- Allow use of C++20 designated initializers.

Otherwise the functionality remains identical.

PiperOrigin-RevId: 564374675
Change-Id: I37c9894566bc39faf217e5aa97a4e2713a70e467

include/mujoco/mjthread.h

@@ -15,33 +15,26 @@
 #ifndef MUJOCO_INCLUDE_MJTHREAD_H_
 #define MUJOCO_INCLUDE_MJTHREAD_H_
 
-// C API for MuJoCo threading
-#ifdef __cplusplus
-extern "C" {
-#endif
+typedef enum mjtTaskStatus_ {  // status values for mjTask
+  mjTASK_NEW = 0,              // newly created
+  mjTASK_QUEUED,               // enqueued in a thread pool
+  mjTASK_COMPLETED             // completed execution
+} mjtTaskStatus;
 
-#include <stddef.h>
-
-#include <mujoco/mjexport.h>
-
-// These types are implemented in C++, they're just used as opaque pointers in C
-// to provide type safety for functions.
-struct mjTask_ {
-  char buffer[24];
-};
-typedef struct mjTask_ mjTask;
+// function pointer type for mjTask
+typedef void* (*mjfTask)(void*);
 
+// An opaque type representing a thread pool.
 struct mjThreadPool_ {
-  char buffer[6208];
+  int nworker;  // number of workers in the pool
 };
 typedef struct mjThreadPool_ mjThreadPool;
 
-typedef void*(*mjStartRoutine_)(void*);
-typedef mjStartRoutine_ mjStartRoutine;
-
-#ifdef __cplusplus
-}
-#endif
-
+struct mjTask_ {        // a task that can be executed by a thread pool.
+  mjfTask func;         // pointer to the function that implements the task
+  void* args;           // arguments to func
+  volatile int status;  // status of the task
+};
+typedef struct mjTask_ mjTask;
 
 #endif  // MUJOCO_INCLUDE_MJTHREAD_H_

include/mujoco/mujoco.h

@@ -1307,19 +1307,21 @@ MJAPI const mjpResourceProvider* mjp_getResourceProviderAtSlot(int slot);
 
 //---------------------- Thread -------------------------------------------------------------------
 
-// Creates a thread pool with the specified number of threads running.
+// Create a thread pool with the specified number of threads running.
 MJAPI mjThreadPool* mju_threadPoolCreate(size_t number_of_threads);
 
-// Enqueues a task in a thread pool.
-MJAPI void mju_threadPoolEnqueue(
-    mjThreadPool* thread_pool, mjTask* task, void*(start_routine)(void*),
-    void* args);
+// Enqueue a task in a thread pool.
+MJAPI void mju_threadPoolEnqueue(mjThreadPool* thread_pool, mjTask* task);
 
-// Waits for a task to complete.
+// Destroy a thread pool.
+MJAPI void mju_threadPoolDestroy(mjThreadPool* thread_pool);
+
+// Initialize an mjTask.
+MJAPI void mju_defaultTask(mjTask* task);
+
+// Wait for a task to complete.
 MJAPI void mju_taskJoin(mjTask* task);
 
-// Destroys a thread pool.
-MJAPI void mju_threadPoolDestroy(mjThreadPool* thread_pool);
 
 #if defined(__cplusplus)
 }