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

test/thread/thread_queue_test.cc

@@ -0,0 +1,46 @@
+// Copyright 2023 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "src/thread/thread_queue.h"
+
+#include <cstddef>
+
+#include <gtest/gtest.h>
+
+namespace mujoco {
+namespace {
+
+constexpr size_t kBufferCapacity = 640;
+
+TEST(TestMujocoLocklessQueue, TestMujocoLocklessQueue) {
+  LocklessQueue<void*, 640> queue;
+  EXPECT_TRUE(queue.empty());
+  int test_integers[kBufferCapacity];
+  for (int h = 0; h < 10; ++h) {
+    for (int i = 0; i < kBufferCapacity; ++i) {
+      test_integers[i] = i;
+      queue.push(&test_integers[i]);
+    }
+    EXPECT_TRUE(queue.full());
+
+    for (int i = 0; i < kBufferCapacity; ++i) {
+      void* output_ptr = queue.pop();
+      ASSERT_EQ(output_ptr, &test_integers[i]);
+    }
+    EXPECT_TRUE(queue.empty());
+  }
+}
+
+}  // namespace
+}  // namespace mujoco