Implement a Worker ThreadPool

This PR implements a worked thread pool. It uses a fixed amount of threads in a pool and allows scheduling tasks
that can be run on threads (and then waited for). It satisfies the following use cases:

* HTML5 thread count is fixed (and similar restrictions are known in consoles) so we need to reuse threads.
* Thread spawning is slow in general, so reusing threads is faster anyway.
* This implementation supports recursive waiting for tasks, making it less prone to deadlocks if threads from the pool also run tasks.

After this is approved and merged, subsequent PRs will be needed to replace the ThreadWorkPool usage by this class.

4 files changed, 630 insertions, 1 deletions

diff --git a/core/object/worker_thread_pool.cpp b/core/object/worker_thread_pool.cpp
new file mode 100644
index 0000000000..c276802f99
--- /dev/null
+++ b/core/object/worker_thread_pool.cpp
@@ -0,0 +1,463 @@
+/*************************************************************************/
+/*  worker_thread_pool.cpp                                               */
+/*************************************************************************/
+/*                       This file is part of:                           */
+/*                           GODOT ENGINE                                */
+/*                      https://godotengine.org                          */
+/*************************************************************************/
+/* Copyright (c) 2007-2022 Juan Linietsky, Ariel Manzur.                 */
+/* Copyright (c) 2014-2022 Godot Engine contributors (cf. AUTHORS.md).   */
+/*                                                                       */
+/* Permission is hereby granted, free of charge, to any person obtaining */
+/* a copy of this software and associated documentation files (the       */
+/* "Software"), to deal in the Software without restriction, including   */
+/* without limitation the rights to use, copy, modify, merge, publish,   */
+/* distribute, sublicense, and/or sell copies of the Software, and to    */
+/* permit persons to whom the Software is furnished to do so, subject to */
+/* the following conditions:                                             */
+/*                                                                       */
+/* The above copyright notice and this permission notice shall be        */
+/* included in all copies or substantial portions of the Software.       */
+/*                                                                       */
+/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,       */
+/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF    */
+/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.*/
+/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY  */
+/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,  */
+/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE     */
+/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
+/*************************************************************************/
+
+#include "worker_thread_pool.h"
+
+#include "core/os/os.h"
+
+WorkerThreadPool *WorkerThreadPool::singleton = nullptr;
+
+void WorkerThreadPool::_process_task_queue() {
+	task_mutex.lock();
+	Task *task = task_queue.first()->self();
+	task_queue.remove(task_queue.first());
+	task_mutex.unlock();
+	_process_task(task);
+}
+
+void WorkerThreadPool::_process_task(Task *p_task) {
+	bool low_priority = p_task->low_priority;
+
+	if (p_task->group) {
+		// Handling a group
+		bool do_post = false;
+		if (p_task->native_group_func) {
+			while (true) {
+				uint32_t work_index = p_task->group->index.postincrement();
+				if (work_index >= p_task->group->max) {
+					do_post = work_index == p_task->group->max; // First one reaching max handles semaphore and clean-up.
+					break;
+				}
+				p_task->native_group_func(p_task->native_func_userdata, work_index);
+			}
+
+		} else {
+			Callable::CallError ce;
+			Variant ret;
+			Variant arg;
+			Variant *argptr = &arg;
+			while (true) {
+				uint32_t work_index = p_task->group->index.postincrement();
+				if (work_index >= p_task->group->max) {
+					do_post = work_index == p_task->group->max; // First one reaching max handles semaphore and clean-up.
+					break;
+				}
+				arg = work_index;
+				p_task->callable.call((const Variant **)&argptr, 1, ret, ce);
+			}
+		}
+
+		if (low_priority && use_native_low_priority_threads) {
+			p_task->completed = true;
+			p_task->done_semaphore.post();
+			if (do_post) {
+				p_task->group->completed.set_to(true);
+			}
+		} else {
+			if (do_post) {
+				p_task->group->done_semaphore.post();
+				p_task->group->completed.set_to(true);
+			}
+			uint32_t max_users = p_task->group->tasks_used + 1; // Add 1 because the thread waiting for it is also user. Read before to avoid another thread freeing task after increment.
+			uint32_t finished_users = p_task->group->finished.increment();
+
+			if (finished_users == max_users) {
+				// Get rid of the group, because nobody else is using it.
+				task_mutex.lock();
+				group_allocator.free(p_task->group);
+				task_mutex.unlock();
+			}
+
+			// For groups, tasks get rid of themselves.
+
+			task_mutex.lock();
+			task_allocator.free(p_task);
+			task_mutex.unlock();
+		}
+	} else {
+		if (p_task->native_func) {
+			p_task->native_func(p_task->native_func_userdata);
+		} else {
+			Callable::CallError ce;
+			Variant ret;
+			p_task->callable.call(nullptr, 0, ret, ce);
+		}
+
+		p_task->completed = true;
+		p_task->done_semaphore.post();
+	}
+
+	if (!use_native_low_priority_threads && low_priority) {
+		// A low prioriry task was freed, so see if we can move a pending one to the high priority queue.
+		bool post = false;
+		task_mutex.lock();
+		if (low_priority_task_queue.first()) {
+			Task *low_prio_task = low_priority_task_queue.first()->self();
+			low_priority_task_queue.remove(low_priority_task_queue.first());
+			task_queue.add_last(&low_prio_task->task_elem);
+			post = true;
+		} else {
+			low_priority_threads_used.decrement();
+		}
+		task_mutex.lock();
+		if (post) {
+			task_available_semaphore.post();
+		}
+	}
+}
+
+void WorkerThreadPool::_thread_function(void *p_user) {
+	while (true) {
+		singleton->task_available_semaphore.wait();
+		if (singleton->exit_threads.is_set()) {
+			break;
+		}
+		singleton->_process_task_queue();
+	}
+}
+
+void WorkerThreadPool::_native_low_priority_thread_function(void *p_user) {
+	Task *task = (Task *)p_user;
+	singleton->_process_task(task);
+}
+
+void WorkerThreadPool::_post_task(Task *p_task, bool p_high_priority) {
+	task_mutex.lock();
+	p_task->low_priority = !p_high_priority;
+	if (!p_high_priority && use_native_low_priority_threads) {
+		task_mutex.unlock();
+		p_task->low_priority_thread = native_thread_allocator.alloc();
+		p_task->low_priority_thread->start(_native_low_priority_thread_function, p_task); // Pask task directly to thread.
+
+	} else if (p_high_priority || low_priority_threads_used.get() < max_low_priority_threads) {
+		task_queue.add_last(&p_task->task_elem);
+		if (!p_high_priority) {
+			low_priority_threads_used.increment();
+		}
+		task_mutex.unlock();
+		task_available_semaphore.post();
+	} else {
+		// Too many threads using low priority, must go to queue.
+		low_priority_task_queue.add_last(&p_task->task_elem);
+		task_mutex.unlock();
+	}
+}
+
+WorkerThreadPool::TaskID WorkerThreadPool::add_native_task(void (*p_func)(void *), void *p_userdata, bool p_high_priority, const String &p_description) {
+	task_mutex.lock();
+	// Get a free task
+	Task *task = task_allocator.alloc();
+	TaskID id = last_task++;
+	task->native_func = p_func;
+	task->native_func_userdata = p_userdata;
+	task->description = p_description;
+	tasks.insert(id, task);
+	task_mutex.unlock();
+
+	_post_task(task, p_high_priority);
+
+	return id;
+}
+
+WorkerThreadPool::TaskID WorkerThreadPool::add_task(const Callable &p_action, bool p_high_priority, const String &p_description) {
+	task_mutex.lock();
+	// Get a free task
+	Task *task = task_allocator.alloc();
+	TaskID id = last_task++;
+	task->callable = p_action;
+	task->description = p_description;
+	tasks.insert(id, task);
+	task_mutex.unlock();
+
+	_post_task(task, p_high_priority);
+
+	return id;
+}
+
+bool WorkerThreadPool::is_task_completed(TaskID p_task_id) const {
+	task_mutex.lock();
+	const Task *const *taskp = tasks.getptr(p_task_id);
+	if (!taskp) {
+		task_mutex.unlock();
+		ERR_FAIL_V_MSG(false, "Invalid Task ID"); // Invalid task
+	}
+
+	bool completed = (*taskp)->completed;
+	task_mutex.unlock();
+
+	return completed;
+}
+
+void WorkerThreadPool::wait_for_task_completion(TaskID p_task_id) {
+	task_mutex.lock();
+	Task **taskp = tasks.getptr(p_task_id);
+	if (!taskp) {
+		task_mutex.unlock();
+		ERR_FAIL_MSG("Invalid Task ID"); // Invalid task
+	}
+	Task *task = *taskp;
+
+	if (task->waiting) {
+		String description = task->description;
+		task_mutex.unlock();
+		if (description.is_empty()) {
+			ERR_FAIL_MSG("Another thread is waiting on this task: " + itos(p_task_id)); // Invalid task
+		} else {
+			ERR_FAIL_MSG("Another thread is waiting on this task: " + description + " (" + itos(p_task_id) + ")"); // Invalid task
+		}
+	}
+
+	task->waiting = true;
+
+	task_mutex.unlock();
+
+	if (use_native_low_priority_threads && task->low_priority) {
+		task->low_priority_thread->wait_to_finish();
+		native_thread_allocator.free(task->low_priority_thread);
+	} else {
+		int *index = thread_ids.getptr(Thread::get_caller_id());
+
+		if (index) {
+			// We are an actual process thread, we must not be blocked so continue processing stuff if available.
+			while (true) {
+				if (task->done_semaphore.try_wait()) {
+					// If done, exit
+					break;
+				}
+				if (task_available_semaphore.try_wait()) {
+					// Solve tasks while they are around.
+					_process_task_queue();
+					continue;
+				}
+				OS::get_singleton()->delay_usec(1); // Microsleep, this could be converted to waiting for multiple objects in supported platforms for a bit more performance.
+			}
+		} else {
+			task->done_semaphore.wait();
+		}
+	}
+
+	task_mutex.lock();
+	tasks.erase(p_task_id);
+	task_allocator.free(task);
+	task_mutex.unlock();
+}
+
+WorkerThreadPool::GroupID WorkerThreadPool::add_native_group_task(void (*p_func)(void *, uint32_t), void *p_userdata, int p_elements, int p_tasks, bool p_high_priority, const String &p_description) {
+	ERR_FAIL_COND_V(p_elements <= 0, INVALID_TASK_ID);
+	if (p_tasks < 0) {
+		p_tasks = threads.size();
+	}
+
+	task_mutex.lock();
+	Group *group = group_allocator.alloc();
+	GroupID id = last_task++;
+	group->max = p_elements;
+	group->self = id;
+	group->tasks_used = p_tasks;
+	Task **tasks_posted = (Task **)alloca(sizeof(Task *) * p_tasks);
+	for (int i = 0; i < p_tasks; i++) {
+		Task *task = task_allocator.alloc();
+		task->native_group_func = p_func;
+		task->native_func_userdata = p_userdata;
+		task->description = p_description;
+		task->group = group;
+		tasks_posted[i] = task;
+		// No task ID is used.
+	}
+	groups[id] = group;
+	task_mutex.unlock();
+
+	if (!p_high_priority && use_native_low_priority_threads) {
+		group->low_priority_native_tasks.resize(p_tasks);
+	}
+
+	for (int i = 0; i < p_tasks; i++) {
+		_post_task(tasks_posted[i], p_high_priority);
+		if (!p_high_priority && use_native_low_priority_threads) {
+			group->low_priority_native_tasks[i] = tasks_posted[i];
+		}
+	}
+
+	return id;
+}
+
+WorkerThreadPool::GroupID WorkerThreadPool::add_group_task(const Callable &p_action, int p_elements, int p_tasks, bool p_high_priority, const String &p_description) {
+	ERR_FAIL_COND_V(p_elements <= 0, INVALID_TASK_ID);
+	if (p_tasks < 0) {
+		p_tasks = threads.size();
+	}
+
+	task_mutex.lock();
+	Group *group = group_allocator.alloc();
+	GroupID id = last_task++;
+	group->max = p_elements;
+	group->self = id;
+	group->tasks_used = p_tasks;
+	Task **tasks_posted = (Task **)alloca(sizeof(Task *) * p_tasks);
+	for (int i = 0; i < p_tasks; i++) {
+		Task *task = task_allocator.alloc();
+		task->callable = p_action;
+		task->description = p_description;
+		task->group = group;
+		tasks_posted[i] = task;
+		// No task ID is used.
+	}
+	groups[id] = group;
+	task_mutex.unlock();
+
+	if (!p_high_priority && use_native_low_priority_threads) {
+		group->low_priority_native_tasks.resize(p_tasks);
+	}
+
+	for (int i = 0; i < p_tasks; i++) {
+		_post_task(tasks_posted[i], p_high_priority);
+		if (!p_high_priority && use_native_low_priority_threads) {
+			group->low_priority_native_tasks[i] = tasks_posted[i];
+		}
+	}
+	return id;
+}
+
+bool WorkerThreadPool::is_group_task_completed(GroupID p_group) const {
+	task_mutex.lock();
+	const Group *const *groupp = groups.getptr(p_group);
+	if (!groupp) {
+		task_mutex.unlock();
+		ERR_FAIL_V_MSG(false, "Invalid Group ID");
+	}
+	bool completed = (*groupp)->completed.is_set();
+	task_mutex.unlock();
+	return completed;
+}
+
+void WorkerThreadPool::wait_for_group_task_completion(GroupID p_group) {
+	task_mutex.lock();
+	Group **groupp = groups.getptr(p_group);
+	task_mutex.unlock();
+	if (!groupp) {
+		ERR_FAIL_MSG("Invalid Group ID");
+	}
+	Group *group = *groupp;
+
+	if (group->low_priority_native_tasks.size() > 0) {
+		for (uint32_t i = 0; i < group->low_priority_native_tasks.size(); i++) {
+			group->low_priority_native_tasks[i]->low_priority_thread->wait_to_finish();
+			native_thread_allocator.free(group->low_priority_native_tasks[i]->low_priority_thread);
+			task_mutex.lock();
+			task_allocator.free(group->low_priority_native_tasks[i]);
+			task_mutex.unlock();
+		}
+
+		task_mutex.lock();
+		group_allocator.free(group);
+		task_mutex.unlock();
+	} else {
+		group->done_semaphore.wait();
+
+		uint32_t max_users = group->tasks_used + 1; // Add 1 because the thread waiting for it is also user. Read before to avoid another thread freeing task after increment.
+		uint32_t finished_users = group->finished.increment(); // fetch happens before inc, so increment later.
+
+		if (finished_users == max_users) {
+			// All tasks using this group are gone (finished before the group), so clear the gorup too.
+			task_mutex.lock();
+			group_allocator.free(group);
+			task_mutex.unlock();
+		}
+	}
+
+	groups.erase(p_group); // Threads do not access this, so safe to erase here.
+}
+
+void WorkerThreadPool::init(int p_thread_count, bool p_use_native_threads_low_priority, float p_low_priority_task_ratio) {
+	ERR_FAIL_COND(threads.size() > 0);
+	if (p_thread_count < 0) {
+		p_thread_count = OS::get_singleton()->get_default_thread_pool_size();
+	}
+
+	if (p_use_native_threads_low_priority) {
+		max_low_priority_threads = 0;
+	} else {
+		max_low_priority_threads = CLAMP(p_thread_count * p_low_priority_task_ratio, 1, p_thread_count);
+	}
+
+	use_native_low_priority_threads = p_use_native_threads_low_priority;
+
+	threads.resize(p_thread_count);
+
+	for (uint32_t i = 0; i < threads.size(); i++) {
+		threads[i].index = i;
+		threads[i].thread.start(&WorkerThreadPool::_thread_function, &threads[i]);
+		thread_ids.insert(threads[i].thread.get_id(), i);
+	}
+}
+
+void WorkerThreadPool::finish() {
+	if (threads.size() == 0) {
+		return;
+	}
+
+	task_mutex.lock();
+	SelfList<Task> *E = low_priority_task_queue.first();
+	while (E) {
+		print_error("Task waiting was never re-claimed: " + E->self()->description);
+		E = E->next();
+	}
+	task_mutex.unlock();
+
+	exit_threads.set_to(true);
+
+	for (uint32_t i = 0; i < threads.size(); i++) {
+		task_available_semaphore.post();
+	}
+
+	for (uint32_t i = 0; i < threads.size(); i++) {
+		threads[i].thread.wait_to_finish();
+	}
+
+	threads.clear();
+}
+
+void WorkerThreadPool::_bind_methods() {
+	ClassDB::bind_method(D_METHOD("add_task", "action", "high_priority", "description"), &WorkerThreadPool::add_task, DEFVAL(false), DEFVAL(String()));
+	ClassDB::bind_method(D_METHOD("is_task_completed", "task_id"), &WorkerThreadPool::is_task_completed);
+	ClassDB::bind_method(D_METHOD("wait_for_task_completion", "task_id"), &WorkerThreadPool::wait_for_task_completion);
+
+	ClassDB::bind_method(D_METHOD("add_group_task", "action", "elements", "tasks_needed", "high_priority", "description"), &WorkerThreadPool::add_group_task, DEFVAL(-1), DEFVAL(false), DEFVAL(String()));
+	ClassDB::bind_method(D_METHOD("is_group_task_completed", "group_id"), &WorkerThreadPool::is_group_task_completed);
+	ClassDB::bind_method(D_METHOD("wait_for_group_task_completion", "group_id"), &WorkerThreadPool::wait_for_group_task_completion);
+}
+
+WorkerThreadPool::WorkerThreadPool() {
+	singleton = this;
+}
+
+WorkerThreadPool::~WorkerThreadPool() {
+	finish();
+}
diff --git a/core/object/worker_thread_pool.h b/core/object/worker_thread_pool.h
new file mode 100644
index 0000000000..dfb0050605
--- /dev/null
+++ b/core/object/worker_thread_pool.h
@@ -0,0 +1,146 @@
+/*************************************************************************/
+/*  worker_thread_pool.h                                                 */
+/*************************************************************************/
+/*                       This file is part of:                           */
+/*                           GODOT ENGINE                                */
+/*                      https://godotengine.org                          */
+/*************************************************************************/
+/* Copyright (c) 2007-2022 Juan Linietsky, Ariel Manzur.                 */
+/* Copyright (c) 2014-2022 Godot Engine contributors (cf. AUTHORS.md).   */
+/*                                                                       */
+/* Permission is hereby granted, free of charge, to any person obtaining */
+/* a copy of this software and associated documentation files (the       */
+/* "Software"), to deal in the Software without restriction, including   */
+/* without limitation the rights to use, copy, modify, merge, publish,   */
+/* distribute, sublicense, and/or sell copies of the Software, and to    */
+/* permit persons to whom the Software is furnished to do so, subject to */
+/* the following conditions:                                             */
+/*                                                                       */
+/* The above copyright notice and this permission notice shall be        */
+/* included in all copies or substantial portions of the Software.       */
+/*                                                                       */
+/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,       */
+/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF    */
+/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.*/
+/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY  */
+/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,  */
+/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE     */
+/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
+/*************************************************************************/
+
+#ifndef WORKER_THREAD_POOL_H
+#define WORKER_THREAD_POOL_H
+
+#include "core/os/memory.h"
+#include "core/os/os.h"
+#include "core/os/semaphore.h"
+#include "core/os/thread.h"
+#include "core/templates/local_vector.h"
+#include "core/templates/paged_allocator.h"
+#include "core/templates/rid.h"
+#include "core/templates/safe_refcount.h"
+
+class WorkerThreadPool : public Object {
+	GDCLASS(WorkerThreadPool, Object)
+public:
+	enum {
+		INVALID_TASK_ID = -1
+	};
+
+	typedef int64_t TaskID;
+	typedef int64_t GroupID;
+
+private:
+	struct Task;
+
+	struct Group {
+		GroupID self;
+		SafeNumeric<uint32_t> index;
+		uint32_t max = 0;
+		Semaphore done_semaphore;
+		SafeFlag completed;
+		SafeNumeric<uint32_t> finished;
+		uint32_t tasks_used = 0;
+		TightLocalVector<Task *> low_priority_native_tasks;
+	};
+
+	struct Task {
+		Callable callable;
+		void (*native_func)(void *) = nullptr;
+		void (*native_group_func)(void *, uint32_t) = nullptr;
+		void *native_func_userdata = nullptr;
+		String description;
+		Semaphore done_semaphore;
+		bool completed = false;
+		Group *group = nullptr;
+		SelfList<Task> task_elem;
+		bool waiting = false; // Waiting for completion
+		bool low_priority = false;
+		Thread *low_priority_thread = nullptr;
+		Task() :
+				task_elem(this) {}
+	};
+
+	PagedAllocator<Task> task_allocator;
+	PagedAllocator<Group> group_allocator;
+	PagedAllocator<Thread> native_thread_allocator;
+
+	SelfList<Task>::List low_priority_task_queue;
+	SelfList<Task>::List task_queue;
+
+	Mutex task_mutex;
+	Semaphore task_available_semaphore;
+
+	struct ThreadData {
+		uint32_t index;
+		Thread thread;
+	};
+
+	TightLocalVector<ThreadData> threads;
+	SafeFlag exit_threads;
+
+	HashMap<Thread::ID, int> thread_ids;
+	HashMap<TaskID, Task *> tasks;
+	HashMap<GroupID, Group *> groups;
+
+	bool use_native_low_priority_threads = false;
+	uint32_t max_low_priority_threads = 0;
+	SafeNumeric<uint32_t> low_priority_threads_used;
+
+	uint64_t last_task = 1;
+
+	static void _thread_function(void *p_user);
+	static void _native_low_priority_thread_function(void *p_user);
+
+	void _process_task_queue();
+	void _process_task(Task *task);
+
+	void _post_task(Task *p_task, bool p_high_priority);
+
+	static WorkerThreadPool *singleton;
+
+protected:
+	static void _bind_methods();
+
+public:
+	TaskID add_native_task(void (*p_func)(void *), void *p_userdata, bool p_high_priority = false, const String &p_description = String());
+	TaskID add_task(const Callable &p_action, bool p_high_priority = false, const String &p_description = String());
+
+	bool is_task_completed(TaskID p_task_id) const;
+	void wait_for_task_completion(TaskID p_task_id);
+
+	GroupID add_native_group_task(void (*p_func)(void *, uint32_t), void *p_userdata, int p_elements, int p_tasks = -1, bool p_high_priority = false, const String &p_description = String());
+	GroupID add_group_task(const Callable &p_action, int p_elements, int p_tasks = -1, bool p_high_priority = false, const String &p_description = String());
+	bool is_group_task_completed(GroupID p_group) const;
+	void wait_for_group_task_completion(GroupID p_group);
+
+	_FORCE_INLINE_ int get_thread_count() const { return threads.size(); }
+
+	static WorkerThreadPool *get_singleton() { return singleton; }
+	void init(int p_thread_count = -1, bool p_use_native_threads_low_priority = true, float p_low_priority_task_ratio = 0.3);
+	void finish();
+	WorkerThreadPool();
+	~WorkerThreadPool();
+};
+
+#endif // WORKER_THREAD_POOL_H
diff --git a/core/register_core_types.cpp b/core/register_core_types.cpp
index e60d325f74..aab4b9a580 100644
--- a/core/register_core_types.cpp
+++ b/core/register_core_types.cpp
@@ -74,6 +74,7 @@
 #include "core/object/class_db.h"
 #include "core/object/script_language_extension.h"
 #include "core/object/undo_redo.h"
+#include "core/object/worker_thread_pool.h"
 #include "core/os/main_loop.h"
 #include "core/os/time.h"
 #include "core/string/optimized_translation.h"
@@ -101,6 +102,8 @@ static IP *ip = nullptr;
 static core_bind::Geometry2D *_geometry_2d = nullptr;
 static core_bind::Geometry3D *_geometry_3d = nullptr;
 
+static WorkerThreadPool *worker_thread_pool = nullptr;
+
 extern Mutex _global_mutex;
 
 static NativeExtensionManager *native_extension_manager = nullptr;
@@ -189,6 +192,8 @@ void register_core_types() {
 	GDREGISTER_CLASS(PacketPeerUDP);
 	GDREGISTER_CLASS(UDPServer);
 
+	GDREGISTER_ABSTRACT_CLASS(WorkerThreadPool);
+
 	ClassDB::register_custom_instance_class<HTTPClient>();
 
 	// Crypto
@@ -271,6 +276,8 @@ void register_core_types() {
 
 	GDREGISTER_NATIVE_STRUCT(AudioFrame, "float left;float right");
 	GDREGISTER_NATIVE_STRUCT(ScriptLanguageExtensionProfilingInfo, "StringName signature;uint64_t call_count;uint64_t total_time;uint64_t self_time");
+
+	worker_thread_pool = memnew(WorkerThreadPool);
 }
 
 void register_core_settings() {
@@ -279,9 +286,18 @@ void register_core_settings() {
 	ProjectSettings::get_singleton()->set_custom_property_info("network/limits/tcp/connect_timeout_seconds", PropertyInfo(Variant::INT, "network/limits/tcp/connect_timeout_seconds", PROPERTY_HINT_RANGE, "1,1800,1"));
 	GLOBAL_DEF_RST("network/limits/packet_peer_stream/max_buffer_po2", (16));
 	ProjectSettings::get_singleton()->set_custom_property_info("network/limits/packet_peer_stream/max_buffer_po2", PropertyInfo(Variant::INT, "network/limits/packet_peer_stream/max_buffer_po2", PROPERTY_HINT_RANGE, "0,64,1,or_greater"));
-
 	GLOBAL_DEF("network/ssl/certificate_bundle_override", "");
 	ProjectSettings::get_singleton()->set_custom_property_info("network/ssl/certificate_bundle_override", PropertyInfo(Variant::STRING, "network/ssl/certificate_bundle_override", PROPERTY_HINT_FILE, "*.crt"));
+
+	int worker_threads = GLOBAL_DEF("threading/worker_pool/max_threads", -1);
+	bool low_priority_use_system_threads = GLOBAL_DEF("threading/worker_pool/use_system_threads_for_low_priority_tasks", true);
+	float low_property_ratio = GLOBAL_DEF("threading/worker_pool/low_priority_thread_ratio", 0.3);
+
+	if (Engine::get_singleton()->is_editor_hint() || Engine::get_singleton()->is_project_manager_hint()) {
+		worker_thread_pool->init();
+	} else {
+		worker_thread_pool->init(worker_threads, low_priority_use_system_threads, low_property_ratio);
+	}
 }
 
 void register_core_singletons() {
@@ -319,6 +335,7 @@ void register_core_singletons() {
 	Engine::get_singleton()->add_singleton(Engine::Singleton("Time", Time::get_singleton()));
 	Engine::get_singleton()->add_singleton(Engine::Singleton("NativeExtensionManager", NativeExtensionManager::get_singleton()));
 	Engine::get_singleton()->add_singleton(Engine::Singleton("ResourceUID", ResourceUID::get_singleton()));
+	Engine::get_singleton()->add_singleton(Engine::Singleton("WorkerThreadPool", worker_thread_pool));
 }
 
 void register_core_extensions() {
@@ -350,6 +367,8 @@ void unregister_core_types() {
 	memdelete(_geometry_2d);
 	memdelete(_geometry_3d);
 
+	memdelete(worker_thread_pool);
+
 	ResourceLoader::remove_resource_format_loader(resource_format_image);
 	resource_format_image.unref();
 
diff --git a/core/templates/safe_refcount.h b/core/templates/safe_refcount.h
index 3148283dca..1f6551762e 100644
--- a/core/templates/safe_refcount.h
+++ b/core/templates/safe_refcount.h
@@ -111,6 +111,7 @@ public:
 			if (tmp >= p_value) {
 				return tmp; // already greater, or equal
 			}
+
 			if (value.compare_exchange_weak(tmp, p_value, std::memory_order_acq_rel)) {
 				return p_value;
 			}