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.

2 files changed, 159 insertions, 0 deletions

diff --git a/tests/core/threads/test_worker_thread_pool.h b/tests/core/threads/test_worker_thread_pool.h
new file mode 100644
index 0000000000..641b293c8a
--- /dev/null
+++ b/tests/core/threads/test_worker_thread_pool.h
@@ -0,0 +1,158 @@
+/*************************************************************************/
+/*  test_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 TEST_WORKER_THREAD_POOL_H
+#define TEST_WORKER_THREAD_POOL_H
+
+#include "core/object/worker_thread_pool.h"
+
+#include "tests/test_macros.h"
+
+namespace TestWorkerThreadPool {
+
+int u32scmp(const char32_t *l, const char32_t *r) {
+	for (; *l == *r && *l && *r; l++, r++) {
+		// Continue.
+	}
+	return *l - *r;
+}
+
+static void static_test(void *p_arg) {
+	SafeNumeric<uint32_t> *counter = (SafeNumeric<uint32_t> *)p_arg;
+	counter->increment();
+}
+
+static SafeNumeric<uint32_t> callable_counter;
+
+static void static_callable_test() {
+	callable_counter.increment();
+}
+
+TEST_CASE("[WorkerThreadPool] Process 256 threads using native task") {
+	const int count = 256;
+	SafeNumeric<uint32_t> counter;
+	WorkerThreadPool::TaskID tasks[count];
+	for (int i = 0; i < count; i++) {
+		tasks[i] = WorkerThreadPool::get_singleton()->add_native_task(static_test, &counter, true);
+	}
+	for (int i = 0; i < count; i++) {
+		WorkerThreadPool::get_singleton()->wait_for_task_completion(tasks[i]);
+	}
+
+	CHECK(counter.get() == count);
+}
+
+TEST_CASE("[WorkerThreadPool] Process 256 threads using native low priority") {
+	const int count = 256;
+	SafeNumeric<uint32_t> counter = SafeNumeric<uint32_t>(0);
+	WorkerThreadPool::TaskID tasks[count];
+	for (int i = 0; i < count; i++) {
+		tasks[i] = WorkerThreadPool::get_singleton()->add_native_task(static_test, &counter, false);
+	}
+	for (int i = 0; i < count; i++) {
+		WorkerThreadPool::get_singleton()->wait_for_task_completion(tasks[i]);
+	}
+
+	CHECK(counter.get() == count);
+}
+
+TEST_CASE("[WorkerThreadPool] Process 256 threads using callable") {
+	const int count = 256;
+	WorkerThreadPool::TaskID tasks[count];
+	callable_counter.set(0);
+	for (int i = 0; i < count; i++) {
+		tasks[i] = WorkerThreadPool::get_singleton()->add_task(callable_mp_static(static_callable_test), true);
+	}
+	for (int i = 0; i < count; i++) {
+		WorkerThreadPool::get_singleton()->wait_for_task_completion(tasks[i]);
+	}
+
+	CHECK(callable_counter.get() == count);
+}
+
+TEST_CASE("[WorkerThreadPool] Process 256 threads using callable low priority") {
+	const int count = 256;
+	WorkerThreadPool::TaskID tasks[count];
+	callable_counter.set(0);
+	for (int i = 0; i < count; i++) {
+		tasks[i] = WorkerThreadPool::get_singleton()->add_task(callable_mp_static(static_callable_test), false);
+	}
+	for (int i = 0; i < count; i++) {
+		WorkerThreadPool::get_singleton()->wait_for_task_completion(tasks[i]);
+	}
+
+	CHECK(callable_counter.get() == count);
+}
+
+static void static_group_test(void *p_arg, uint32_t p_index) {
+	SafeNumeric<uint32_t> *counter = (SafeNumeric<uint32_t> *)p_arg;
+	counter->exchange_if_greater(p_index);
+}
+
+TEST_CASE("[WorkerThreadPool] Process 256 elements on native task group") {
+	const int count = 256;
+	SafeNumeric<uint32_t> counter;
+	WorkerThreadPool::GroupID group = WorkerThreadPool::get_singleton()->add_native_group_task(static_group_test, &counter, count, -1, true);
+	WorkerThreadPool::get_singleton()->wait_for_group_task_completion(group);
+	CHECK(counter.get() == count - 1);
+}
+
+TEST_CASE("[WorkerThreadPool] Process 256 elements on native task group low priority") {
+	const int count = 256;
+	SafeNumeric<uint32_t> counter;
+	WorkerThreadPool::GroupID group = WorkerThreadPool::get_singleton()->add_native_group_task(static_group_test, &counter, count, -1, false);
+	WorkerThreadPool::get_singleton()->wait_for_group_task_completion(group);
+	CHECK(counter.get() == count - 1);
+}
+
+static SafeNumeric<uint32_t> callable_group_counter;
+
+static void static_callable_group_test(uint32_t p_index) {
+	callable_group_counter.exchange_if_greater(p_index);
+}
+
+TEST_CASE("[WorkerThreadPool] Process 256 elements on native task group") {
+	const int count = 256;
+	WorkerThreadPool::GroupID group = WorkerThreadPool::get_singleton()->add_group_task(callable_mp_static(static_callable_group_test), count, -1, true);
+	WorkerThreadPool::get_singleton()->wait_for_group_task_completion(group);
+	CHECK(callable_group_counter.get() == count - 1);
+}
+
+TEST_CASE("[WorkerThreadPool] Process 256 elements on native task group low priority") {
+	const int count = 256;
+	callable_group_counter.set(0);
+	WorkerThreadPool::GroupID group = WorkerThreadPool::get_singleton()->add_group_task(callable_mp_static(static_callable_group_test), count, -1, false);
+	WorkerThreadPool::get_singleton()->wait_for_group_task_completion(group);
+	CHECK(callable_group_counter.get() == count - 1);
+}
+
+} // namespace TestWorkerThreadPool
+
+#endif // TEST_WORKER_THREAD_POOL_H
diff --git a/tests/test_main.cpp b/tests/test_main.cpp
index 79cda7e512..e71f2edba3 100644
--- a/tests/test_main.cpp
+++ b/tests/test_main.cpp
@@ -70,6 +70,7 @@
 #include "tests/core/test_crypto.h"
 #include "tests/core/test_hashing_context.h"
 #include "tests/core/test_time.h"
+#include "tests/core/threads/test_worker_thread_pool.h"
 #include "tests/core/variant/test_array.h"
 #include "tests/core/variant/test_dictionary.h"
 #include "tests/core/variant/test_variant.h"