Modernize Thread

- Based on C++11's `thread` and `thread_local`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed (except for the few cases of non-portable functions)
- Simpler for `NO_THREADS`
- Thread ids are now the same across platforms (main is 1; others follow)

core/core_bind.cpp

@@ -1990,43 +1990,27 @@ Error _Thread::start(Object *p_instance, const StringName &p_method, const Varia
 
 	Thread::Settings s;
 	s.priority = (Thread::Priority)p_priority;
-	thread = Thread::create(_start_func, ud, s);
-	if (!thread) {
-		active = false;
-		target_method = StringName();
-		target_instance = nullptr;
-		userdata = Variant();
-		return ERR_CANT_CREATE;
-	}
+	thread.start(_start_func, ud, s);
 
 	return OK;
 }
 
 String _Thread::get_id() const {
-	if (!thread) {
-		return String();
-	}
-
-	return itos(thread->get_id());
+	return itos(thread.get_id());
 }
 
 bool _Thread::is_active() const {
 	return active;
 }
 
 Variant _Thread::wait_to_finish() {
-	ERR_FAIL_COND_V_MSG(!thread, Variant(), "Thread must exist to wait for its completion.");
 	ERR_FAIL_COND_V_MSG(!active, Variant(), "Thread must be active to wait for its completion.");
-	Thread::wait_to_finish(thread);
+	thread.wait_to_finish();
 	Variant r = ret;
 	active = false;
 	target_method = StringName();
 	target_instance = nullptr;
 	userdata = Variant();
-	if (thread) {
-		memdelete(thread);
-	}
-	thread = nullptr;
 
 	return r;
 }
@@ -2042,10 +2026,6 @@ void _Thread::_bind_methods() {
 	BIND_ENUM_CONSTANT(PRIORITY_HIGH);
 }
 
-_Thread::~_Thread() {
-	ERR_FAIL_COND_MSG(active, "Reference to a Thread object was lost while the thread is still running...");
-}
-
 ////// _ClassDB //////
 
 PackedStringArray _ClassDB::get_class_list() const {

core/core_bind.h

@@ -553,7 +553,7 @@ class _Thread : public Reference {
 	volatile bool active = false;
 	Object *target_instance = nullptr;
 	StringName target_method;
-	Thread *thread = nullptr;
+	Thread thread;
 	static void _bind_methods();
 	static void _start_func(void *ud);
 
@@ -569,9 +569,6 @@ class _Thread : public Reference {
 	String get_id() const;
 	bool is_active() const;
 	Variant wait_to_finish();
-
-	_Thread() {}
-	~_Thread();
 };
 
 VARIANT_ENUM_CAST(_Thread::Priority);

core/debugger/remote_debugger_peer.cpp

@@ -65,12 +65,8 @@ int RemoteDebuggerPeerTCP::get_max_message_size() const {
 }
 
 void RemoteDebuggerPeerTCP::close() {
-	if (thread) {
-		running = false;
-		Thread::wait_to_finish(thread);
-		memdelete(thread);
-		thread = nullptr;
-	}
+	running = false;
+	thread.wait_to_finish();
 	tcp_client->disconnect_from_host();
 	out_buf.resize(0);
 	in_buf.resize(0);
@@ -85,7 +81,7 @@ RemoteDebuggerPeerTCP::RemoteDebuggerPeerTCP(Ref<StreamPeerTCP> p_tcp) {
 		connected = true;
 #ifndef NO_THREADS
 		running = true;
-		thread = Thread::create(_thread_func, this);
+		thread.start(_thread_func, this);
 #endif
 	} else {
 		tcp_client.instance();
@@ -188,7 +184,7 @@ Error RemoteDebuggerPeerTCP::connect_to_host(const String &p_host, uint16_t p_po
 	connected = true;
 #ifndef NO_THREADS
 	running = true;
-	thread = Thread::create(_thread_func, this);
+	thread.start(_thread_func, this);
 #endif
 	return OK;
 }

core/debugger/remote_debugger_peer.h

@@ -58,7 +58,7 @@ class RemoteDebuggerPeerTCP : public RemoteDebuggerPeer {
 private:
 	Ref<StreamPeerTCP> tcp_client;
 	Mutex mutex;
-	Thread *thread = nullptr;
+	Thread thread;
 	List<Array> in_queue;
 	List<Array> out_queue;
 	int out_left = 0;

core/io/file_access_network.cpp

@@ -201,7 +201,7 @@ Error FileAccessNetworkClient::connect(const String &p_host, int p_port, const S
 		return ERR_INVALID_PARAMETER;
 	}
 
-	thread = Thread::create(_thread_func, this);
+	thread.start(_thread_func, this);
 
 	return OK;
 }
@@ -214,12 +214,9 @@ FileAccessNetworkClient::FileAccessNetworkClient() {
 }
 
 FileAccessNetworkClient::~FileAccessNetworkClient() {
-	if (thread) {
-		quit = true;
-		sem.post();
-		Thread::wait_to_finish(thread);
-		memdelete(thread);
-	}
+	quit = true;
+	sem.post();
+	thread.wait_to_finish();
 }
 
 void FileAccessNetwork::_set_block(int p_offset, const Vector<uint8_t> &p_block) {

core/io/file_access_network.h

@@ -48,7 +48,7 @@ class FileAccessNetworkClient {
 	List<BlockRequest> block_requests;
 
 	Semaphore sem;
-	Thread *thread = nullptr;
+	Thread thread;
 	bool quit = false;
 	Mutex mutex;
 	Mutex blockrequest_mutex;

core/io/ip.cpp

@@ -71,7 +71,7 @@ struct _IP_ResolverPrivate {
 	Mutex mutex;
 	Semaphore sem;
 
-	Thread *thread;
+	Thread thread;
 	//Semaphore* semaphore;
 	bool thread_abort;
 
@@ -141,7 +141,7 @@ IP::ResolverID IP::resolve_hostname_queue_item(const String &p_hostname, IP::Typ
 	} else {
 		resolver->queue[id].response = IP_Address();
 		resolver->queue[id].status = IP::RESOLVER_STATUS_WAITING;
-		if (resolver->thread) {
+		if (resolver->thread.is_started()) {
 			resolver->sem.post();
 		} else {
 			resolver->resolve_queues();
@@ -285,26 +285,14 @@ IP::IP() {
 	singleton = this;
 	resolver = memnew(_IP_ResolverPrivate);
 
-#ifndef NO_THREADS
-
 	resolver->thread_abort = false;
-
-	resolver->thread = Thread::create(_IP_ResolverPrivate::_thread_function, resolver);
-#else
-	resolver->thread = nullptr;
-#endif
+	resolver->thread.start(_IP_ResolverPrivate::_thread_function, resolver);
 }
 
 IP::~IP() {
-#ifndef NO_THREADS
-	if (resolver->thread) {
-		resolver->thread_abort = true;
-		resolver->sem.post();
-		Thread::wait_to_finish(resolver->thread);
-		memdelete(resolver->thread);
-	}
-
-#endif
+	resolver->thread_abort = true;
+	resolver->sem.post();
+	resolver->thread.wait_to_finish();
 
 	memdelete(resolver);
 }

core/io/resource_loader.cpp

@@ -362,7 +362,8 @@ Error ResourceLoader::load_threaded_request(const String &p_path, const String &
 
 		print_lt("REQUEST: load count: " + itos(thread_loading_count) + " / wait count: " + itos(thread_waiting_count) + " / suspended count: " + itos(thread_suspended_count) + " / active: " + itos(thread_loading_count - thread_suspended_count));
 
-		load_task.thread = Thread::create(_thread_load_function, &thread_load_tasks[local_path]);
+		load_task.thread = memnew(Thread);
+		load_task.thread->start(_thread_load_function, &thread_load_tasks[local_path]);
 		load_task.loader_id = load_task.thread->get_id();
 	}
 
@@ -489,7 +490,7 @@ RES ResourceLoader::load_threaded_get(const String &p_path, Error *r_error) {
 
 	if (load_task.requests == 0) {
 		if (load_task.thread) { //thread may not have been used
-			Thread::wait_to_finish(load_task.thread);
+			load_task.thread->wait_to_finish();
 			memdelete(load_task.thread);
 		}
 		thread_load_tasks.erase(local_path);

core/os/thread.cpp

@@ -30,30 +30,70 @@
 
 #include "thread.h"
 
-Thread *(*Thread::create_func)(ThreadCreateCallback, void *, const Settings &) = nullptr;
-Thread::ID (*Thread::get_thread_id_func)() = nullptr;
-void (*Thread::wait_to_finish_func)(Thread *) = nullptr;
+#include "core/object/script_language.h"
+
+#if !defined(NO_THREADS)
+
 Error (*Thread::set_name_func)(const String &) = nullptr;
+void (*Thread::set_priority_func)(Thread::Priority) = nullptr;
+void (*Thread::init_func)() = nullptr;
+void (*Thread::term_func)() = nullptr;
+
+Thread::ID Thread::main_thread_id = 1;
+Thread::ID Thread::last_thread_id = 1;
+thread_local Thread::ID Thread::caller_id = 1;
 
-Thread::ID Thread::_main_thread_id = 0;
+void Thread::_set_platform_funcs(
+		Error (*p_set_name_func)(const String &),
+		void (*p_set_priority_func)(Thread::Priority),
+		void (*p_init_func)(),
+		void (*p_term_func)()) {
+	Thread::set_name_func = p_set_name_func;
+	Thread::set_priority_func = p_set_priority_func;
+	Thread::init_func = p_init_func;
+	Thread::term_func = p_term_func;
+}
 
-Thread::ID Thread::get_caller_id() {
-	if (get_thread_id_func) {
-		return get_thread_id_func();
+void Thread::callback(Thread *p_self, const Settings &p_settings, Callback p_callback, void *p_userdata) {
+	Thread::caller_id = p_self->id;
+	if (set_priority_func) {
+		set_priority_func(p_settings.priority);
+	}
+	if (init_func) {
+		init_func();
+	}
+	ScriptServer::thread_enter(); //scripts may need to attach a stack
+	p_callback(p_userdata);
+	ScriptServer::thread_exit();
+	if (term_func) {
+		term_func();
 	}
-	return 0;
 }
 
-Thread *Thread::create(ThreadCreateCallback p_callback, void *p_user, const Settings &p_settings) {
-	if (create_func) {
-		return create_func(p_callback, p_user, p_settings);
+void Thread::start(Thread::Callback p_callback, void *p_user, const Settings &p_settings) {
+	if (id != 0) {
+#ifdef DEBUG_ENABLED
+		WARN_PRINT("A Thread object has been re-started without wait_to_finish() having been called on it. Please do so to ensure correct cleanup of the thread.");
+#endif
+		thread.detach();
+		std::thread empty_thread;
+		thread.swap(empty_thread);
 	}
-	return nullptr;
+	id = atomic_increment(&last_thread_id);
+	std::thread new_thread(&Thread::callback, this, p_settings, p_callback, p_user);
+	thread.swap(new_thread);
 }
 
-void Thread::wait_to_finish(Thread *p_thread) {
-	if (wait_to_finish_func) {
-		wait_to_finish_func(p_thread);
+bool Thread::is_started() const {
+	return id != 0;
+}
+
+void Thread::wait_to_finish() {
+	if (id != 0) {
+		thread.join();
+		std::thread empty_thread;
+		thread.swap(empty_thread);
+		id = 0;
 	}
 }
 
@@ -64,3 +104,14 @@ Error Thread::set_name(const String &p_name) {
 
 	return ERR_UNAVAILABLE;
 }
+
+Thread::~Thread() {
+	if (id != 0) {
+#ifdef DEBUG_ENABLED
+		WARN_PRINT("A Thread object has been destroyed without wait_to_finish() having been called on it. Please do so to ensure correct cleanup of the thread.");
+#endif
+		thread.detach();
+	}
+}
+
+#endif