AsyncTask::setResult(): permit returning ThreadSafe objects to the main thread

this is now supported thanks to the object rescue feature implemented in pthreads 5.1, making returning of thread-safe values from async tasks possible.
This needs to be explicitly supported, since otherwise it will attempt to serialize them, which isn't supported anymore.

tests/phpunit/scheduler/ThreadSafeResultAsyncTask.php

@@ -0,0 +1,55 @@
+<?php
+
+/*
+ *
+ *  ____            _        _   __  __ _                  __  __ ____
+ * |  _ \ ___   ___| | _____| |_|  \/  (_)_ __   ___      |  \/  |  _ \
+ * | |_) / _ \ / __| |/ / _ \ __| |\/| | | '_ \ / _ \_____| |\/| | |_) |
+ * |  __/ (_) | (__|   <  __/ |_| |  | | | | | |  __/_____| |  | |  __/
+ * |_|   \___/ \___|_|\_\___|\__|_|  |_|_|_| |_|\___|     |_|  |_|_|
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * @author PocketMine Team
+ * @link http://www.pocketmine.net/
+ *
+ *
+ */
+
+declare(strict_types=1);
+
+namespace pocketmine\scheduler;
+
+use pmmp\thread\ThreadSafe;
+use pmmp\thread\ThreadSafeArray;
+use pocketmine\promise\PromiseResolver;
+
+class ThreadSafeResultAsyncTask extends AsyncTask{
+	private const TLS_KEY_PROMISE = "promise";
+
+	/**
+	 * @phpstan-param PromiseResolver<ThreadSafe> $promise
+	 */
+	public function __construct(
+		PromiseResolver $promise
+	){
+		$this->storeLocal(self::TLS_KEY_PROMISE, $promise);
+	}
+
+	public function onRun() : void{
+		//this only works in pthreads 5.1+ and pmmpthread
+		//in prior versions the ThreadSafe would be destroyed before onCompletion is called
+		$result = new ThreadSafeArray();
+		$result[] = "foo";
+		$this->setResult($result);
+	}
+
+	public function onCompletion() : void{
+		/** @var PromiseResolver<ThreadSafe> $promise */
+		$promise = $this->fetchLocal(self::TLS_KEY_PROMISE);
+		$promise->resolve($this->getResult());
+	}
+}