Fixed player spawning in ungenerated terrain (#4087)

fixes #4044
fixes #2724

this is significantly more complex than I hoped for, but it's a start... and it works.

src/world/ChunkPopulationPromise.php

@@ -0,0 +1,76 @@
+<?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\world;
+
+use function spl_object_id;
+
+final class ChunkPopulationPromise{
+	/**
+	 * @var \Closure[]
+	 * @phpstan-var array<int, \Closure() : void>
+	 */
+	private array $onSuccess = [];
+	/**
+	 * @var \Closure[]
+	 * @phpstan-var array<int, \Closure() : void>
+	 */
+	private array $onFailure = [];
+
+	private ?bool $success = null;
+
+	/**
+	 * @phpstan-param \Closure() : void $onSuccess
+	 * @phpstan-param \Closure() : void $onFailure
+	 */
+	public function onCompletion(\Closure $onSuccess, \Closure $onFailure) : void{
+		if($this->success !== null){
+			$this->success ? $onSuccess() : $onFailure();
+		}else{
+			$this->onSuccess[spl_object_id($onSuccess)] = $onSuccess;
+			$this->onFailure[spl_object_id($onFailure)] = $onFailure;
+		}
+	}
+
+	public function resolve() : void{
+		$this->success = true;
+		foreach($this->onSuccess as $callback){
+			$callback();
+		}
+		$this->onSuccess = [];
+		$this->onFailure = [];
+	}
+
+	public function reject() : void{
+		$this->success = false;
+		foreach($this->onFailure as $callback){
+			$callback();
+		}
+		$this->onSuccess = [];
+		$this->onFailure = [];
+	}
+
+	public function isCompleted() : bool{
+		return $this->success !== null;
+	}
+}