Check for closed target Levels in Position (#241)

More foolproof

src/pocketmine/level/Level.php

@@ -260,6 +260,8 @@ class Level implements ChunkManager, Metadatable{
 	/** @var Generator */
 	private $generatorInstance;
 
+	private $closed = false;
+
 	public static function chunkHash(int $x, int $z){
 		return PHP_INT_SIZE === 8 ? (($x & 0xFFFFFFFF) << 32) | ($z & 0xFFFFFFFF) : $x . ":" . $z;
 	}
@@ -404,7 +406,12 @@ class Level implements ChunkManager, Metadatable{
 		return $this->levelId;
 	}
 
+	public function isClosed() : bool{
+		return $this->closed;
+	}
+
 	public function close(){
+		assert(!$this->closed, "Tried to close a level which is already closed");
 
 		if($this->getAutoSave()){
 			$this->save();
@@ -421,6 +428,8 @@ class Level implements ChunkManager, Metadatable{
 		$this->blockMetadata = null;
 		$this->blockCache = [];
 		$this->temporalPosition = null;
+
+		$this->closed = true;
 	}
 
 	public function addSound(Sound $sound, array $players = null){

src/pocketmine/level/Position.php

@@ -23,6 +23,7 @@ namespace pocketmine\level;
 
 use pocketmine\math\Vector3;
 use pocketmine\utils\LevelException;
+use pocketmine\utils\MainLogger;
 
 class Position extends Vector3{
 
@@ -47,19 +48,40 @@ class Position extends Vector3{
 	}
 
 	/**
-	 * @return Level
+	 * Returns the target Level, or null if the target is not valid.
+	 * If a reference exists to a Level which is closed, the reference will be destroyed and null will be returned.
+	 *
+	 * @return Level|null
 	 */
 	public function getLevel(){
+		if($this->level !== null and $this->level->isClosed()){
+			MainLogger::getLogger()->debug("Position was holding a reference to an unloaded Level");
+			$this->level = null;
+		}
+
 		return $this->level;
 	}
 
-	public function setLevel(Level $level){
+	/**
+	 * Sets the target Level of the position.
+	 *
+	 * @param Level|null $level
+	 *
+	 * @return $this
+	 *
+	 * @throws \InvalidArgumentException if the specified Level has been closed
+	 */
+	public function setLevel(Level $level = null){
+		if($level !== null and $level->isClosed()){
+			throw new \InvalidArgumentException("Specified level has been unloaded and cannot be used");
+		}
+
 		$this->level = $level;
 		return $this;
 	}
 
 	/**
-	 * Checks if this object has a valid reference to a Level
+	 * Checks if this object has a valid reference to a loaded Level
 	 *
 	 * @return bool
 	 */

src/pocketmine/level/WeakPosition.php

@@ -53,7 +53,18 @@ class WeakPosition extends Position{
 		return Server::getInstance()->getLevel($this->levelId);
 	}
 
-	public function setLevel(Level $level){
+	/**
+	 * @param Level|null $level
+	 *
+	 * @return $this
+	 *
+	 * @throws \InvalidArgumentException if the specified Level has been closed
+	 */
+	public function setLevel(Level $level = null){
+		if($level !== null and $level->isClosed()){
+			throw new \InvalidArgumentException("Specified level has been unloaded and cannot be used");
+		}
+
 		$this->levelId = ($level !== null ? $level->getId() : -1);
 		return $this;
 	}