Move server-full/banned/whitelisted controls into PlayerPreLoginEvent

This allows plugins to more easily control the behaviour of server-full, whitelisting and banning. A message can be assigned for each, with a plugin custom reason taking the final priority if set.

This system solves several edge case problems in the Bukkit version by allowing kick reasons to be combined, so that removing one kick reason will roll back the final reason to the next highest, instead of just allowing the player through completely.
Only one message will be shown at point of disconnection, for consistency with the old behaviour.
The message priority is as follows (for all cases, only if set):
- Plugin reason
- Server full
- Whitelist enabled
- Player is banned

This also brings us one step closer to separating Player and NetworkSession.

src/pocketmine/Player.php

@@ -1732,26 +1732,20 @@ class Player extends Human implements CommandSender, ChunkLoader, IPlayer{
 		$this->setSkin($packet->skin);
 
 
-		$ev = new PlayerPreLoginEvent($this, "Plugin reason", $this->server->requiresAuthentication());
+		$ev = new PlayerPreLoginEvent($this, $this->server->requiresAuthentication());
+		if(count($this->server->getOnlinePlayers()) >= $this->server->getMaxPlayers()){
+			$ev->setKickReason(PlayerPreLoginEvent::KICK_REASON_SERVER_FULL, "disconnectionScreen.serverFull");
+		}
+		if(!$this->server->isWhitelisted($this->username)){
+			$ev->setKickReason(PlayerPreLoginEvent::KICK_REASON_SERVER_WHITELISTED, "Server is whitelisted");
+		}
+		if($this->isBanned() or $this->server->getIPBans()->isBanned($this->getAddress())){
+			$ev->setKickReason(PlayerPreLoginEvent::KICK_REASON_BANNED, "You are banned");
+		}
+
 		$ev->call();
-		if($ev->isCancelled()){
-			$this->close("", $ev->getKickMessage());
-
-			return true;
-		}
-
-		if(count($this->server->getOnlinePlayers()) >= $this->server->getMaxPlayers() and $this->kick("disconnectionScreen.serverFull", false)){
-			return true;
-		}
-
-		if(!$this->server->isWhitelisted($this->username) and $this->kick("Server is white-listed", false)){
-			return true;
-		}
-
-		if(
-			($this->isBanned() or $this->server->getIPBans()->isBanned($this->getAddress())) and
-			$this->kick("You are banned", false)
-		){
+		if(!$ev->isAllowed()){
+			$this->close("", $ev->getFinalKickMessage());
 			return true;
 		}
 

src/pocketmine/event/player/PlayerPreLoginEvent.php

@@ -23,7 +23,6 @@ declare(strict_types=1);
 
 namespace pocketmine\event\player;
 
-use pocketmine\event\Cancellable;
 use pocketmine\Player;
 
 /**
@@ -39,37 +38,34 @@ use pocketmine\Player;
  * WARNING: Due to internal bad architecture, the player is not fully constructed at this stage, and errors might occur
  * when calling API methods on the player. Tread with caution.
  */
-class PlayerPreLoginEvent extends PlayerEvent implements Cancellable{
-	/** @var string */
-	protected $kickMessage;
+class PlayerPreLoginEvent extends PlayerEvent{
+	public const KICK_REASON_PLUGIN = 0;
+	public const KICK_REASON_SERVER_FULL = 1;
+	public const KICK_REASON_SERVER_WHITELISTED = 2;
+	public const KICK_REASON_BANNED = 3;
+
+	public const KICK_REASON_PRIORITY = [
+		self::KICK_REASON_PLUGIN, //Plugin reason should always take priority over anything else
+		self::KICK_REASON_SERVER_FULL,
+		self::KICK_REASON_SERVER_WHITELISTED,
+		self::KICK_REASON_BANNED
+	];
+
 	/** @var bool */
 	protected $authRequired;
 
+	/** @var string[] reason const => associated message */
+	protected $kickReasons = [];
+
 	/**
 	 * @param Player $player
-	 * @param string $kickMessage
 	 * @param bool   $authRequired
 	 */
-	public function __construct(Player $player, string $kickMessage, bool $authRequired){
+	public function __construct(Player $player, bool $authRequired){
 		$this->player = $player;
-		$this->kickMessage = $kickMessage;
 		$this->authRequired = $authRequired;
 	}
 
-	/**
-	 * @param string $kickMessage
-	 */
-	public function setKickMessage(string $kickMessage) : void{
-		$this->kickMessage = $kickMessage;
-	}
-
-	/**
-	 * @return string
-	 */
-	public function getKickMessage() : string{
-		return $this->kickMessage;
-	}
-
 	/**
 	 * @return bool
 	 */
@@ -83,4 +79,92 @@ class PlayerPreLoginEvent extends PlayerEvent implements Cancellable{
 	public function setAuthRequired(bool $v) : void{
 		$this->authRequired = $v;
 	}
+
+	/**
+	 * Returns an array of kick reasons currently assigned.
+	 *
+	 * @return int[]
+	 */
+	public function getKickReasons() : array{
+		return array_keys($this->kickReasons);
+	}
+
+	/**
+	 * Returns whether the given kick reason is set for this event.
+	 *
+	 * @param int $flag
+	 *
+	 * @return bool
+	 */
+	public function isKickReasonSet(int $flag) : bool{
+		return isset($this->kickReasons[$flag]);
+	}
+
+	/**
+	 * Sets a reason to disallow the player to continue continue authenticating, with a message.
+	 * This can also be used to change kick messages for already-set flags.
+	 *
+	 * @param int    $flag
+	 * @param string $message
+	 */
+	public function setKickReason(int $flag, string $message) : void{
+		$this->kickReasons[$flag] = $message;
+	}
+
+	/**
+	 * Clears a specific kick flag if it was set. This allows fine-tuned kick reason removal without impacting other
+	 * reasons (for example, a ban can be bypassed without accidentally allowing a player to join a full server).
+	 *
+	 * @param int $flag Specific flag to clear.
+	 */
+	public function clearKickReason(int $flag) : void{
+		unset($this->kickReasons[$flag]);
+	}
+
+	/**
+	 * Clears all pre-assigned kick reasons, allowing the player to continue logging in.
+	 */
+	public function clearAllKickReasons() : void{
+		$this->kickReasons = [];
+	}
+
+	/**
+	 * Returns whether the player is allowed to continue logging in.
+	 *
+	 * @return bool
+	 */
+	public function isAllowed() : bool{
+		return empty($this->kickReasons);
+	}
+
+	/**
+	 * Returns the kick message provided for the given kick flag, or null if not set.
+	 *
+	 * @param int $flag
+	 *
+	 * @return string|null
+	 */
+	public function getKickMessage(int $flag) : ?string{
+		return $this->kickReasons[$flag] ?? null;
+	}
+
+	/**
+	 * Returns the final kick message which will be shown on the disconnect screen.
+	 *
+	 * Note: Only one message (the highest priority one) will be shown. See priority order to decide how to set your
+	 * messages.
+	 *
+	 * @see PlayerPreLoginEvent::KICK_REASON_PRIORITY
+	 *
+	 * @return string
+	 */
+	public function getFinalKickMessage() : string{
+		foreach(self::KICK_REASON_PRIORITY as $p){
+			if(isset($this->kickReasons[$p])){
+				return $this->kickReasons[$p];
+			}
+		}
+
+		return "";
+	}
 }