PermissionAttachment may now reference zero or more permissibles

this makes PermissionAttachment more and more like Permission, except that it doesn't have a name.
Right now, the only value of this API change is that it allows breaking references to dead Permissibles, but in the future it should be possible to reuse a single PermissionAttachment on multiple Permissibles.

src/permission/PermissibleBase.php

@@ -105,12 +105,14 @@ class PermissibleBase implements Permissible{
 			throw new PluginException("Plugin " . $plugin->getDescription()->getName() . " is disabled");
 		}
 
-		$result = new PermissionAttachment($plugin, $this);
+		$result = new PermissionAttachment($plugin);
 		$this->attachments[spl_object_id($result)] = $result;
 		if($name !== null and $value !== null){
 			$result->setPermission($name, $value);
 		}
 
+		$result->subscribePermissible($this);
+
 		$this->recalculatePermissions();
 
 		return $result;
@@ -119,8 +121,9 @@ class PermissibleBase implements Permissible{
 	public function removeAttachment(PermissionAttachment $attachment) : void{
 		if(isset($this->attachments[spl_object_id($attachment)])){
 			unset($this->attachments[spl_object_id($attachment)]);
+			$attachment->unsubscribePermissible($this);
 			if(($ex = $attachment->getRemovalCallback()) !== null){
-				$ex->attachmentRemoved($attachment);
+				$ex->attachmentRemoved($this, $attachment);
 			}
 
 			$this->recalculatePermissions();
@@ -213,7 +216,10 @@ class PermissibleBase implements Permissible{
 	public function destroyCycles() : void{
 		PermissionManager::getInstance()->unsubscribeFromAllPermissions($this);
 		$this->permissions = []; //PermissionAttachmentInfo doesn't reference Permissible anymore, but it references PermissionAttachment which does
-		$this->attachments = []; //this might still be a problem if the attachments are still referenced, but we can't do anything about that
+		foreach($this->attachments as $attachment){
+			$attachment->unsubscribePermissible($this);
+		}
+		$this->attachments = [];
 		$this->permissionRecalculationCallbacks->clear();
 	}
 }

src/permission/PermissionAttachment.php

@@ -25,6 +25,7 @@ namespace pocketmine\permission;
 
 use pocketmine\plugin\Plugin;
 use pocketmine\plugin\PluginException;
+use function spl_object_id;
 
 class PermissionAttachment{
 	/** @var PermissionRemovedExecutor|null */
@@ -33,8 +34,11 @@ class PermissionAttachment{
 	/** @var bool[] */
 	private $permissions = [];
 
-	/** @var Permissible */
-	private $permissible;
+	/**
+	 * @var Permissible[]
+	 * @phpstan-var array<int, Permissible>
+	 */
+	private $subscribers = [];
 
 	/** @var Plugin */
 	private $plugin;
@@ -42,12 +46,11 @@ class PermissionAttachment{
 	/**
 	 * @throws PluginException
 	 */
-	public function __construct(Plugin $plugin, Permissible $permissible){
+	public function __construct(Plugin $plugin){
 		if(!$plugin->isEnabled()){
 			throw new PluginException("Plugin " . $plugin->getDescription()->getName() . " is disabled");
 		}
 
-		$this->permissible = $permissible;
 		$this->plugin = $plugin;
 	}
 
@@ -63,9 +66,11 @@ class PermissionAttachment{
 		return $this->removed;
 	}
 
-	public function getPermissible() : Permissible{
-		return $this->permissible;
-	}
+	/**
+	 * @return Permissible[]
+	 * @phpstan-return array<int, Permissible>
+	 */
+	public function getSubscribers() : array{ return $this->subscribers; }
 
 	/**
 	 * @return bool[]
@@ -74,9 +79,15 @@ class PermissionAttachment{
 		return $this->permissions;
 	}
 
+	private function recalculatePermissibles() : void{
+		foreach($this->subscribers as $permissible){
+			$permissible->recalculatePermissions();
+		}
+	}
+
 	public function clearPermissions() : void{
 		$this->permissions = [];
-		$this->permissible->recalculatePermissions();
+		$this->recalculatePermissibles();
 	}
 
 	/**
@@ -86,7 +97,7 @@ class PermissionAttachment{
 		foreach($permissions as $key => $value){
 			$this->permissions[$key] = $value;
 		}
-		$this->permissible->recalculatePermissions();
+		$this->recalculatePermissibles();
 	}
 
 	/**
@@ -96,7 +107,7 @@ class PermissionAttachment{
 		foreach($permissions as $node){
 			unset($this->permissions[$node]);
 		}
-		$this->permissible->recalculatePermissions();
+		$this->recalculatePermissibles();
 	}
 
 	/**
@@ -120,7 +131,7 @@ class PermissionAttachment{
 			unset($this->permissions[$name]);
 		}
 		$this->permissions[$name] = $value;
-		$this->permissible->recalculatePermissions();
+		$this->recalculatePermissibles();
 	}
 
 	/**
@@ -130,11 +141,21 @@ class PermissionAttachment{
 		$name = $name instanceof Permission ? $name->getName() : $name;
 		if(isset($this->permissions[$name])){
 			unset($this->permissions[$name]);
-			$this->permissible->recalculatePermissions();
+			$this->recalculatePermissibles();
 		}
 	}
 
-	public function remove() : void{
-		$this->permissible->removeAttachment($this);
+	/**
+	 * @internal
+	 */
+	public function subscribePermissible(Permissible $permissible) : void{
+		$this->subscribers[spl_object_id($permissible)] = $permissible;
+	}
+
+	/**
+	 * @internal
+	 */
+	public function unsubscribePermissible(Permissible $permissible) : void{
+		unset($this->subscribers[spl_object_id($permissible)]);
 	}
 }

src/permission/PermissionRemovedExecutor.php

@@ -25,5 +25,5 @@ namespace pocketmine\permission;
 
 interface PermissionRemovedExecutor{
 
-	public function attachmentRemoved(PermissionAttachment $attachment) : void;
+	public function attachmentRemoved(Permissible $permissible, PermissionAttachment $attachment) : void;
 }