Fix race condition in old invitation closing

src/mibew/libs/invitation.php

@@ -21,6 +21,7 @@
 use Mibew\EventDispatcher\EventDispatcher;
 use Mibew\EventDispatcher\Events;
 use Mibew\Database;
+use Mibew\ProcessLock;
 use Mibew\Settings;
 use Mibew\Thread;
 
@@ -248,6 +249,9 @@ function invitation_reject($visitor_id)
  */
 function invitation_close_old()
 {
+    // Run only one instance of cleaning process.
+    $lock = new ProcessLock('invitations_close_old');
+    if ($lock->get()) {
         $db = Database::getInstance();
 
         // Get all threads to close
@@ -299,6 +303,10 @@ function invitation_close_old()
 
             unset($thread);
         }
+
+        // Release the lock
+        $lock->release();
+    }
 }
 
 /**