Added background scheduler and scheduled thumbnail generation job

cps/services/background_scheduler.py

@@ -0,0 +1,52 @@
+# -*- coding: utf-8 -*-
+
+#   This file is part of the Calibre-Web (https://github.com/janeczku/calibre-web)
+#     Copyright (C) 2020 mmonkey
+#
+#   This program is free software: you can redistribute it and/or modify
+#   it under the terms of the GNU General Public License as published by
+#   the Free Software Foundation, either version 3 of the License, or
+#   (at your option) any later version.
+#
+#   This program is distributed in the hope that it will be useful,
+#   but WITHOUT ANY WARRANTY; without even the implied warranty of
+#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#   GNU General Public License for more details.
+#
+#   You should have received a copy of the GNU General Public License
+#   along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+from __future__ import division, print_function, unicode_literals
+import atexit
+
+from .. import logger
+from .worker import WorkerThread
+from apscheduler.schedulers.background import BackgroundScheduler as BScheduler
+
+
+class BackgroundScheduler:
+    _instance = None
+
+    def __new__(cls):
+        if cls._instance is None:
+            cls._instance = super(BackgroundScheduler, cls).__new__(cls)
+
+            scheduler = BScheduler()
+            atexit.register(lambda: scheduler.shutdown())
+
+            cls.log = logger.create()
+            cls.scheduler = scheduler
+            cls.scheduler.start()
+
+        return cls._instance
+
+    def add(self, func, trigger, **trigger_args):
+        self.scheduler.add_job(func=func, trigger=trigger, **trigger_args)
+
+    def add_task(self, user, task, trigger, **trigger_args):
+        def scheduled_task():
+            worker_task = task()
+            self.log.info('Running scheduled task in background: ' + worker_task.name + ': ' + worker_task.message)
+            WorkerThread.add(user, worker_task)
+
+        self.add(func=scheduled_task, trigger=trigger, **trigger_args)