fix: Global flock file accessible by every user / Introducing FlockContext manager

CHANGES

@@ -1,6 +1,7 @@
 Back In Time
 
 Version 1.4.4-dev (development of upcoming release)
+* Fix: Global flock for multiple users (#1122, #1676)
 * Fix: Validation of diff command settings in compare snapshots dialog (#1662) (@stcksmsh Kosta Vukicevic)
 * Fix bug: Open symlinked folders in file view (#1476)
 * Fix bug: Respect dark mode using color roles (#1601)

common/applicationinstance.py

@@ -1,23 +1,24 @@
-#    Back In Time
-#    Copyright (C) 2008-2022 Oprea Dan, Bart de Koning, Richard Bailey, Germar Reitze
+# Back In Time
+# Copyright (C) 2008-2022 Oprea Dan, Bart de Koning, Richard Bailey,
+# Germar Reitze
 #
-#    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 2 of the License, or
-#    (at your option) any later version.
+# 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 2 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.
+# 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, write to the Free Software Foundation, Inc.,
-#    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+# You should have received a copy of the GNU General Public License along
+# with this program; if not, write to the Free Software Foundation, Inc.,
+# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 
 """
 This module holds the ApplicationInstance class, used to handle
-the one application instance mechanism 
+the one application instance mechanism.
 """
 
 import os

common/flock.py

@@ -0,0 +1,64 @@
+# SPDX-FileCopyrightText: © 2024 Christian BUHTZ <c.buhtz@posteo.jp>
+#
+# SPDX-License-Identifier: GPL-2.0
+#
+# This file is part of the program "Back In time" which is released under GNU
+# General Public License v2 (GPLv2).
+# See file LICENSE or go to <https://www.gnu.org/licenses/#GPL>.
+import os
+import fcntl
+from pathlib import Path
+import logger
+
+
+class FlockContext:
+    """Context manager to manage flock.
+
+    Usage example ::
+
+        with FlockContext('backintime.lock'):
+             do_fancy_things()
+
+    """
+    def __init__(self, filename: str, folder: Path = None):
+        # default
+        if folder is None:
+            folder = Path(Path.cwd().root) / 'run' / 'lock'
+
+            # out-dated default
+            if not folder.exists():
+                folder = Path(Path.cwd().root) / 'var' / 'lock'
+
+        self._file_path = folder / filename
+        """Path to used for flock"""
+
+    def __enter__(self):
+        self._log('Set')
+
+        # Open (and create if needed) the file
+        mode = 'r' if self._file_path.exists() else 'w'
+        self._flock_handle = self._file_path.open(mode)
+
+        # blocks (waits) until an existing flock is released
+        fcntl.flock(self._flock_handle, fcntl.LOCK_EX)
+
+        # If new created file set itspermissions to "rw-rw-rw".
+        # otherwise a foreign user is not able to use it.
+        if mode == 'w':
+            self._file_path.chmod(int('0o666', 8))
+
+        return self
+
+    def __exit__(self, exc_type, exc_value, exc_tb):
+        self._log('Release')
+        fcntl.fcntl(self._flock_handle, fcntl.LOCK_UN)
+        self._flock_handle.close()
+
+    def _log(self, prefix: str):
+        logger.debug(f'{prefix} flock {self._file_path} by PID {os.getpid()}',
+                     self)
+
+
+class GlobalFlock(FlockContext):
+    def __init__(self):
+        super().__init__('backintime.lock')