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

.codespellrc

@@ -1,6 +1,6 @@
 [codespell]
 # Folders and files to skip
-skip = .codespellrc,*.po,Makefile,*.desktop,.git,__pycache__,*.pyc,languages.py
+skip = .codespellrc,*.po,Makefile,*.desktop,.git,__pycache__,*.pyc,languages.py,*.html,_build
 # Print N lines of surrounding context
 context = 1
 # Check hidden files also (empty means True)

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 bug: "Backup folders" list does reflect the selected snapshot (#1585) (@rafaelhdr Rafael Hurpia da Rocha)
 * Breaking Change: GUI started with --debug does no longer add --debug to the crontab for scheduled profiles.
                    Use the new "enable logging for debug messages" in the 'Schedule' section of the 'Manage profiles' GUI instead.

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
@@ -30,6 +31,8 @@
 #      one app instance eg. if a restore is running and another
 #      backup starts).
 #      Rename it to eg. LockFileManager
+# TODO2 When refactoring have a look at "common/flock.py" still implementing
+#       a contxt manager for that problem.
 class ApplicationInstance:
     """
     Class used to handle one application instance mechanism.
@@ -154,15 +157,20 @@ def flockExclusiv(self):
         (so that only the last creator wins).
 
         Dev notes:
-        ---------
+        ----------
         buhtz (2023-09):
         Not sure but just log an ERROR without doing anything else is
         IMHO not enough.
+
         aryoda (2023-12):
         It seems the purpose of this additional lock file using an exclusive lock
         is to block the other process to continue until this exclusive lock
         is released (= serialize execution).
         Therefore advisory locks are used via fcntl.flock (see: man 2 fcntl)
+
+        buhtz (2024-05):
+        Have a look at the new :mod:`flock` module providing an flock context
+        manager.
         """
 
         flock_file_URI = self.pidFile + '.flock'

common/doc-dev/flock.rst

@@ -0,0 +1,7 @@
+flock module
+============
+
+.. automodule:: flock
+    :members:
+    :undoc-members:
+    :show-inheritance:

common/doc-dev/modules.rst

@@ -14,6 +14,7 @@ common
    diagnostics
    encfstools
    exceptions
+   flock
    guiapplicationinstance
    logger
    mount

common/flock.py

@@ -0,0 +1,74 @@
+# 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 file locks (flock).
+
+    The flock file is stored in the folder `/run/lock` or if not present in
+    `/var/lock`.
+
+    Usage example ::
+
+        class MyFlock(_FlockContext):
+            def __init__(self):
+                super().__init__('my.lock')
+
+        with MyFlock():
+             do_fancy_things()
+
+    """
+    def __init__(self, filename: str, folder: Path = None):
+        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):
+        """Generate a log message including the current lock files path and the
+        process ID.
+        """
+        logger.debug(f'{prefix} flock {self._file_path} by PID {os.getpid()}',
+                     self)
+
+
+class GlobalFlock(_FlockContext):
+    """Flock context manager used for global flock in Back In Time."""
+    def __init__(self):
+        super().__init__('backintime.lock')