Add lockfile attribute

rbackup/plugins/packagemanager.py

@@ -11,7 +11,8 @@ from pathlib import Path
 from tempfile import NamedTemporaryFile
 
 # ========== Constants ==========
-VALID_DB_COMPRESS_MODES = ["bzip2", "gz", "lzma", "xz"]
+LOCKFILE_MODE = 0o0000
+VALID_DB_COMPRESS_MODES = [None, "bzip2", "gz", "lzma", "xz"]
 
 # ========== Logging Setup ==========
 syslog = logging.getLogger(__name__)
@@ -19,20 +20,62 @@ syslog = logging.getLogger(__name__)
 
 # ========== Classes ==========
 class PackageManager:
-    def __init__(self, cachedir, db_path, pkglist_cmd):
+    """Class for abstracting package manager-based operations.
+
+    The package manager can be used in conjunction with a ``Snapshot`` for backups.
+
+    Lockfile Management
+    ^^^^^^^^^^^^^^^^^^^
+
+    This class can be used as a context manager for creating a lockfile for the
+    specific package manager. This is to prevent transactions from occurring during
+    backup operations which would most likely leave the package manager's database in
+    an inconsistent state on the backup.
+
+        .. note:: Subclasses can override the context manager and implement i.e. blocking until
+            the process is complete with a timeout.
+    """
+
+    def __init__(self, cachedir=None, db_path=None, lockfile=None, pkglist_cmd=None):
         """Default constructor for the PackageManager class.
 
         :param cachedir: path to the package manager cache directory
         :type cachedir: str or path-like object
         :param db_path: path to the package manager database
         :type db_path: str or path-like object
+        :param lockfile: path to this package manager's lockfile
+        :type lockfile: str
         :param pkglist_cmd: command to list installed packages to stdout
         :type pkglist_cmd: str or iterable of str
         """
         self._cachedir = Path(cachedir)
         self._db_path = Path(db_path)
+        self._lockfile = Path(lockfile)
         self._pkglist_cmd = pkglist_cmd
 
+    def __enter__(self):
+        """Create the package manager's lockfile. This prevents transactions
+        from occurring during the backup which could leave the database backup
+        in an inconsistent state.
+
+            The existence of this lockfile is an error, and its meaning is up to
+            the package manager. For example, pacman's db.lck indicates
+            either there is an ongoing transaction in progress or a previous transaction
+            failed and the database is in an inconsistent state.
+
+        :returns: self
+        :rtype: ``PackageManager`` object
+        :raises FileExistsError: if lockfile exists when this method is called
+        """
+        self._lockfile.touch(mode=0o000)
+        yield self
+
+    def __exit__(self):
+        """Remove the package manager's lockfile. After this lockfile is closed,
+        the package manager this class abstracts can perform transactions once again.
+        """
+        self._lockfile.unlink()
+
     def gen_pkglist(self):
         """Generate a text file listing installed packages
         on the system and return the path to that file.
@@ -109,6 +152,14 @@ class PackageManager:
         """
         return self._db_path
 
+    @property
+    def lockfile(self):
+        """
+        :returns: the lockfile path of this package manager
+        :rtype: path-like object
+        """
+        return self._lockfile
+
     @property
     def pkglist_cmd(self):
         """

tests/test_packagemanager.py

@@ -60,8 +60,11 @@ class TestPackageManagerMethods(unittest.TestCase):
 
         self.cachedir = "/var/cache/pacman/"
         self.db_path = "/var/lib/pacman"
+        self.lockfile = "/var/lib/pacman/db.lck"
         self.pkglist_cmd = ["pacman", "-Qqe"]
-        self.p = PackageManager(self.cachedir, self.db_path, self.pkglist_cmd)
+        self.p = PackageManager(
+            self.cachedir, self.db_path, self.lockfile, self.pkglist_cmd
+        )
 
     def test_pkglist(self):
         self.mocked_run.return_value.stdout = "packages"