add module for filesystem transactions

daklib.fstransactions allows to copy, remove, and move files around in
transactions, that is all operations will succeed or be reverted.

daklib/fstransactions.py

+# Copyright (C) 2012, Ansgar Burchardt <ansgar@debian.org>
+#
+# 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.
+#
+# 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.
+
+"""Transactions for filesystem actions
+"""
+
+import errno
+import os
+import shutil
+
+class _FilesystemAction(object):
+    @property
+    def temporary_name(self):
+        raise NotImplementedError()
+
+    def check_for_temporary(self):
+        try:
+            if os.path.exists(self.temporary_name):
+                raise IOError("Temporary file '{0}' already exists.".format(self.temporary_name))
+        except NotImplementedError:
+            pass
+
+class _FilesystemCopyAction(_FilesystemAction):
+    def __init__(self, source, destination, link=True, symlink=False, mode=None):
+        self.destination = destination
+        self.need_cleanup = False
+
+        self.check_for_temporary()
+        destdir = os.path.dirname(self.destination)
+        if not os.path.exists(destdir):
+            os.makedirs(destdir, 0o2775)
+        if symlink:
+            os.symlink(source, self.destination)
+        elif link:
+            try:
+                os.link(source, self.destination)
+            except OSError:
+                shutil.copy2(source, self.destination)
+        else:
+            shutil.copy2(source, self.destination)
+
+        self.need_cleanup = True
+        if mode is not None:
+            os.chmod(self.destination, mode)
+
+    @property
+    def temporary_name(self):
+        return self.destination
+
+    def commit(self):
+        pass
+
+    def rollback(self):
+        if self.need_cleanup:
+            os.unlink(self.destination)
+            self.need_cleanup = False
+
+class _FilesystemUnlinkAction(_FilesystemAction):
+    def __init__(self, path):
+        self.path = path
+        self.need_cleanup = False
+
+        self.check_for_temporary()
+        os.rename(self.path, self.temporary_name)
+        self.need_cleanup = True
+
+    @property
+    def temporary_name(self):
+        return "{0}.dak-rm".format(self.path)
+
+    def commit(self):
+        if self.need_cleanup:
+            os.unlink(self.temporary_name)
+            self.need_cleanup = False
+
+    def rollback(self):
+        if self.need_cleanup:
+            os.rename(self.temporary_name, self.path)
+            self.need_cleanup = False
+
+class _FilesystemCreateAction(_FilesystemAction):
+    def __init__(self, path):
+        self.path = path
+        self.need_cleanup = True
+
+    @property
+    def temporary_name(self):
+        return self.path
+
+    def commit(self):
+        pass
+
+    def rollback(self):
+        if self.need_cleanup:
+            os.unlink(self.path)
+            self.need_cleanup = False
+
+class FilesystemTransaction(object):
+    """transactions for filesystem actions"""
+    def __init__(self):
+        self.actions = []
+
+    def copy(self, source, destination, link=True, symlink=False, mode=None):
+        """copy `source` to `destination`
+
+        Args:
+           source (str): source file
+           destination (str): destination file
+
+        Kwargs:
+           link (bool): Try hardlinking, falling back to copying.
+           symlink (bool): Create a symlink instead
+           mode (int): Permissions to change `destination` to.
+        """
+        self.actions.append(_FilesystemCopyAction(source, destination, link=link, symlink=symlink, mode=mode))
+
+    def move(self, source, destination, mode=None):
+        """move `source` to `destination`
+
+        Args:
+           source (str): source file
+           destination (str): destination file
+
+        Kwargs:
+           mode (int): Permissions to change `destination` to.
+        """
+        self.copy(source, destination, link=True, mode=mode)
+        self.unlink(source)
+
+    def unlink(self, path):
+        """unlink `path`
+
+        Args:
+           path (str): file to unlink
+        """
+        self.actions.append(_FilesystemUnlinkAction(path))
+
+    def create(self, path, mode=None):
+        """create `filename` and return file handle
+
+        Args:
+           filename (str): file to create
+
+        Kwargs:
+           mode (int): Permissions for the new file
+
+        Returns:
+           file handle of the new file
+        """
+        destdir = os.path.dirname(path)
+        if not os.path.exists(destdir):
+            os.makedirs(destdir, 0o2775)
+        if os.path.exists(path):
+            raise IOError("File '{0}' already exists.".format(path))
+        fh = open(path, 'w')
+        self.actions.append(_FilesystemCreateAction(path))
+        if mode is not None:
+            os.chmod(path, mode)
+        return fh
+
+    def commit(self):
+        """Commit all recorded actions."""
+        try:
+            for action in self.actions:
+                action.commit()
+        except:
+            self.rollback()
+            raise
+        finally:
+            self.actions = []
+
+    def rollback(self):
+        """Undo all recorded actions."""
+        try:
+            for action in self.actions:
+                action.rollback()
+        finally:
+            self.actions = []
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, type, value, traceback):
+        if type is None:
+            self.commit()
+        else:
+            self.rollback()
+        return None

tests/test_daklib_fstransactions.py

+#! /usr/bin/env python
+#
+# Copyright (C) 2012, Ansgar Burchardt <ansgar@debian.org>
+#
+# 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.
+#
+# 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.
+
+from base_test import DakTestCase
+from daklib.fstransactions import FilesystemTransaction
+
+from unittest import main
+
+import os
+import shutil
+import tempfile
+
+
+class TemporaryDirectory:
+    def __init__(self):
+        self.directory = None
+    def __str__(self):
+        return self.directory
+    def filename(self, suffix):
+        return os.path.join(self.directory, suffix)
+    def __enter__(self):
+        self.directory = tempfile.mkdtemp()
+        return self
+    def __exit__(self, *args):
+        if self.directory is not None:
+            shutil.rmtree(self.directory)
+            self.directory = None
+        return None
+
+class FilesystemTransactionTestCase(DakTestCase):
+    def _copy_a_b(self, tmp, fs, **kwargs):
+        fs.copy(tmp.filename('a'), tmp.filename('b'), **kwargs)
+
+    def _write_to_a(self, tmp):
+        with open(tmp.filename('a'), 'w') as fh:
+            print >>fh, 'a'
+
+    def test_copy_non_existing(self):
+        def copy():
+            with TemporaryDirectory() as t:
+                with FilesystemTransaction() as fs:
+                    self._copy_a_b(t, fs)
+
+        self.assertRaises(IOError, copy)
+
+    def test_copy_existing_and_commit(self):
+        with TemporaryDirectory() as t:
+            self._write_to_a(t)
+
+            with FilesystemTransaction() as fs:
+                self._copy_a_b(t, fs)
+                self.assert_(os.path.exists(t.filename('a')))
+                self.assert_(os.path.exists(t.filename('b')))
+
+            self.assert_(os.path.exists(t.filename('a')))
+            self.assert_(os.path.exists(t.filename('b')))
+
+    def test_copy_existing_and_rollback(self):
+        with TemporaryDirectory() as t:
+            self._write_to_a(t)
+
+            class TestException(Exception):
+                pass
+            try:
+                with FilesystemTransaction() as fs:
+                    self._copy_a_b(t, fs)
+                    self.assert_(os.path.exists(t.filename('a')))
+                    self.assert_(os.path.exists(t.filename('b')))
+                    raise TestException()
+            except TestException:
+                pass
+
+            self.assert_(os.path.exists(t.filename('a')))
+            self.assert_(not os.path.exists(t.filename('b')))
+
+    def test_unlink_and_commit(self):
+        with TemporaryDirectory() as t:
+            self._write_to_a(t)
+            a = t.filename('a')
+            with FilesystemTransaction() as fs:
+                self.assert_(os.path.exists(a))
+                fs.unlink(a)
+                self.assert_(not os.path.exists(a))
+            self.assert_(not os.path.exists(a))
+
+    def test_unlink_and_rollback(self):
+        with TemporaryDirectory() as t:
+            self._write_to_a(t)
+            a = t.filename('a')
+            class TestException(Exception):
+                pass
+
+            try:
+                with FilesystemTransaction() as fs:
+                    self.assert_(os.path.exists(a))
+                    fs.unlink(a)
+                    self.assert_(not os.path.exists(a))
+                    raise TestException()
+            except TestException:
+                pass
+            self.assert_(os.path.exists(a))
+
+if __name__ == '__main__':
+    main()