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# persister: persist Python objects safely to pickle files
# Copyright 2003, 2004, 2005, 2013, 2014 Adam Sampson <ats@offog.org>
#
# rawdog is free software; you can redistribute and/or modify it
# under the terms of that license as published by the Free Software
# Foundation; either version 2 of the License, or (at your option)
# any later version.
#
# rawdog 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 rawdog; see the file COPYING. If not, write to the Free
# Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
# MA 02110-1301, USA, or see http://www.gnu.org/.
import cPickle as pickle
import errno
import fcntl
import os
import sys
class Persistable:
"""An object which can be persisted."""
def __init__(self):
self._modified = False
def modified(self, state=True):
"""Mark the object as having been modified (or not)."""
self._modified = state
def is_modified(self):
return self._modified
class Persisted:
"""Context manager for a persistent object. The object being persisted
must implement the Persistable interface."""
def __init__(self, klass, filename, persister):
self.klass = klass
self.filename = filename
self.persister = persister
self.lock_file = None
self.object = None
self.refcount = 0
def rename(self, new_filename):
"""Rename the persisted file. This works whether the file is
currently open or not."""
self.persister._rename(self.filename, new_filename)
for ext in ("", ".lock"):
try:
os.rename(self.filename + ext,
new_filename + ext)
except OSError, e:
# If the file doesn't exist (yet),
# that's OK.
if e.errno != errno.ENOENT:
raise e
self.filename = new_filename
def __enter__(self):
"""As open()."""
return self.open()
def __exit__(self, type, value, tb):
"""As close(), unless an exception occurred in which case do
nothing."""
if tb is None:
self.close()
def open(self, no_block=True):
"""Return the persistent object, loading it from its file if it
isn't already open. You must call close() once you're finished
with the object.
If no_block is True, then this will return None if loading the
object would otherwise block (i.e. if it's locked by another
process)."""
if self.refcount > 0:
# Already loaded.
self.refcount += 1
return self.object
try:
self._open(no_block)
except KeyboardInterrupt:
sys.exit(1)
except:
print "An error occurred while reading state from " + os.path.abspath(self.filename) + "."
print "This usually means the file is corrupt, and removing it will fix the problem."
sys.exit(1)
self.refcount = 1
return self.object
def _get_lock(self, no_block):
if not self.persister.use_locking:
return True
self.lock_file = open(self.filename + ".lock", "w+")
try:
mode = fcntl.LOCK_EX
if no_block:
mode |= fcntl.LOCK_NB
fcntl.lockf(self.lock_file.fileno(), mode)
except IOError, e:
if no_block and e.errno in (errno.EACCES, errno.EAGAIN):
return False
raise e
return True
def _open(self, no_block):
self.persister.log("Loading state file: ", self.filename)
if not self._get_lock(no_block):
return None
try:
f = open(self.filename, "rb")
except IOError:
# File can't be opened.
# Create a new object.
self.object = self.klass()
self.object.modified()
return
self.object = pickle.load(f)
self.object.modified(False)
f.close()
def close(self):
"""Reduce the reference count of the persisted object, saving
it back to its file if necessary."""
self.refcount -= 1
if self.refcount > 0:
# Still in use.
return
if self.object.is_modified():
self.persister.log("Saving state file: ", self.filename)
newname = "%s.new-%d" % (self.filename, os.getpid())
newfile = open(newname, "w")
pickle.dump(self.object, newfile, pickle.HIGHEST_PROTOCOL)
newfile.close()
os.rename(newname, self.filename)
if self.lock_file is not None:
self.lock_file.close()
self.persister._remove(self.filename)
class Persister:
"""Manage the collection of persisted files."""
def __init__(self, config):
self.files = {}
self.log = config.log
self.use_locking = config.locking
def get(self, klass, filename):
"""Get a context manager for a persisted file.
If the file is already open, this will return
the existing context manager."""
if filename in self.files:
return self.files[filename]
p = Persisted(klass, filename, self)
self.files[filename] = p
return p
def _rename(self, old_filename, new_filename):
self.files[new_filename] = self.files[old_filename]
del self.files[old_filename]
def _remove(self, filename):
del self.files[filename]
def delete(self, filename):
"""Delete a persisted file, along with its lock file,
if they exist."""
for ext in ("", ".lock"):
try:
os.unlink(filename + ext)
except OSError:
pass
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