import errno

import os

import tempfile

from django.conf import settings

from django.contrib.sessions.backends.base import SessionBase, CreateError

from django.core.exceptions import SuspiciousOperation, ImproperlyConfigured

class SessionStore(SessionBase):

    """

    Implements a file based session store.

    """

    def __init__(self, session_key=None):

        self.storage_path = getattr(settings, "SESSION_FILE_PATH", None)

        if not self.storage_path:

            self.storage_path = tempfile.gettempdir()

        # Make sure the storage path is valid.

        if not os.path.isdir(self.storage_path):

            raise ImproperlyConfigured(

                "The session storage path %r doesn't exist. Please set your"

                " SESSION_FILE_PATH setting to an existing directory in which"

                " Django can store session data." % self.storage_path)

        self.file_prefix = settings.SESSION_COOKIE_NAME

        super(SessionStore, self).__init__(session_key)

    def _key_to_file(self, session_key=None):

        """

        Get the file associated with this session key.

        """

        if session_key is None:

            session_key = self.session_key

        # Make sure we're not vulnerable to directory traversal. Session keys

        # should always be md5s, so they should never contain directory

        # components.

        if os.path.sep in session_key:

            raise SuspiciousOperation(

                "Invalid characters (directory components) in session key")

        return os.path.join(self.storage_path, self.file_prefix + session_key)

    def load(self):

        session_data = {}

        try:

            session_file = open(self._key_to_file(), "rb")

            try:

                file_data = session_file.read()

                # Don't fail if there is no data in the session file.

                # We may have opened the empty placeholder file.

                if file_data:

                    try:

                        session_data = self.decode(file_data)

                    except (EOFError, SuspiciousOperation):

                        self.create()

            finally:

                session_file.close()

        except IOError:

            pass

        return session_data

    def create(self):

        while True:

            self._session_key = self._get_new_session_key()

            try:

                self.save(must_create=True)

            except CreateError:

                continue

            self.modified = True

            self._session_cache = {}

            return

    def save(self, must_create=False):

        # Get the session data now, before we start messing

        # with the file it is stored within.

        session_data = self._get_session(no_load=must_create)

        session_file_name = self._key_to_file()

        try:

            # Make sure the file exists.  If it does not already exist, an

            # empty placeholder file is created.

            flags = os.O_WRONLY | os.O_CREAT | getattr(os, 'O_BINARY', 0)

            if must_create:

                flags |= os.O_EXCL

            fd = os.open(session_file_name, flags)

            os.close(fd)

        except OSError, e:

            if must_create and e.errno == errno.EEXIST:

                raise CreateError

            raise

        # Write the session file without interfering with other threads

        # or processes.  By writing to an atomically generated temporary

        # file and then using the atomic os.rename() to make the complete

        # file visible, we avoid having to lock the session file, while

        # still maintaining its integrity.

        #

        # Note: Locking the session file was explored, but rejected in part

        # because in order to be atomic and cross-platform, it required a

        # long-lived lock file for each session, doubling the number of

        # files in the session storage directory at any given time.  This

        # rename solution is cleaner and avoids any additional overhead

        # when reading the session data, which is the more common case

        # unless SESSION_SAVE_EVERY_REQUEST = True.

        #

        # See ticket #8616.

        dir, prefix = os.path.split(session_file_name)

        try:

            output_file_fd, output_file_name = tempfile.mkstemp(dir=dir,

                prefix=prefix + '_out_')

            renamed = False

            try:

                try:

                    os.write(output_file_fd, self.encode(session_data))

                finally:

                    os.close(output_file_fd)

                os.rename(output_file_name, session_file_name)

                renamed = True

            finally:

                if not renamed:

                    os.unlink(output_file_name)

        except (OSError, IOError, EOFError):

            pass

    def exists(self, session_key):

        if os.path.exists(self._key_to_file(session_key)):

            return True

        return False

    def delete(self, session_key=None):

        if session_key is None:

            if self._session_key is None:

                return

            session_key = self._session_key

        try:

            os.unlink(self._key_to_file(session_key))

        except OSError:

            pass

    def clean(self):

        pass