"Thread-safe in-memory cache backend."

import time

try:

    import cPickle as pickle

except ImportError:

    import pickle

from django.core.cache.backends.base import BaseCache

from django.utils.synch import RWLock

class CacheClass(BaseCache):

    def __init__(self, _, params):

        BaseCache.__init__(self, params)

        self._cache = {}

        self._expire_info = {}

        max_entries = params.get('max_entries', 300)

        try:

            self._max_entries = int(max_entries)

        except (ValueError, TypeError):

            self._max_entries = 300

        cull_frequency = params.get('cull_frequency', 3)

        try:

            self._cull_frequency = int(cull_frequency)

        except (ValueError, TypeError):

            self._cull_frequency = 3

        self._lock = RWLock()

    def add(self, key, value, timeout=None):

        self._lock.writer_enters()

        try:

            exp = self._expire_info.get(key)

            if exp is None or exp <= time.time():

                try:

                    self._set(key, pickle.dumps(value), timeout)

                    return True

                except pickle.PickleError:

                    pass

            return False

        finally:

            self._lock.writer_leaves()

    def get(self, key, default=None):

        self._lock.reader_enters()

        try:

            exp = self._expire_info.get(key)

            if exp is None:

                return default

            elif exp > time.time():

                try:

                    return pickle.loads(self._cache[key])

                except pickle.PickleError:

                    return default

        finally:

            self._lock.reader_leaves()

        self._lock.writer_enters()

        try:

            try:

                del self._cache[key]

                del self._expire_info[key]

            except KeyError:

                pass

            return default

        finally:

            self._lock.writer_leaves()

    def _set(self, key, value, timeout=None):

        if len(self._cache) >= self._max_entries:

            self._cull()

        if timeout is None:

            timeout = self.default_timeout

        self._cache[key] = value

        self._expire_info[key] = time.time() + timeout

    def set(self, key, value, timeout=None):

        self._lock.writer_enters()

        # Python 2.4 doesn't allow combined try-except-finally blocks.

        try:

            try:

                self._set(key, pickle.dumps(value), timeout)

            except pickle.PickleError:

                pass

        finally:

            self._lock.writer_leaves()

    def has_key(self, key):

        self._lock.reader_enters()

        try:

            exp = self._expire_info.get(key)

            if exp is None:

                return False

            elif exp > time.time():

                return True

        finally:

            self._lock.reader_leaves()

        self._lock.writer_enters()

        try:

            try:

                del self._cache[key]

                del self._expire_info[key]

            except KeyError:

                pass

            return False

        finally:

            self._lock.writer_leaves()

    def _cull(self):

        if self._cull_frequency == 0:

            self.clear()

        else:

            doomed = [k for (i, k) in enumerate(self._cache) if i % self._cull_frequency == 0]

            for k in doomed:

                self._delete(k)

    def _delete(self, key):

        try:

            del self._cache[key]

        except KeyError:

            pass

        try:

            del self._expire_info[key]

        except KeyError:

            pass

    def delete(self, key):

        self._lock.writer_enters()

        try:

            self._delete(key)

        finally:

            self._lock.writer_leaves()

    def clear(self):

        self._cache.clear()

        self._expire_info.clear()