diff --git a/synapse/util/expiringcache.py b/synapse/util/expiringcache.py
new file mode 100644
index 0000000000..fb5b27ad10
--- /dev/null
+++ b/synapse/util/expiringcache.py
@@ -0,0 +1,115 @@
+# -*- coding: utf-8 -*-
+# Copyright 2015 OpenMarket Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import logging
+
+
+logger = logging.getLogger(__name__)
+
+
+class ExpiringCache(object):
+ def __init__(self, cache_name, clock, max_len=0, expiry_ms=0,
+ reset_expiry_on_get=False):
+ """
+ Args:
+ cache_name (str): Name of this cache, used for logging.
+ clock (Clock)
+ max_len (int): Max size of dict. If the dict grows larger than this
+ then the oldest items get automatically evicted. Default is 0,
+ which indicates there is no max limit.
+ expiry_ms (int): How long before an item is evicted from the cache
+ in milliseconds. Default is 0, indicating items never get
+ evicted based on time.
+ reset_expiry_on_get (bool): If true, will reset the expiry time for
+ an item on access. Defaults to False.
+
+ """
+ self._cache_name = cache_name
+
+ self._clock = clock
+
+ self._max_len = max_len
+ self._expiry_ms = expiry_ms
+
+ self._reset_expiry_on_get = reset_expiry_on_get
+
+ self._cache = {}
+
+ def start(self):
+ if not self._expiry_ms:
+ # Don't bother starting the loop if things never expire
+ return
+
+ def f():
+ self._prune_cache()
+
+ self._clock.looping_call(f, self._expiry_ms)
+
+ def __setitem__(self, key, value):
+ now = self._clock.time_msec()
+ self._cache[key] = _CacheEntry(now, value)
+
+ # Evict if there are now too many items
+ if self._max_len and len(self._cache.keys()) > self._max_len:
+ sorted_entries = sorted(
+ self._cache.items(),
+ key=lambda k, v: v.time,
+ )
+
+ for k, _ in sorted_entries[self._max_len:]:
+ self._cache.pop(k)
+
+ def __getitem__(self, key):
+ entry = self._cache[key]
+
+ if self._reset_expiry_on_get:
+ entry.time = self._clock.time_msec()
+
+ return entry.value
+
+ def get(self, key, default=None):
+ try:
+ return self[key]
+ except KeyError:
+ return default
+
+ def _purge_cache(self):
+ if not self._expiry_ms:
+ # zero expiry time means don't expire. This should never get called
+ # since we have this check in start too.
+ return
+ begin_length = len(self._cache)
+
+ now = self._clock.time_msec()
+
+ keys_to_delete = set()
+
+ for key, cache_entry in self._cache.items():
+ if now - cache_entry.time > self._expiry_ms:
+ keys_to_delete.add(key)
+
+ for k in keys_to_delete:
+ self._cache.pop(k)
+
+ logger.debug(
+ "[%s] _prune_cache before: %d, after len: %d",
+ self._cache_name, begin_length, len(self._cache.keys())
+ )
+
+
+class _CacheEntry(object):
+ def __init__(self, time, value):
+ self.time = time
+ self.value = value
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