Allow a choice of LRU behaviour for Cache() by using LruCache() or OrderedDict()

synapse/storage/_base.py

@@ -55,10 +55,14 @@ cache_counter = metrics.register_cache(
 
 class Cache(object):
 
-    def __init__(self, name, max_entries=1000, keylen=1):
+    def __init__(self, name, max_entries=1000, keylen=1, lru=False):
+        if lru:
+            self.cache = LruCache(max_size=max_entries)
+            self.max_entries = None
+        else:
             self.cache = OrderedDict()
-
             self.max_entries = max_entries
+
         self.name = name
         self.keylen = keylen
 
@@ -82,7 +86,8 @@ class Cache(object):
         if len(keyargs) != self.keylen:
             raise ValueError("Expected a key to have %d items", self.keylen)
 
-        while len(self.cache) > self.max_entries:
+        if self.max_entries is not None:
+            while len(self.cache) >= self.max_entries:
                 self.cache.popitem(last=False)
 
         self.cache[keyargs] = value
@@ -94,9 +99,7 @@ class Cache(object):
         self.cache.pop(keyargs, None)
 
 
-# TODO(paul):
-#  * consider other eviction strategies - LRU?
-def cached(max_entries=1000, num_args=1):
+def cached(max_entries=1000, num_args=1, lru=False):
     """ A method decorator that applies a memoizing cache around the function.
 
     The function is presumed to take zero or more arguments, which are used in
@@ -115,6 +118,7 @@ def cached(max_entries=1000, num_args=1):
             name=orig.__name__,
             max_entries=max_entries,
             keylen=num_args,
+            lru=lru,
         )
 
         @functools.wraps(orig)

tests/storage/test__base.py

@@ -69,6 +69,28 @@ class CacheTestCase(unittest.TestCase):
         cache.get(2)
         cache.get(3)
 
+    def test_eviction_lru(self):
+        cache = Cache("test", max_entries=2, lru=True)
+
+        cache.prefill(1, "one")
+        cache.prefill(2, "two")
+
+        # Now access 1 again, thus causing 2 to be least-recently used
+        cache.get(1)
+
+        cache.prefill(3, "three")
+
+        failed = False
+        try:
+            cache.get(2)
+        except KeyError:
+            failed = True
+
+        self.assertTrue(failed)
+
+        cache.get(1)
+        cache.get(3)
+
 
 class CacheDecoratorTestCase(unittest.TestCase):