Caches should be bound to instances.

Before, caches were global and so different instances of the stores
would share caches. This caused problems in the unit tests.

synapse/storage/_base.py

@@ -127,7 +127,7 @@ class Cache(object):
         self.cache.clear()
 
 
-def cached(max_entries=1000, num_args=1, lru=False):
+class CacheDescriptor(object):
     """ 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
@@ -141,25 +141,32 @@ def cached(max_entries=1000, num_args=1, lru=False):
     which can be used to insert values into the cache specifically, without
     calling the calculation function.
     """
-    def wrap(orig):
+    def __init__(self, orig, max_entries=1000, num_args=1, lru=False):
+        self.orig = orig
+
+        self.max_entries = max_entries
+        self.num_args = num_args
+        self.lru = lru
+
+    def __get__(self, obj, objtype=None):
         cache = Cache(
-            name=orig.__name__,
-            max_entries=max_entries,
-            keylen=num_args,
-            lru=lru,
+            name=self.orig.__name__,
+            max_entries=self.max_entries,
+            keylen=self.num_args,
+            lru=self.lru,
         )
 
-        @functools.wraps(orig)
+        @functools.wraps(self.orig)
         @defer.inlineCallbacks
-        def wrapped(self, *keyargs):
+        def wrapped(*keyargs):
             try:
-                cached_result = cache.get(*keyargs)
+                cached_result = cache.get(*keyargs[:self.num_args])
                 if DEBUG_CACHES:
-                    actual_result = yield orig(self, *keyargs)
+                    actual_result = yield self.orig(obj, *keyargs)
                     if actual_result != cached_result:
                         logger.error(
                             "Stale cache entry %s%r: cached: %r, actual %r",
-                            orig.__name__, keyargs,
+                            self.orig.__name__, keyargs,
                             cached_result, actual_result,
                         )
                         raise ValueError("Stale cache entry")
@@ -170,18 +177,28 @@ def cached(max_entries=1000, num_args=1, lru=False):
                 # while the SELECT is executing (SYN-369)
                 sequence = cache.sequence
 
-                ret = yield orig(self, *keyargs)
+                ret = yield self.orig(obj, *keyargs)
 
-                cache.update(sequence, *keyargs + (ret,))
+                cache.update(sequence, *keyargs[:self.num_args] + (ret,))
 
                 defer.returnValue(ret)
 
         wrapped.invalidate = cache.invalidate
         wrapped.invalidate_all = cache.invalidate_all
         wrapped.prefill = cache.prefill
+
+        obj.__dict__[self.orig.__name__] = wrapped
+
         return wrapped
 
-    return wrap
+
+def cached(max_entries=1000, num_args=1, lru=False):
+    return lambda orig: CacheDescriptor(
+        orig,
+        max_entries=max_entries,
+        num_args=num_args,
+        lru=lru
+    )
 
 
 class LoggingTransaction(object):