Revert "Revert "move _state_group_cache to statestore""

We're going to fix this properly on this branch, so that the _state_group_cache
can end up in StateGroupReadStore.

This reverts commit ab335edb02.

synapse/storage/_base.py

@@ -16,8 +16,6 @@ import logging
 
 from synapse.api.errors import StoreError
 from synapse.util.logcontext import LoggingContext, PreserveLoggingContext
-from synapse.util.caches import CACHE_SIZE_FACTOR
-from synapse.util.caches.dictionary_cache import DictionaryCache
 from synapse.util.caches.descriptors import Cache
 from synapse.storage.engines import PostgresEngine
 import synapse.metrics
@@ -180,10 +178,6 @@ class SQLBaseStore(object):
         self._get_event_cache = Cache("*getEvent*", keylen=3,
                                       max_entries=hs.config.event_cache_size)
 
-        self._state_group_cache = DictionaryCache(
-            "*stateGroupCache*", 100000 * CACHE_SIZE_FACTOR
-        )
-
         self._event_fetch_lock = threading.Condition()
         self._event_fetch_list = []
         self._event_fetch_ongoing = 0

synapse/storage/state.py

@@ -13,16 +13,17 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from ._base import SQLBaseStore
-from synapse.util.caches.descriptors import cached, cachedList
-from synapse.util.caches import intern_string
-from synapse.util.stringutils import to_ascii
-from synapse.storage.engines import PostgresEngine
+from collections import namedtuple
+import logging
 
 from twisted.internet import defer
-from collections import namedtuple
 
-import logging
+from synapse.storage.engines import PostgresEngine
+from synapse.util.caches import intern_string, CACHE_SIZE_FACTOR
+from synapse.util.caches.descriptors import cached, cachedList
+from synapse.util.caches.dictionary_cache import DictionaryCache
+from synapse.util.stringutils import to_ascii
+from ._base import SQLBaseStore
 
 logger = logging.getLogger(__name__)
 
@@ -81,6 +82,10 @@ class StateStore(SQLBaseStore):
             where_clause="type='m.room.member'",
         )
 
+        self._state_group_cache = DictionaryCache(
+            "*stateGroupCache*", 100000 * CACHE_SIZE_FACTOR
+        )
+
     @cached(max_entries=100000, iterable=True)
     def get_current_state_ids(self, room_id):
         """Get the current state event ids for a room based on the