Disable partial state group caching for wildcard lookups

When _get_state_for_groups is given a wildcard filter, just do a complete
lookup. Hopefully this will give us the best of both worlds by not filling up
the ram if we only need one or two keys, but also making the cache still work
for the federation reader usecase.

2 files changed, 55 insertions, 26 deletions

diff --git a/synapse/storage/state.py b/synapse/storage/state.py
index 986a20400c..cd9821c270 100644
--- a/synapse/storage/state.py
+++ b/synapse/storage/state.py
@@ -526,10 +526,23 @@ class StateGroupWorkerStore(SQLBaseStore):
 
     @defer.inlineCallbacks
     def _get_state_for_groups(self, groups, types=None):
-        """Given list of groups returns dict of group -> list of state events
-        with matching types. `types` is a list of `(type, state_key)`, where
-        a `state_key` of None matches all state_keys. If `types` is None then
-        all events are returned.
+        """Gets the state at each of a list of state groups, optionally
+        filtering by type/state_key
+
+        Args:
+            groups (iterable[int]): list of state groups for which we want
+                to get the state.
+            types (None|iterable[(str, None|str)]):
+                indicates the state type/keys required. If None, the whole
+                state is fetched and returned.
+
+                Otherwise, each entry should be a `(type, state_key)` tuple to
+                include in the response. A `state_key` of None is a wildcard
+                meaning that we require all state with that type.
+
+        Returns:
+            Deferred[dict[int, dict[(type, state_key), EventBase]]]
+                a dictionary mapping from state group to state dictionary.
         """
         if types:
             types = frozenset(types)
@@ -538,7 +551,7 @@ class StateGroupWorkerStore(SQLBaseStore):
         if types is not None:
             for group in set(groups):
                 state_dict_ids, _, got_all = self._get_some_state_from_cache(
-                    group, types
+                    group, types,
                 )
                 results[group] = state_dict_ids
 
@@ -559,22 +572,40 @@ class StateGroupWorkerStore(SQLBaseStore):
             # Okay, so we have some missing_types, lets fetch them.
             cache_seq_num = self._state_group_cache.sequence
 
+            # the DictionaryCache knows if it has *all* the state, but
+            # does not know if it has all of the keys of a particular type,
+            # which makes wildcard lookups expensive unless we have a complete
+            # cache. Hence, if we are doing a wildcard lookup, populate the
+            # cache fully so that we can do an efficient lookup next time.
+
+            if types and any(k is None for (t, k) in types):
+                types_to_fetch = None
+            else:
+                types_to_fetch = types
+
             group_to_state_dict = yield self._get_state_groups_from_groups(
-                missing_groups, types
+                missing_groups, types_to_fetch,
             )
 
-            # Now we want to update the cache with all the things we fetched
-            # from the database.
             for group, group_state_dict in iteritems(group_to_state_dict):
                 state_dict = results[group]
-                state_dict.update(group_state_dict)
 
+                # update the result, filtering by `types`.
+                if types:
+                    for k, v in iteritems(group_state_dict):
+                        (typ, _) = k
+                        if k in types or (typ, None) in types:
+                            state_dict[k] = v
+                else:
+                    state_dict.update(group_state_dict)
+
+                # update the cache with all the things we fetched from the
+                # database.
                 self._state_group_cache.update(
                     cache_seq_num,
                     key=group,
-                    value=state_dict,
-                    full=(types is None),
-                    known_absent=types,
+                    value=group_state_dict,
+                    fetched_keys=types_to_fetch,
                 )
 
         defer.returnValue(results)
@@ -681,7 +712,6 @@ class StateGroupWorkerStore(SQLBaseStore):
                 self._state_group_cache.sequence,
                 key=state_group,
                 value=dict(current_state_ids),
-                full=True,
             )
 
             return state_group
diff --git a/synapse/util/caches/dictionary_cache.py b/synapse/util/caches/dictionary_cache.py
index bdc21e348f..95793d466d 100644
--- a/synapse/util/caches/dictionary_cache.py
+++ b/synapse/util/caches/dictionary_cache.py
@@ -107,29 +107,28 @@ class DictionaryCache(object):
         self.sequence += 1
         self.cache.clear()
 
-    def update(self, sequence, key, value, full=False, known_absent=None):
+    def update(self, sequence, key, value, fetched_keys=None):
         """Updates the entry in the cache
 
         Args:
             sequence
-            key
-            value (dict): The value to update the cache with.
-            full (bool): Whether the given value is the full dict, or just a
-                partial subset there of. If not full then any existing entries
-                for the key will be updated.
-            known_absent (set): Set of keys that we know don't exist in the full
-                dict.
+            key (K)
+            value (dict[X,Y]): The value to update the cache with.
+            fetched_keys (None|set[X]): All of the dictionary keys which were
+                fetched from the database.
+
+                If None, this is the complete value for key K. Otherwise, it
+                is used to infer a list of keys which we know don't exist in
+                the full dict.
         """
         self.check_thread()
         if self.sequence == sequence:
             # Only update the cache if the caches sequence number matches the
             # number that the cache had before the SELECT was started (SYN-369)
-            if known_absent is None:
-                known_absent = set()
-            if full:
-                self._insert(key, value, known_absent)
+            if fetched_keys is None:
+                self._insert(key, value, set())
             else:
-                self._update_or_insert(key, value, known_absent)
+                self._update_or_insert(key, value, fetched_keys)
 
     def _update_or_insert(self, key, value, known_absent):
         # We pop and reinsert as we need to tell the cache the size may have