1 files changed, 31 insertions, 2 deletions
diff --git a/synapse/state.py b/synapse/state.py
index cd792afed1..4520fa0415 100644
--- a/synapse/state.py
+++ b/synapse/state.py
@@ -55,12 +55,15 @@ def _gen_state_id():
 
 
 class _StateCacheEntry(object):
-    __slots__ = ["state", "state_group", "state_id"]
+    __slots__ = ["state", "state_group", "state_id", "prev_group", "delta_ids"]
 
-    def __init__(self, state, state_group):
+    def __init__(self, state, state_group, prev_group=None, delta_ids=None):
         self.state = state
         self.state_group = state_group
 
+        self.prev_group = prev_group
+        self.delta_ids = delta_ids
+
         # The `state_id` is a unique ID we generate that can be used as ID for
         # this collection of state. Usually this would be the same as the
         # state group, but on worker instances we can't generate a new state
@@ -245,11 +248,20 @@ class StateHandler(object):
             if key in context.prev_state_ids:
                 replaces = context.prev_state_ids[key]
                 event.unsigned["replaces_state"] = replaces
+
             context.current_state_ids = dict(context.prev_state_ids)
             context.current_state_ids[key] = event.event_id
+
+            context.prev_group = entry.prev_group
+            context.delta_ids = entry.delta_ids
+            if context.delta_ids is not None:
+                context.delta_ids[key] = event.event_id
         else:
             context.current_state_ids = context.prev_state_ids
 
+            context.prev_group = entry.prev_group
+            context.delta_ids = entry.delta_ids
+
         context.prev_state_events = []
         defer.returnValue(context)
 
@@ -283,6 +295,8 @@ class StateHandler(object):
             defer.returnValue(_StateCacheEntry(
                 state=state_list,
                 state_group=name,
+                prev_group=name,
+                delta_ids={},
             ))
 
         with (yield self.resolve_linearizer.queue(group_names)):
@@ -340,9 +354,24 @@ class StateHandler(object):
                 if hasattr(self.store, "get_next_state_group"):
                     state_group = self.store.get_next_state_group()
 
+            prev_group = None
+            delta_ids = None
+            for old_group, old_ids in state_groups_ids.items():
+                if not set(new_state.iterkeys()) - set(old_ids.iterkeys()):
+                    n_delta_ids = {
+                        k: v
+                        for k, v in new_state.items()
+                        if old_ids.get(k) != v
+                    }
+                    if not delta_ids or len(n_delta_ids) < len(delta_ids):
+                        prev_group = old_group
+                        delta_ids = n_delta_ids
+
             cache = _StateCacheEntry(
                 state=new_state,
                 state_group=state_group,
+                prev_group=prev_group,
+                delta_ids=delta_ids,
             )
 
             if self._state_cache is not None: