diff --git a/synapse/storage/state.py b/synapse/storage/state.py
index 3f4cbd61c4..b88c7dc091 100644
--- a/synapse/storage/state.py
+++ b/synapse/storage/state.py
@@ -1041,6 +1041,56 @@ class StateGroupWorkerStore(EventsWorkerStore, SQLBaseStore):
return count
+ def _is_state_group_referenced(self, txn, state_group):
+ """Checks if a given state group is referenced, or is safe to delete.
+
+ A state groups is referenced if it or any of its descendants are
+ pointed at by an event. (A descendant is a group which has the given
+ state_group as a prev group)
+ """
+
+ # We check this by doing a depth first search to look for any
+ # descendant referenced by `event_to_state_groups`.
+
+ # State groups we need to check, contains state groups that are
+ # descendants of `state_group`
+ state_groups_to_search = [state_group]
+
+ # Set of state groups we've already checked
+ state_groups_searched = set()
+
+ while state_groups_to_search:
+ state_group = state_groups_to_search.pop() # Next state group to check
+
+ is_referenced = self._simple_select_one_onecol_txn(
+ txn,
+ table="event_to_state_groups",
+ keyvalues={"state_group": state_group},
+ retcol="event_id",
+ allow_none=True,
+ )
+ if is_referenced:
+ # A descendant is referenced by event_to_state_groups, so
+ # original state group is referenced.
+ return True
+
+ state_groups_searched.add(state_group)
+
+ # Find all children of current state group and add to search
+ references = self._simple_select_onecol_txn(
+ txn,
+ table="state_group_edges",
+ keyvalues={"prev_state_group": state_group},
+ retcol="state_group",
+ )
+ state_groups_to_search.extend(references)
+
+ # Lets be paranoid and check for cycles
+ if state_groups_searched.intersection(references):
+ raise Exception("State group %s has cyclic dependency", state_group)
+
+ return False
+
class StateStore(StateGroupWorkerStore, BackgroundUpdateStore):
""" Keeps track of the state at a given event.
|
