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