Ensure we don't use unpersisted state group as prev group

synapse/storage/state.py

@@ -223,6 +223,19 @@ class StateStore(SQLBaseStore):
             # We persist as a delta if we can, while also ensuring the chain
             # of deltas isn't tooo long, as otherwise read performance degrades.
             if context.prev_group:
+                is_in_db = self._simple_select_one_onecol_txn(
+                    txn,
+                    table="state_groups",
+                    keyvalues={"id": context.prev_group},
+                    retcol="id",
+                    allow_none=True,
+                )
+                if not is_in_db:
+                    raise Exception(
+                        "Trying to persist state with unpersisted prev_group: %r"
+                        % (context.prev_group,)
+                    )
+
                 potential_hops = self._count_state_group_hops_txn(
                     txn, context.prev_group
                 )