handle a subtle iteration issue in the cache with extra locking

dbt/adapters/cache.py

@@ -184,13 +184,14 @@ def dump_graph(self):
  known relation is a key with a value of a list of keys it is referenced
  by.
  """
- # we have to hold the lock while iterating, if other threads modify
- # self.relations during iteration it's a runtime error
+ # we have to hold the lock for the entire dump, if other threads modify
+ # self.relations or any cache entry's referenced_by during iteration
+ # it's a runtime error!
  with self.lock:
- # consume the full iterator inside the lock
- items = list(self.relations.items())
-
- return {dot_separated(k): v.dump_graph_entry() for k, v in items}
+ return {
+  dot_separated(k): v.dump_graph_entry()
+ for k, v in self.relations.items()
+  }
 
  def _setdefault(self, relation):
  """Add a relation to the cache, or return it if it already exists.