file_db: support default data if file not present

artiq/protocols/file_db.py

@@ -5,9 +5,16 @@ from artiq.protocols.sync_struct import Notifier
 
 
 class FlatFileDB:
-    def __init__(self, filename):
+    def __init__(self, filename, default_data=None):
         self.filename = filename
-        self.data = Notifier(pyon.load_file(self.filename))
+        try:
+            data = pyon.load_file(self.filename)
+        except FileNotFoundError:
+            if default_data is None:
+                raise
+            else:
+                data = default_data
+        self.data = Notifier(data)
         self.hooks = []
 
     def save(self):