worker_db: support read-only mode

artiq/language/db.py

@@ -82,7 +82,7 @@ class AutoDB:
                         raise KeyError("Device '{}' not found".format(k))
                     object.__setattr__(self, k, dev)
         self.build()
-        if self.dbh is not None:
+        if self.dbh is not None and self.realtime_results:
             self.dbh.add_rt_results(self.realtime_results)
 
     def __getattr__(self, name):

artiq/master/worker_db.py

@@ -84,13 +84,14 @@ def _create_device(desc, dbh):
 class DBHub:
     """Connects device, parameter and result databases to experiment.
     Handle device driver creation and destruction.
-
     """
-    def __init__(self, ddb, pdb, rdb):
+    def __init__(self, ddb, pdb, rdb, read_only=False):
         self.ddb = ddb
         self.active_devices = OrderedDict()
 
         self.get_parameter = pdb.request
+
+        if not read_only:
             self.set_parameter = pdb.set
             self.add_rt_results = rdb.add_rt_results
             self.get_result = rdb.request