worker: also return DummyDevice from ExamineDeviceMgr

artiq/language/environment.py

@@ -213,8 +213,12 @@ class HasEnvironment:
         Other initialization steps such as requesting devices may also be
         performed here.
 
-        When the repository is scanned, any requested devices and arguments
-        are set to ``None``.
+        There are two situations where the requested devices are replaced by
+        ``DummyDevice()`` and arguments are set to their defaults (or ``None``)
+        instead: when the repository is scanned to build the list of
+        available experiments and when the dataset browser ``artiq_browser``
+        is used to open or run the analysis stage of an experiment. Do not
+        rely on being able to operate on devices or arguments in ``build()``.
 
         Datasets are read-only in this method.
 

artiq/master/worker_impl.py

@@ -11,7 +11,7 @@ import artiq
 from artiq.protocols import pipe_ipc, pyon
 from artiq.protocols.packed_exceptions import raise_packed_exc
 from artiq.tools import multiline_log_config, file_import
-from artiq.master.worker_db import DeviceManager, DatasetManager
+from artiq.master.worker_db import DeviceManager, DatasetManager, DummyDevice
 from artiq.language.environment import (is_experiment, TraceArgumentManager,
                                         ProcessArgumentManager)
 from artiq.language.core import set_watchdog_factory, TerminationRequested
@@ -124,7 +124,7 @@ class ExamineDeviceMgr:
     get_device_db = make_parent_action("get_device_db")
 
     def get(name):
-        return None
+        return DummyDevice()
 
 
 def examine(device_mgr, dataset_mgr, file):