with DeviceCollector doesn't work in plans, so error rather than hang (…

…#612)

src/ophyd_async/core/_device.py

@@ -12,7 +12,7 @@
 )
 
 from bluesky.protocols import HasName
-from bluesky.run_engine import call_in_bluesky_event_loop
+from bluesky.run_engine import call_in_bluesky_event_loop, in_bluesky_event_loop
 
 from ._utils import DEFAULT_TIMEOUT, NotConnected, wait_for_connection
 
@@ -224,6 +224,11 @@ async def __aexit__(self, type, value, traceback):
         await self._on_exit()
 
     def __exit__(self, type_, value, traceback):
+        if in_bluesky_event_loop():
+            raise RuntimeError(
+                "Cannot use DeviceConnector inside a plan, instead use "
+                "`yield from ophyd_async.plan_stubs.ensure_connected(device)`"
+            )
         self._objects_on_exit = self._caller_locals()
         try:
             fut = call_in_bluesky_event_loop(self._on_exit())

tests/core/test_device_collector.py

@@ -67,6 +67,16 @@ def test_sync_device_connector_run_engine_created_connects(RE):
     assert working_device.connected
 
 
+def test_connecting_in_plan_raises(RE):
+    def bad_plan():
+        yield from bps.null()
+        with DeviceCollector():
+            working_device = WorkingDevice("somename")  # noqa: F841
+
+    with pytest.raises(RuntimeError, match="Cannot use DeviceConnector inside a plan"):
+        RE(bad_plan())
+
+
 def test_async_device_connector_run_engine_same_event_loop():
     async def set_up_device():
         async with DeviceCollector(mock=True):