Read hardware on every read of the aperture scatterguard (#789)

* Make sure that the aperture scatterguard position is set correctly and add tests

* Add helper signal that is backed by some function

* Use a hardware backed signal for the aperturescatterguard

* Apply suggestions from code review

Co-authored-by: DiamondJoseph <[email protected]>

* Make get_current_aperture_position private

* Fix linting

* Add some documentation pointing to the future cleaner derived signals

---------

Co-authored-by: DiamondJoseph <[email protected]>

docs/reference/device-standards.rst

@@ -21,6 +21,17 @@ should think about where to place them in the following order:
 
 This is in an effort to avoid duplication across facilities/beamlines. 
 
+Device Best Practices
+----------------------------
+
+Ophyd-async directory contains a flowchart_ for a simplified decision tree about what interfaces
+should a given device implement. In addition to this the following guidelines are strongly recommended:
+
+#. Devices should contain only the PV suffixes that are generic for any instance of the device. See `PV Suffixes`_
+#. Anything in a device that is expected to be set externally should be a signal. See `Use of signals`_
+#. Devices should not hold state, when they are read they should read the hardware. See `Holding State`_
+
+
 PV Suffixes
 -----------
 
@@ -86,16 +97,78 @@ Instead you should make a soft signal:
     
     class MyDevice(Device):
         def __init__(self):
-             self.param = create_soft_signal(str, "", self.name)
+             self.param = soft_signal_rw(str)
     
     my_device = MyDevice()
     def my_plan():
         yield from bps.mv(my_device.param, "new_value")
 
-Ophyd Devices best practices
-----------------------------
 
-Ophyd-async directory contains a flowchart_ for a simplified decision tree about what interfaces
-should a given device implement.
+Holding State
+-------------
+
+Devices should avoid holding state as much as possible. Ophyd devices are mostly trying to reflect the state of hardware and so when the device is read that hardware should be read.
+
+If the device holds the state itself it is likely to not reflect the real hardware if:
+* The device has just been initialised
+* The hardware has changed independently e.g. via EPICS directly
+* The hardware has failed to do what the device expected
+
+For example, if I have a device that I would like to treat as moving in/out based on an underlying axis then it would be incorrect to implement it like this:
+
+.. code-block:: python
+
+    class InOut(Enum):
+        IN = 0
+        OUT = 0
+
+    class MyDevice(Device):
+        def __init__(self):
+            self.underlying_motor = Motor("MOTOR")
+            with self.add_children_as_readables():
+                self.in_out, self._in_out_setter = soft_signal_r_and_setter(InOut)
+                
+
+        @AsyncStatus.wrap
+        async def set(self, value: InOut):
+            if value == InOut.IN:
+                await self.underlying_motor.set(100)
+            else:
+                await self.underlying_motor.set(0)
+            self._in_out_setter(value)
+
+While this may appear to work fine during normal operation the state of in_out is only ever updated if the ophyd device is set. It is incorrect to assume that underlying_motor only changes
+based on this and so this has the issues listed above. Instead you should make sure to update in_out whenever the device is read e.g.
+
+.. code-block:: python
+
+    class InOut(Enum):
+        IN = 0
+        OUT = 0
+
+    class MyDevice(Device):
+        def __init__(self):
+            self.underlying_motor = Motor("MOTOR")
+            with self.add_children_as_readables():
+                self.in_out = create_hardware_backed_soft_signal(InOut, self._get_in_out_from_hardware)
+                
+        async def _get_in_out_from_hardware(self):
+            current_position = await self.underlying_motor.get_value()
+            if isclose(current_position, 0):
+                return InOut.IN
+            elif isclose(current_position, 100):
+                return InOut.OUT
+            else:
+                raise ValueError()
+
+
+        @AsyncStatus.wrap
+        async def set(self, value: InOut):
+            if value == InOut.IN:
+                await self.underlying_motor.set(100)
+            else:
+                await self.underlying_motor.set(0)
+
+This will be simplified by https://github.com/bluesky/ophyd-async/issues/525
 
 .. _flowchart: https://blueskyproject.io/ophyd-async/main/how-to/choose-interfaces-for-devices.html

pull_request_template.md

@@ -6,6 +6,6 @@ Fixes #ISSUE
 
 ### Checks for reviewer
 - [ ] Would the PR title make sense to a scientist on a set of release notes
-- [ ] If a new device has been added does it follow the [standards](https://github.com/DiamondLightSource/dodal/wiki/Device-Standards)
+- [ ] If a new device has been added does it follow the [standards](https://diamondlightsource.github.io/dodal/main/reference/device-standards.html)
 - [ ] If changing the API for a pre-existing device, ensure that any beamlines using this device have updated their Bluesky plans accordingly
 - [ ] Have the connection tests for the relevant beamline(s) been run via `dodal connect ${BEAMLINE}`

src/dodal/common/signal_utils.py

@@ -0,0 +1,53 @@
+from collections.abc import Callable, Coroutine
+from typing import Any, TypeVar
+
+from bluesky.protocols import Reading
+from ophyd_async.core import SignalR, SoftSignalBackend
+from ophyd_async.core._soft_signal_backend import SignalMetadata
+
+T = TypeVar("T")
+
+
+class HarwareBackedSoftSignalBackend(SoftSignalBackend[T]):
+    def __init__(
+        self,
+        get_from_hardware_func: Callable[[], Coroutine[Any, Any, T]],
+        *args,
+        **kwargs,
+    ) -> None:
+        self.get_from_hardware_func = get_from_hardware_func
+        super().__init__(*args, **kwargs)
+
+    async def _update_value(self):
+        new_value = await self.get_from_hardware_func()
+        await self.put(new_value)
+
+    async def get_reading(self) -> Reading:
+        await self._update_value()
+        return await super().get_reading()
+
+    async def get_value(self) -> T:
+        await self._update_value()
+        return await super().get_value()
+
+
+def create_hardware_backed_soft_signal(
+    datatype: type[T],
+    get_from_hardware_func: Callable[[], Coroutine[Any, Any, T]],
+    units: str | None = None,
+    precision: int | None = None,
+):
+    """Creates a soft signal that, when read will call the function passed into
+    `get_from_hardware_func` and return this.
+
+    This will allow you to make soft signals derived from arbitrary hardware signals.
+    However, calling subscribe on this signal does not give you a sensible value and
+    the signal is currently read only. See https://github.com/bluesky/ophyd-async/issues/525
+    for a more full solution.
+    """
+    metadata = SignalMetadata(units=units, precision=precision)
+    return SignalR(
+        backend=HarwareBackedSoftSignalBackend(
+            get_from_hardware_func, datatype, metadata=metadata
+        )
+    )

src/dodal/devices/aperturescatterguard.py

@@ -8,11 +8,11 @@
     AsyncStatus,
     HintedSignal,
     StandardReadable,
-    soft_signal_rw,
 )
 from pydantic import BaseModel, Field
 
 from dodal.common.beamlines.beamline_parameters import GDABeamlineParameters
+from dodal.common.signal_utils import create_hardware_backed_soft_signal
 from dodal.devices.aperture import Aperture
 from dodal.devices.scatterguard import Scatterguard
 
@@ -105,7 +105,9 @@ def __init__(
     ) -> None:
         self.aperture = Aperture(prefix + "-MO-MAPT-01:")
         self.scatterguard = Scatterguard(prefix + "-MO-SCAT-01:")
-        self.radius = soft_signal_rw(float, units="µm")
+        self.radius = create_hardware_backed_soft_signal(
+            float, self._get_current_radius, units="µm"
+        )
         self._loaded_positions = loaded_positions
         self._tolerances = tolerances
         self.add_readables(
@@ -119,7 +121,9 @@ def __init__(
             ],
         )
         with self.add_children_as_readables(HintedSignal):
-            self.selected_aperture = soft_signal_rw(ApertureValue)
+            self.selected_aperture = create_hardware_backed_soft_signal(
+                ApertureValue, self._get_current_aperture_position
+            )
 
         super().__init__(name)
 
@@ -136,9 +140,6 @@ async def set(self, value: ApertureValue):
     @AsyncStatus.wrap
     async def _set_raw_unsafe(self, position: AperturePosition):
         """Accept the risks and move in an unsafe way. Collisions possible."""
-        if position.radius is not None:
-            await self.radius.set(position.radius)
-
         aperture_x, aperture_y, aperture_z, scatterguard_x, scatterguard_y = (
             position.values
         )
@@ -150,13 +151,8 @@ async def _set_raw_unsafe(self, position: AperturePosition):
             self.scatterguard.x.set(scatterguard_x),
             self.scatterguard.y.set(scatterguard_y),
         )
-        try:
-            value = await self.get_current_aperture_position()
-            self.selected_aperture.set(value)
-        except InvalidApertureMove:
-            self.selected_aperture.set(None)  # type: ignore
 
-    async def get_current_aperture_position(self) -> ApertureValue:
+    async def _get_current_aperture_position(self) -> ApertureValue:
         """
         Returns the current aperture position using readback values
         for SMALL, MEDIUM, LARGE. ROBOT_LOAD position defined when
@@ -176,6 +172,10 @@ async def get_current_aperture_position(self) -> ApertureValue:
 
         raise InvalidApertureMove("Current aperture/scatterguard state unrecognised")
 
+    async def _get_current_radius(self) -> float | None:
+        current_value = await self._get_current_aperture_position()
+        return self._loaded_positions[current_value].radius
+
     async def _safe_move_within_datacollection_range(
         self, position: AperturePosition, value: ApertureValue
     ):
@@ -203,8 +203,6 @@ async def _safe_move_within_datacollection_range(
             )
 
         current_ap_y = await self.aperture.y.user_readback.get_value()
-        if position.radius is not None:
-            await self.radius.set(position.radius)
 
         aperture_x, aperture_y, aperture_z, scatterguard_x, scatterguard_y = (
             position.values
@@ -231,4 +229,3 @@ async def _safe_move_within_datacollection_range(
                 self.scatterguard.x.set(scatterguard_x),
                 self.scatterguard.y.set(scatterguard_y),
             )
-        await self.selected_aperture.set(value)

tests/devices/unit_tests/test_aperture_scatterguard.py

@@ -8,6 +8,7 @@
 from bluesky.run_engine import RunEngine
 from ophyd_async.core import (
     DeviceCollector,
+    callback_on_mock_put,
     get_mock_put,
     set_mock_value,
 )
@@ -115,6 +116,7 @@ async def aperture_in_medium_pos_w_call_log(
     await ap_sg._set_raw_unsafe(aperture_positions[ApertureValue.MEDIUM])
 
     set_mock_value(ap_sg.aperture.medium, 1)
+
     yield ap_sg, call_log
 
 
@@ -144,9 +146,6 @@ def _assert_position_in_reading(
     position: AperturePosition,
     device_name: str,
 ):
-    if position.radius is not None:
-        assert reading[f"{device_name}-radius"]["value"] == position.radius
-    assert reading[f"{device_name}-selected_aperture"]["value"] == aperture
     assert reading[f"{device_name}-aperture-x"]["value"] == position.aperture_x
     assert reading[f"{device_name}-aperture-y"]["value"] == position.aperture_y
     assert reading[f"{device_name}-aperture-z"]["value"] == position.aperture_z
@@ -244,19 +243,27 @@ def set_underlying_motors(ap_sg: ApertureScatterguard, position: AperturePositio
         motor.set(pos)
 
 
-async def test_aperture_positions_large(ap_sg: ApertureScatterguard):
-    set_mock_value(ap_sg.aperture.large, 1)
-    assert await ap_sg.get_current_aperture_position() == ApertureValue.LARGE
-
-
-async def test_aperture_positions_medium(ap_sg: ApertureScatterguard):
-    set_mock_value(ap_sg.aperture.medium, 1)
-    assert await ap_sg.get_current_aperture_position() == ApertureValue.MEDIUM
-
-
-async def test_aperture_positions_small(ap_sg: ApertureScatterguard):
-    set_mock_value(ap_sg.aperture.small, 1)
-    assert await ap_sg.get_current_aperture_position() == ApertureValue.SMALL
+@pytest.mark.parametrize(
+    "read_pv, aperture",
+    [
+        ("large", ApertureValue.LARGE),
+        ("medium", ApertureValue.MEDIUM),
+        ("small", ApertureValue.SMALL),
+    ],
+)
+async def test_aperture_positions(
+    ap_sg: ApertureScatterguard,
+    aperture_positions: dict[ApertureValue, AperturePosition],
+    read_pv: str,
+    aperture: ApertureValue,
+):
+    set_mock_value(getattr(ap_sg.aperture, read_pv), 1)
+    reading = await ap_sg.read()
+    assert isinstance(reading, dict)
+    assert (
+        reading[f"{ap_sg.name}-radius"]["value"] == aperture_positions[aperture].radius
+    )
+    assert reading[f"{ap_sg.name}-selected_aperture"]["value"] == aperture
 
 
 async def test_aperture_positions_robot_load(
@@ -268,7 +275,12 @@ async def test_aperture_positions_robot_load(
     set_mock_value(ap_sg.aperture.small, 0)
     robot_load = aperture_positions[ApertureValue.ROBOT_LOAD]
     await ap_sg.aperture.y.set(robot_load.aperture_y)
-    assert await ap_sg.get_current_aperture_position() == ApertureValue.ROBOT_LOAD
+    reading = await ap_sg.read()
+    assert isinstance(reading, dict)
+    assert reading[f"{ap_sg.name}-radius"]["value"] == 0.0
+    assert (
+        reading[f"{ap_sg.name}-selected_aperture"]["value"] == ApertureValue.ROBOT_LOAD
+    )
 
 
 async def test_aperture_positions_robot_load_within_tolerance(
@@ -282,7 +294,12 @@ async def test_aperture_positions_robot_load_within_tolerance(
     set_mock_value(ap_sg.aperture.medium, 0)
     set_mock_value(ap_sg.aperture.small, 0)
     await ap_sg.aperture.y.set(robot_load_ap_y + tolerance)
-    assert await ap_sg.get_current_aperture_position() == ApertureValue.ROBOT_LOAD
+    reading = await ap_sg.read()
+    assert isinstance(reading, dict)
+    assert reading[f"{ap_sg.name}-radius"]["value"] == 0.0
+    assert (
+        reading[f"{ap_sg.name}-selected_aperture"]["value"] == ApertureValue.ROBOT_LOAD
+    )
 
 
 async def test_aperture_positions_robot_load_outside_tolerance(
@@ -297,7 +314,7 @@ async def test_aperture_positions_robot_load_outside_tolerance(
     set_mock_value(ap_sg.aperture.small, 0)
     await ap_sg.aperture.y.set(robot_load_ap_y + tolerance)
     with pytest.raises(InvalidApertureMove):
-        await ap_sg.get_current_aperture_position()
+        await ap_sg.read()
 
 
 async def test_aperture_positions_robot_load_unsafe(
@@ -308,7 +325,7 @@ async def test_aperture_positions_robot_load_unsafe(
     set_mock_value(ap_sg.aperture.small, 0)
     await ap_sg.aperture.y.set(50.0)
     with pytest.raises(InvalidApertureMove):
-        await ap_sg.get_current_aperture_position()
+        await ap_sg.read()
 
 
 @pytest.mark.skip(
@@ -360,6 +377,14 @@ async def test_ap_sg_in_runengine(
     ap = aperture_in_medium_pos.aperture
     sg = aperture_in_medium_pos.scatterguard
     test_loc = aperture_positions[ApertureValue.SMALL]
+
+    def set_small_readback_pv(value, *args, **kwargs):
+        set_mock_value(ap.small, 1)
+        set_mock_value(ap.medium, 0)
+        set_mock_value(ap.y.user_readback, value)
+
+    callback_on_mock_put(ap.y.user_setpoint, set_small_readback_pv)
+
     RE(bps.abs_set(aperture_in_medium_pos, ApertureValue.SMALL, wait=True))
     assert await ap.x.user_readback.get_value() == test_loc.aperture_x
     assert await ap.y.user_readback.get_value() == test_loc.aperture_y