fix(api): Fix liquid probes always causing opentrons_simulate to raise MustHomeError

api/src/opentrons/protocol_runner/create_simulating_orchestrator.py

@@ -47,7 +47,11 @@ async def create_simulating_orchestrator(
         robot_type=robot_type
     )
 
-    # TODO(mc, 2021-08-25): move initial home to protocol engine
+    # TODO(mm, 2024-08-06): This home has theoretically been replaced by Protocol Engine
+    # `home` commands within the `RunOrchestrator` or `ProtocolRunner`. However, it turns
+    # out that this `HardwareControlAPI`-level home is accidentally load-bearing,
+    # working around Protocol Engine bugs where *both* layers need to be homed for
+    # certain commands to work. https://opentrons.atlassian.net/browse/EXEC-646
     await simulating_hardware_api.home()
 
     protocol_engine = await create_protocol_engine(

api/src/opentrons/simulate.py

@@ -327,6 +327,12 @@ def get_protocol_api(
     # Intentional difference from execute.get_protocol_api():
     # For the caller's convenience, we home the virtual hardware so they don't get MustHomeErrors.
     # Since this hardware is virtual, there's no harm in commanding this "movement" implicitly.
+    #
+    # Calling `checked_hardware_sync.home()` is a hack. It ought to be redundant with
+    # `context.home()`. We need it here to work around a Protocol Engine simulation bug
+    # where both the `HardwareControlAPI` level and the `ProtocolEngine` level need to
+    # be homed for certain commands to work. https://opentrons.atlassian.net/browse/EXEC-646
+    checked_hardware.sync.home()
     context.home()
 
     return context
@@ -936,6 +942,13 @@ async def run(protocol_source: ProtocolSource) -> _SimulateResult:
             ),
         )
 
+        # TODO(mm, 2024-08-06): This home is theoretically redundant with Protocol
+        # Engine `home` commands within the `RunOrchestrator`. However, we need this to
+        # work around Protocol Engine bugs where both the `HardwareControlAPI` level
+        # and the `ProtocolEngine` level need to be homed for certain commands to work.
+        # https://opentrons.atlassian.net/browse/EXEC-646
+        await hardware_api_wrapped.home()
+
         scraper = _CommandScraper(stack_logger, log_level, protocol_runner.broker)
         with scraper.scrape():
             result = await orchestrator.run(

api/tests/opentrons/test_simulate.py

@@ -296,6 +296,45 @@ def test_get_protocol_api_usable_without_homing(api_version: APIVersion) -> None
     pipette.pick_up_tip(tip_rack["A1"])  # Should not raise.
 
 
+def test_liquid_probe_get_protocol_api() -> None:
+    """Covers `simulate.get_protocol_api()`-specific issues with liquid probes.
+
+    See https://opentrons.atlassian.net/browse/EXEC-646.
+    """
+    protocol = simulate.get_protocol_api(version="2.20", robot_type="Flex")
+    pipette = protocol.load_instrument("flex_1channel_1000", mount="left")
+    tip_rack = protocol.load_labware("opentrons_flex_96_tiprack_1000ul", "A1")
+    well_plate = protocol.load_labware(
+        "opentrons_96_wellplate_200ul_pcr_full_skirt", "A2"
+    )
+    pipette.pick_up_tip(tip_rack["A1"])
+    pipette.require_liquid_presence(well_plate["A1"])  # Should not raise MustHomeError.
+
+
+def test_liquid_probe_simulate_file() -> None:
+    """Covers `opentrons_simulate`-specific issues with liquid probes.
+
+    See https://opentrons.atlassian.net/browse/EXEC-646.
+    """
+    protocol_contents = textwrap.dedent(
+        """\
+        requirements = {"robotType": "Flex", "apiLevel": "2.20"}
+        def run(protocol):
+            pipette = protocol.load_instrument("flex_1channel_1000", mount="left")
+            tip_rack = protocol.load_labware("opentrons_flex_96_tiprack_1000ul", "A1")
+            well_plate = protocol.load_labware(
+                "opentrons_96_wellplate_200ul_pcr_full_skirt", "A2"
+            )
+            pipette.pick_up_tip(tip_rack["A1"])
+            pipette.require_liquid_presence(well_plate["A1"])
+        """
+    )
+    protocol_contents_stream = io.StringIO(protocol_contents)
+    simulate.simulate(
+        protocol_file=protocol_contents_stream
+    )  # Should not raise MustHomeError.
+
+
 class TestGetProtocolAPILabware:
     """Tests for making sure get_protocol_api() handles extra labware correctly."""
 

hardware-testing/hardware_testing/gravimetric/helpers.py

@@ -109,6 +109,11 @@ async def _thread_manager_build_hw_api(
             extra_labware=extra_labware,
             hardware_simulator=ThreadManager(_thread_manager_build_hw_api),
             robot_type="Flex",
+            # use_virtual_hardware=False makes this simulation work unlike
+            # opentrons_simulate, app-side analysis, and server-side analysis.
+            # We need to do this because some of our hardware testing scripts still
+            # interact directly with the OT3API and there is no way to tell Protocol
+            # Engine's hardware virtualization about those updates.
             use_virtual_hardware=False,
         )
     else: