Allow standalone system simulation component

This involves changing the SystemSimulation input parameters, and making
it so that the slave scheduler runs an 'initial' tick in the same way as
master.

Author: Rose Yemelyanova

examples/configs/standalone-system-sim-counter.yaml

@@ -0,0 +1,11 @@
+- tickit.core.components.system_simulation.SystemSimulation:
+    name: internal_tickit
+    inputs: {}
+    components:
+      - examples.devices.counter.Counter:
+          name: counter  
+          inputs: {}
+      - tickit.devices.sink.Sink:
+          name: counter_sink
+          inputs:
+            input: counter:value

examples/configs/standalone-system-sim.yaml

@@ -0,0 +1,12 @@
+- tickit.core.components.system_simulation.SystemSimulation:
+    name: internal_tickit
+    inputs: {}
+    components:
+      - tickit.devices.source.Source:
+          name: internal_source
+          inputs: {}
+          value: 42
+      - tickit.devices.sink.Sink:
+          name: internal_sink
+          inputs:
+            input: internal_source:value

examples/configs/test.yaml

@@ -0,0 +1,8 @@
+- tickit.devices.source.Source:
+    name: internal_source
+    inputs: {}
+    value: 42
+- tickit.devices.sink.Sink:
+    name: internal_sink
+    inputs:
+      input: internal_source:value

src/tickit/core/components/system_simulation.py

@@ -102,10 +102,8 @@ async def stop_component(self) -> None:
 class SystemSimulation(ComponentConfig):
     """Simulation of a nested set of components."""
 
-    name: ComponentID
-    inputs: Dict[PortID, ComponentPort]
     components: List[ComponentConfig]
-    expose: Dict[PortID, ComponentPort]
+    expose: Dict[PortID, ComponentPort] = field(default_factory=dict)
 
     def __call__(self) -> Component:  # noqa: D102
         return SystemSimulationComponent(

src/tickit/core/management/schedulers/slave.py

@@ -1,3 +1,4 @@
+import asyncio
 import logging
 from typing import Awaitable, Callable, Dict, Optional, Set, Tuple, Type, Union
 
@@ -48,10 +49,22 @@ def __init__(
         wiring = self.add_exposing_wiring(wiring, expose)
         super().__init__(wiring, state_consumer, state_producer)
 
+        reads_from_external: bool = False
+        for component in self._wiring:
+            input_components = [
+                p.component for p in self._wiring[component].values()  # type: ignore
+            ]
+            if "external" in input_components:
+                reads_from_external = True
+                break
+
+        self.reads_from_external = reads_from_external
         self.raise_interrupt = raise_interrupt
         self.interrupts: Set[ComponentID] = set()
         self.component_error: ComponentException
 
+        self.first_tick: asyncio.Event = asyncio.Event()
+
     @staticmethod
     def add_exposing_wiring(
         wiring: Union[Wiring, InverseWiring],
@@ -127,17 +140,20 @@ async def on_tick(
         wakeup_components = {
             component for component, when in self.wakeups.items() if when <= time
         }
-        root_components: Set[ComponentID] = {
-            *self.interrupts,
-            *wakeup_components,
-            ComponentID("external"),
-        }
+
+        root_components: Set[ComponentID] = {*self.interrupts, *wakeup_components}
+
+        if self.reads_from_external:
+            root_components.update([ComponentID("external")])
+
         for component in wakeup_components:
             del self.wakeups[component]
         self.interrupts.clear()
 
         self.input_changes = changes
         self.output_changes = Changes(Map())
+
+        await asyncio.wait_for(self.first_tick.wait(), timeout=30)
         await self.ticker(time, root_components)
 
         _, call_at = self.get_first_wakeups()
@@ -146,6 +162,11 @@ async def on_tick(
     async def run_forever(self) -> None:
         """Delegates to setup which instantiates the ticker and state interfaces."""
         await self.setup()
+        await self.ticker(
+            SimTime(0),
+            self.ticker.components,
+        )
+        self.first_tick.set()
 
     async def schedule_interrupt(self, source: ComponentID) -> None:
         """Schedules the interrupt of a component immediately.

tests/core/management/schedulers/test_slave_scheduler.py

@@ -179,6 +179,8 @@ async def test_slave_scheduler_run_forever_method(slave_scheduler: SlaveSchedule
 async def test_slave_scheduler_on_tick_method(
     slave_scheduler: SlaveScheduler, mock_ticker: Mock
 ):
+    await slave_scheduler.run_forever()
+
     changes = Changes(Map({PortID("67"): 67}))
     slave_scheduler.ticker = mock_ticker
     output_changes, call_at = await slave_scheduler.on_tick(SimTime(8), changes)
@@ -190,6 +192,7 @@ async def test_slave_scheduler_on_tick_method(
 async def test_slave_scheduler_on_tick_method_with_wakeups(
     slave_scheduler: SlaveScheduler, mock_ticker: Mock
 ):
+    await slave_scheduler.run_forever()
     changes = Changes(Map({PortID("67"): 67}))
     slave_scheduler.ticker = mock_ticker
     slave_scheduler.wakeups = {