Store a reference to the engine in receiver objects (#478)

This PR fixes a bug which occurs when creating and using a higher order
formula engine, without holding a reference to it.

RELEASE_NOTES.md

@@ -37,3 +37,5 @@ New `Quantity` types! These types can have units (power, current, voltage, etc.)
 - Two bugs in the ring buffer which is used by the `MovingWindow` class were fixed:
   - `len(buffer)` was not considering potentially existing gaps (areas without elements) in the buffer.
   - A off-by-one error in the gap calculation logic was fixed that recorded a gap when there was none if an element with a future timestamp was added that would create a gap of exactly 1.
+
+- A formula engine lifetime issue, when creating higher order formula receivers without holding on to a reference to the engine, was fixed.

src/frequenz/sdk/timeseries/_formula_engine/_formula_engine.py

@@ -360,7 +360,19 @@ def new_receiver(
         if self._task is None:
             self._task = asyncio.create_task(self._run())
 
-        return self._channel.new_receiver(name, max_size)
+        recv = self._channel.new_receiver(name, max_size)
+
+        # This is a hack to ensure that the lifetime of the engine is tied to the
+        # lifetime of the receiver.  This is necessary because the engine is a task that
+        # runs forever, and in cases where higher order built for example with the below
+        # idiom, the user would hold no references to the engine and it could get
+        # garbage collected before the receiver.  This behaviour is explained in the
+        # `asyncio.create_task` docs here:
+        # https://docs.python.org/3/library/asyncio-task.html#asyncio.create_task
+        #
+        #     formula = (grid_power_engine + bat_power_engine).build().new_receiver()
+        recv._engine_reference = self  # type: ignore # pylint: disable=protected-access
+        return recv
 
 
 class FormulaEngine3Phase(