Store a reference to the engine in receiver objects

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.

    formula = (grid_power_engine + bat_power_engine).build().new_receiver()

Signed-off-by: Sahas Subramanian <[email protected]>

Author: shsms

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(