Close receiver when stopping LatestValueCache (#495)

Author: shsms

RELEASE_NOTES.md

@@ -6,7 +6,8 @@
 
 ## Upgrading
 
-<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
+- `LatestValueCache` now closes the receiver when it is stopped.
+- Fetching values from stopped `LatestValueCache` instances is now disallowed.
 
 ## New Features
 

pyproject.toml

@@ -59,6 +59,7 @@ dev-mkdocs = [
   "mkdocstrings[python] == 0.30.1",
   "mkdocstrings-python == 1.18.2",
   "pymdownx-superfence-filter-lines == 0.1.0",
+  "griffe==1.15.0",
 ]
 dev-mypy = [
   # For checking the noxfile, docs/ script, and tests

src/frequenz/channels/_latest_value_cache.py

@@ -53,6 +53,9 @@ class LatestValueCache(typing.Generic[T_co]):
 
     It provides a way to look up the latest value in a stream without any delay,
     as long as there has been one value received.
+
+    Takes ownership of the receiver.  When the cache is stopped, the receiver
+    will be closed.
     """
 
     def __init__(
@@ -66,12 +69,13 @@ def __init__(
                 provided, a unique identifier will be generated from the object's
                 [`id()`][id]. It is used mostly for debugging purposes.
         """
-        self._receiver = receiver
+        self._receiver: Receiver[T_co] = receiver
         self._unique_id: str = hex(id(self)) if unique_id is None else unique_id
         self._latest_value: T_co | _Sentinel = _Sentinel()
-        self._task = asyncio.create_task(
+        self._task: asyncio.Task[None] = asyncio.create_task(
             self._run(), name=f"LatestValueCache«{self._unique_id}»"
         )
+        self._stopped: bool = False
 
     @property
     def unique_id(self) -> str:
@@ -93,6 +97,8 @@ def get(self) -> T_co:
         """
         if isinstance(self._latest_value, _Sentinel):
             raise ValueError("No value has been received yet.")
+        if self._stopped:
+            raise ValueError("Cache has been stopped.")
         return self._latest_value
 
     def has_value(self) -> bool:
@@ -108,7 +114,9 @@ async def _run(self) -> None:
             self._latest_value = value
 
     async def stop(self) -> None:
-        """Stop the cache."""
+        """Stop the cache and close the owned receiver."""
+        self._receiver.close()
+        self._stopped = True
         if not self._task.done():
             self._task.cancel()
             try: