Fix NopReceiver.ready() to properly terminate when receiver is closed. (#427)

The ready() method was not terminating when close() was called, causing
it to hang indefinitely. This fix ensures ready() returns False when the
receiver is closed.

Author: ela-kotulska-frequenz

RELEASE_NOTES.md

@@ -14,4 +14,4 @@
 
 ## Bug Fixes
 
-<!-- Here goes notable bug fixes that are worth a special mention or explanation -->
+- Fix `NopReceiver.ready()` to properly terminate when receiver is closed.

src/frequenz/channels/experimental/_nop_receiver.py

@@ -20,7 +20,9 @@ class NopReceiver(Receiver[ReceiverMessageT_co]):
 
     def __init__(self) -> None:
         """Initialize this instance."""
-        self._closed: bool = False
+        self._ready_future: asyncio.Future[bool] = (
+            asyncio.get_running_loop().create_future()
+        )
 
     @override
     async def ready(self) -> bool:
@@ -29,10 +31,9 @@ async def ready(self) -> bool:
         Returns:
             Whether the receiver is still active.
         """
-        if self._closed:
+        if self._ready_future.done():
             return False
-        await asyncio.Future()
-        return False
+        return await self._ready_future
 
     @override
     def consume(self) -> ReceiverMessageT_co:  # noqa: DOC503 (raised indirectly)
@@ -47,11 +48,12 @@ def consume(self) -> ReceiverMessageT_co:  # noqa: DOC503 (raised indirectly)
             ReceiverStoppedError: If the receiver stopped producing messages.
             ReceiverError: If there is some problem with the underlying receiver.
         """
-        if self._closed:
+        if self._ready_future.done():
             raise ReceiverStoppedError(self)
         raise ReceiverError("`consume()` must be preceded by a call to `ready()`", self)
 
     @override
     def close(self) -> None:
         """Stop the receiver."""
-        self._closed = True
+        if not self._ready_future.done():
+            self._ready_future.set_result(False)

tests/experimental/test_nop_receiver.py

@@ -38,3 +38,28 @@ async def test_consuming_raises() -> None:
     receiver.close()
     with pytest.raises(ReceiverStoppedError):
         receiver.consume()
+
+
+async def test_close_method_effect_on_ready() -> None:
+    """Test `ready()` terminates when the receiver is closed.
+
+    When the receiver is closed, `ready()` should return False.
+    This test verifies that:
+    1. `ready()` returns False immediately after `close()` is called
+    2. `ready()` and `close()` can be called multiple times
+    """
+    receiver = NopReceiver[int]()
+
+    # Create a task that waits for the receiver to be ready.
+    task = asyncio.create_task(receiver.ready())
+
+    # Wait for the task to start.
+    await asyncio.sleep(0.1)
+
+    # Close the receiver and wait for the task to complete.
+    receiver.close()
+    assert await asyncio.wait_for(task, timeout=0.1) is False
+
+    # Second call to `close()` or `ready()` should not raise an error.
+    receiver.close()
+    assert await asyncio.wait_for(receiver.ready(), timeout=0.1) is False