bleak: fix leaking of ensure_future()

As noted in the Python docs, a reference to the return value of
`asyncio.ensure_future()` must be held in order to prevent it from
being garbage collected before the task completes.

This applies the recommended fix from the docs of holding the reference
in a global set and then discarding the reference when the task
completes.

Also change `asyncio.ensure_future()` to `asyncio.create_task()` while
we are touching this since the minimum supported Python version is now
3.7.

Fixes: #1258

CHANGELOG.rst

@@ -10,6 +10,12 @@ and this project adheres to `Semantic Versioning <https://semver.org/spec/v2.0.0
 `Unreleased`_
 =============
 
+Fixed
+-----
+* Fixed possible garbage collection of running async callback from ``BleakClient.start_notify()``.
+* Fixed possible garbage collection of running async callback from ``BleakScanner(detection_callback=)``.
+* Fixed possible garbage collection of disconnect monitor in BlueZ backend. Fixed #1258.
+
 `0.20.0`_ (2023-03-17)
 ======================
 

bleak/__init__.py

@@ -22,6 +22,7 @@
     Iterable,
     List,
     Optional,
+    Set,
     Tuple,
     Type,
     Union,
@@ -70,6 +71,10 @@
     _logger.setLevel(logging.DEBUG)
 
 
+# prevent tasks from being garbage collected
+_background_tasks: Set[asyncio.Task] = set()
+
+
 class BleakScanner:
     """
     Interface for Bleak Bluetooth LE Scanners.
@@ -702,7 +707,9 @@ def callback(sender: BleakGATTCharacteristic, data: bytearray):
         if inspect.iscoroutinefunction(callback):
 
             def wrapped_callback(data):
-                asyncio.ensure_future(callback(characteristic, data))
+                task = asyncio.create_task(callback(characteristic, data))
+                _background_tasks.add(task)
+                task.add_done_callback(_background_tasks.discard)
 
         else:
             wrapped_callback = functools.partial(callback, characteristic)

bleak/backends/bluezdbus/client.py

@@ -35,6 +35,9 @@
 
 logger = logging.getLogger(__name__)
 
+# prevent tasks from being garbage collected
+_background_tasks: Set[asyncio.Task] = set()
+
 
 class BleakClientBlueZDBus(BaseBleakClient):
     """A native Linux Bleak Client
@@ -243,13 +246,15 @@ def on_value_changed(char_path: str, value: bytes) -> None:
                         self._is_connected = True
 
                         # Create a task that runs until the device is disconnected.
-                        asyncio.ensure_future(
+                        task = asyncio.create_task(
                             self._disconnect_monitor(
                                 self._bus,
                                 self._device_path,
                                 local_disconnect_monitor_event,
                             )
                         )
+                        _background_tasks.add(task)
+                        task.add_done_callback(_background_tasks.discard)
 
                         #
                         # We will try to use the cache if it exists and `dangerous_use_bleak_cache`

bleak/backends/scanner.py

@@ -11,13 +11,17 @@
     List,
     NamedTuple,
     Optional,
+    Set,
     Tuple,
     Type,
 )
 
 from ..exc import BleakError
 from .device import BLEDevice
 
+# prevent tasks from being garbage collected
+_background_tasks: Set[asyncio.Task] = set()
+
 
 class AdvertisementData(NamedTuple):
     """
@@ -165,7 +169,9 @@ def register_detection_callback(
         if inspect.iscoroutinefunction(callback):
 
             def detection_callback(s, d):
-                asyncio.ensure_future(callback(s, d))
+                task = asyncio.create_task(callback(s, d))
+                _background_tasks.add(task)
+                task.add_done_callback(_background_tasks.discard)
 
         else:
             detection_callback = callback