Fix race with _disconnect_monitor event in BlueZ (#999)

If the device disconnects right away for any reason, the
on_connected_changed callback fires and clears the
disconnect_monitor_event which results in the following
exception:

```
  File "/usr/local/lib/python3.10/site-packages/bleak/backends/bluezdbus/client.py", line 182, in connect
    asyncio.ensure_future(self._disconnect_monitor())
  File "/usr/local/lib/python3.10/asyncio/tasks.py", line 615, in ensure_future
    return _ensure_future(coro_or_future, loop=loop)
  File "/usr/local/lib/python3.10/asyncio/tasks.py", line 636, in _ensure_future
    return loop.create_task(coro_or_future)
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/site-packages/bleak/backends/bluezdbus/client.py", line 240, in _disconnect_monitor
    await self._disconnect_monitor_event.wait()
AttributeError: NoneType object has no attribute wait
```

CHANGELOG.rst

@@ -35,6 +35,7 @@ Fixed
 * Fixed inconsistent return types for ``properties`` and ``descriptors`` properties of ``BleakGATTCharacteristic``.
 * Handle device being removed before GetManagedObjects returns in BlueZ backend. Fixes #996.
 * Fixed crash in ``max_pdu_size_changed_handler`` in WinRT backend. Fixes #998.
+* Fixes a race in the BlueZ D-Bus backend where the disconnect monitor would be removed before it could be awaited.
 
 Removed
 -------

bleak/backends/bluezdbus/client.py

@@ -179,7 +179,11 @@ def on_value_changed(char_path: str, value: bytes) -> None:
 
                 # Create a task that runs until the device is disconnected.
                 self._disconnect_monitor_event = asyncio.Event()
-                asyncio.ensure_future(self._disconnect_monitor())
+                asyncio.ensure_future(
+                    self._disconnect_monitor(
+                        self._bus, self._device_path, self._disconnect_monitor_event
+                    )
+                )
 
                 #
                 # We will try to use the cache if it exists and `dangerous_use_bleak_cache`
@@ -226,7 +230,10 @@ def on_value_changed(char_path: str, value: bytes) -> None:
             self._cleanup_all()
             raise
 
-    async def _disconnect_monitor(self) -> None:
+    @staticmethod
+    async def _disconnect_monitor(
+        bus: MessageBus, device_path: str, disconnect_monitor_event: asyncio.Event
+    ) -> None:
         # This task runs until the device is disconnected. If the task is
         # cancelled, it probably means that the event loop crashed so we
         # try to disconnected the device. Otherwise BlueZ will keep the device
@@ -237,16 +244,16 @@ async def _disconnect_monitor(self) -> None:
         # task will still be running and asyncio complains if a loop with running
         # tasks is stopped.
         try:
-            await self._disconnect_monitor_event.wait()
+            await disconnect_monitor_event.wait()
         except asyncio.CancelledError:
             try:
                 # by using send() instead of call(), we ensure that the message
                 # gets sent, but we don't wait for a reply, which could take
                 # over one second while the device disconnects.
-                await self._bus.send(
+                await bus.send(
                     Message(
                         destination=defs.BLUEZ_SERVICE,
-                        path=self._device_path,
+                        path=device_path,
                         interface=defs.DEVICE_INTERFACE,
                         member="Disconnect",
                     )