bluez: Create a bluez manager instance per-event-loop

Avoids "got Future <Future pending> attached to a different loop" if Bleak
instances are constructed in context of more than one event loop.

CHANGELOG.rst

@@ -14,6 +14,11 @@ Added
 -----
 * Added support for Python 3.11.
 
+Fixed
+-----
+* On BlueZ, support creating additional instances running on a different event
+  loops (i.e. multiple pytest-asyncio cases)
+
 `0.18.1`_ (2022-09-25)
 ======================
 

bleak/backends/bluezdbus/manager.py

@@ -21,6 +21,7 @@
     Set,
     cast,
 )
+from weakref import WeakKeyDictionary
 
 from dbus_fast import BusType, Message, MessageType, Variant
 from dbus_fast.aio.message_bus import MessageBus
@@ -843,15 +844,20 @@ def _run_advertisement_callbacks(
             callback(device_path, device.copy())
 
 
+_global_instances: Dict[Any, BlueZManager] = WeakKeyDictionary()
+
+
 async def get_global_bluez_manager() -> BlueZManager:
     """
-    Gets the initialized global BlueZ manager instance.
+    Gets an existing initialized global BlueZ manager instance associated with the current event loop,
+    or initializes a new instance.
     """
 
-    if not hasattr(get_global_bluez_manager, "instance"):
-        setattr(get_global_bluez_manager, "instance", BlueZManager())
-
-    instance: BlueZManager = getattr(get_global_bluez_manager, "instance")
+    loop = asyncio.get_running_loop()
+    try:
+        instance = _global_instances[loop]
+    except KeyError:
+        instance = _global_instances[loop] = BlueZManager()
 
     await instance.async_init()
 

docs/backends/linux.rst

@@ -3,12 +3,10 @@
 Linux backend
 =============
 
-The Linux backend of Bleak is written using the
-`TxDBus <https://github.com/cocagne/txdbus>`_
-package. It is written for
-`Twisted <https://twistedmatrix.com/trac/>`_, but by using the
-`twisted.internet.asyncioreactor <https://twistedmatrix.com/documents/current/api/twisted.internet.asyncioreactor.html>`_
-one can use it with `asyncio`.
+The Linux backend of Bleak communicates with `BlueZ <http://www.bluez.org/>`_
+over DBus. Communication uses the `dbus-fast
+<https://github.com/Bluetooth-Devices/dbus-fast>`_ package for async access to
+DBus messaging.
 
 
 Special handling for ``write_gatt_char``
@@ -33,6 +31,15 @@ return the cached services without waiting for them to be resolved again. This
 is useful when you know services have not changed, and you want to use the
 services immediately, but don't want to wait for them to be resolved again.
 
+Parallel Access
+---------------
+
+Each Bleak object should be created and used from a single `asyncio event
+loop`_. Simple asyncio programs will only have a single event loop. It's also
+possible to use Bleak with multiple event loops, even at the same time, but
+individual Bleak objects should not be shared between event loops. Otherwise,
+RuntimeErrors similar to ``[...] got Future <Future pending> attached to a
+different loop`` will be thrown.
 
 API
 ---
@@ -48,3 +55,5 @@ Client
 
 .. automodule:: bleak.backends.bluezdbus.client
     :members:
+
+.. _`asyncio event loop`: https://docs.python.org/3/library/asyncio-eventloop.html