The LoopRunner is accepting an event loop that has been created externally. This is a pattern that has been deprecated by CPython and our current implementation.

Requirements

• LoopRunner no longer accepts event loops but creates and owns them itself.
• There must still be a way to attach multiple servers (e.g. Scheduler and Client) to the same event loop.
• The LoopRunner guarantees that all existing tornado.IOLoop instances are properly closed if the underlying asyncio event loop is stopped when loop is started and stopped without tornado API (e.g. asyncio.run).
• It is not required to implement a deprecation cycle

More context in #6049

Implementation details

# Current interface

class LoopRunner:
    def __init__(self, loop=None, asynchronous=False):
        ...

    def start(self):
        ...
    
    def stop(self, timeout=10):
       ...

    def run_sync(self, func, *args, **kwargs):
        ...

• always bind the Scheduler to the IOLoop.current() - and deprecate the Scheduler(loop= kwarg #6443
• deprecate the io_loop and loop kwarg to Server, Worker, and Nanny #6473
• port test_local.LocalTest to pytest tests to allow use of loop fixture #6523
• Use loop_in_thread fixture for all tests that use local cluster, client, etc. to make sure they are using the correct event loop
• Deprecate passing a stopped loop to the LoopRunner
• Remove deprecated code after one to two release cycle