Error on shutdown when prescence is enabled

[DMRobertson]

Spotted when I was reading through sytest logs.

21082-2021-12-06 18:04:19,715 - synapse.metrics.background_process_metrics - 246 - ERROR - generic_presence.on_shutdown-0 - Background process 'generic_presence.on_shutdown' threw an exception
21083-Traceback (most recent call last):
21084-  File "/venv/lib/python3.7/site-packages/synapse/metrics/background_process_metrics.py", line 242, in run
21085-    return await func(*args, **kwargs)
21086-TypeError: object NoneType can't be used in 'await' expression

The problem is that we register a call to run_as_background_process before shutdown here. We pass it the callable self._on_shutdown. The latter is a synchronous function, but run_as_background_process expects an async function.

synapse/synapse/handlers/presence.py

Lines 416 to 428 in 75ca0a6

	hs.get_reactor().addSystemEventTrigger(
	"before",
	"shutdown",
	run_as_background_process,
	"generic_presence.on_shutdown",
	self._on_shutdown,
	)
	def _on_shutdown(self) -> None:
	if self._presence_enabled:
	self.hs.get_tcp_replication().send_command(
	ClearUserSyncsCommand(self.instance_id)
	)

The symptom was introduced in #10847 when we removed maybe_awaitable. But the underlying cause is that _on_shutdown didn't return a Deferred back when we expected it to. (Plus addSystemEventTrigger obscures the call from mypy. Probably extra confusion when this was rewritten from twisted defers to asyncs.)