to_device messages aimed at a local device are not removed after that device is deleted

[anoadragon453]

When a user's device is deleted - either manually, when they log out or when their account is deactivated - to_device messages aimed at that device are not deleted, and end up sitting in the database forever.

synapse/synapse/handlers/device.py

Lines 343 to 417 in a78016d

	async def delete_device(self, user_id: str, device_id: str) -> None:
	""" Delete the given device
	Args:
	user_id: The user to delete the device from.
	device_id: The device to delete.
	"""
	try:
	await self.store.delete_device(user_id, device_id)
	except errors.StoreError as e:
	if e.code == 404:
	# no match
	set_tag("error", True)
	log_kv(
	{"reason": "User doesn't have device id.", "device_id": device_id}
	)
	pass
	else:
	raise
	await self._auth_handler.delete_access_tokens_for_user(
	user_id, device_id=device_id
	)
	await self.store.delete_e2e_keys_by_device(user_id=user_id, device_id=device_id)
	await self.notify_device_update(user_id, [device_id])
	@trace
	async def delete_all_devices_for_user(
	self, user_id: str, except_device_id: Optional[str] = None
	) -> None:
	"""Delete all of the user's devices
	Args:
	user_id: The user to remove all devices from
	except_device_id: optional device id which should not be deleted
	"""
	device_map = await self.store.get_devices_by_user(user_id)
	device_ids = list(device_map)
	if except_device_id is not None:
	device_ids = [d for d in device_ids if d != except_device_id]
	await self.delete_devices(user_id, device_ids)
	async def delete_devices(self, user_id: str, device_ids: List[str]) -> None:
	""" Delete several devices
	Args:
	user_id: The user to delete devices from.
	device_ids: The list of device IDs to delete
	"""
	try:
	await self.store.delete_devices(user_id, device_ids)
	except errors.StoreError as e:
	if e.code == 404:
	# no match
	set_tag("error", True)
	set_tag("reason", "User doesn't have that device id.")
	pass
	else:
	raise
	# Delete access tokens and e2e keys for each device. Not optimised as it is not
	# considered as part of a critical path.
	for device_id in device_ids:
	await self._auth_handler.delete_access_tokens_for_user(
	user_id, device_id=device_id
	)
	await self.store.delete_e2e_keys_by_device(
	user_id=user_id, device_id=device_id
	)
	await self.notify_device_update(user_id, device_ids)

There's no point in keeping them around, and should get rid of them when a device goes pop.

Related: #3599

[anoadragon453]

After deleting these manually I found that ~48% of the device_inbox table was taken up by messages to devices that no longer existed.