It is not possible to determine if a server channel can be applied to a device or not.

[UniversalSuperBox]

The dbus method com.canonical.SystemImage GetChannels returns all channels from the system-image server. This would be acceptable if every device had its own system-image server, but we have many devices and channels on a single server. This causes a huge list of channels to be returned for the call:

qml: CHANNEL IS 16.04,arm64,mainline,rc
qml: CHANNEL IS 16.04,arm64,mainline,stable
qml: CHANNEL IS 16.04,armhf,hybris,devel
qml: CHANNEL IS 16.04,community,anpok,devel
qml: CHANNEL IS 16.04,community,fredldotme,devel
qml: CHANNEL IS 16.04,community,guf,devel
qml: CHANNEL IS 16.04,community,kuailexs,devel
qml: CHANNEL IS 16.04,community,walid,devel
qml: CHANNEL IS ubports-touch,16.04,devel
qml: CHANNEL IS ubports-touch,16.04,edge
qml: CHANNEL IS ubports-touch,16.04,rc
qml: CHANNEL IS ubports-touch,16.04,stable

This is not desirable, as it puts the work on the client to determine if a channel is appropriate for the device or not. This is not possible using any of the data which can be returned by system-image via dbus. Instead, we should only return a list of channels which are applicable for the current device.

The current code for returning this list inside system-image is not terribly smart. It only checks to make sure the channel is not hidden, an alias, or a redirect. If none of those are the case, the channel is fine as far as it's concerned.

system-image/systemimage/dbus.py

Lines 403 to 412 in aabc2fd

	def GetChannels(self):
	"""Get channels from system server."""
	ret = list()
	channels = self._api.get_channels()
	if channels:
	for key in channels:
	if not key["hidden"] and not key["alias"] and not key["redirect"]:
	ret.append(key["name"])
	log.info('Channels {}', ret)
	return ret

The code which it eventually calls back to is in the Mediator:

system-image/systemimage/api.py

Lines 124 to 131 in aabc2fd

	self._channels = list()
	for key in sorted(self._state.channels):
	self._channels.append(dict(
	hidden=self._state.channels[key].get('hidden'),
	alias=self._state.channels[key].get('alias'),
	redirect=self._state.channels[key].get('redirect'),
	name=key
	))

And, through a couple more levels of indirection, the Channels class:

system-image/systemimage/channel.py

Lines 42 to 58 in aabc2fd

	class Channels(Bag):
	@classmethod
	def from_json(cls, data):
	mapping = json.loads(data)
	channels = {}
	for channel_name, mapping_1 in mapping.items():
	hidden = mapping_1.pop('hidden', None)
	if hidden is None:
	hidden = False
	else:
	assert hidden in (True, False), (
	"Unexpected value for 'hidden': {}".format(hidden))
	mapping_1['hidden'] = hidden
	device_mapping = mapping_1.pop('devices')
	mapping_1['devices'] = _parse_device_mappings(device_mapping)
	channels[channel_name] = Bag(**mapping_1)
	return cls(**channels)

We should be able to change GetChannels so it only returns the channels applicable for the current device.