Add synchronous method to determine if there are known updates

[Ken VanDine]

There are times when auto-checking of updates occurs outside of system-settings, which could add an emblem to the system-settings launcher icon indicating available updates. However, when launching system-settings we have no way of determining if there are already updates that system-image-dbus knows about before calling CheckForUpdates(). This causes a delay in toggling visibility of the updates available entry in system-settings, which shifts the UI around after start. See bug #1355803

We need a synchronous method to call when system-settings starts that returns a boolean if there are known updates, which we can use to determine visibility of the entry at start time.

[Barry Warsaw (barry)]

See also LP: #1355803 for the UX impetus behind this request.

[Barry Warsaw (barry)]

We can probably do this without a new API. We have the synchronous method Information() which returns a mapping. We could add a new key to this mapping called 'target_build_number' (or available_build_number, or new_build_number, or something else). If the value is a number > the current_build_number, you know that si-dbus knows there's an update available. If the value is "none" or 0 (or missing?) then you know that si-dbus doesn't know whether an update is available or not, in which case, you'd have to call CheckForUpdate().

Does this seem reasonable?

[Ken VanDine (ken-vandine)]

That sounds good to me!

[Barry Warsaw (barry)]

<barry> kenvandine: thanks. my big question is what to do in the
        "we-don't-know" case. should the target_build_number key just be
        missing? should it return a non-number like "none", or should it
        return a nonsense number like -1 (or 0?). my initial preference would
        be to omit the key, but i don't know how much trouble the different
        options would cause the client
<kenvandine> barry, i think i'd prefer -1

[Barry Warsaw (barry)]

One other thing: If we know there is no update available, what should the value be? The obvious choices would be 0 or current_build_number. With the latter, you'd just check info['current_build_number'] == info['target_build_number']. If that's true, there's no update available. I think I prefer that.

[Ken VanDine (ken-vandine)]

info['current_build_number'] == info['target_build_number'] is fine with me

[Ken VanDine (ken-vandine)]

This works for me, thanks!