Unable to move back from storage to mirror screen when certain conditions are met

[Olivier Gayot]

when testing semi offline installs, I ended up triggering a bug which prevents the user from going back from the Storage screen.

The order of the screens is "... -> Mirror -> Refresh -> Storage -> ...".

After moving from Mirror to Storage, hitting Back moves us the Refresh page again ; which in certain circumstances does not display a UI and instead moves forward to the Storage screen automatically.

https://asciinema.org/a/gF5VuZxVdakZrQ72hmyvRj48P

Steps to reproduce:

* Do a fully offline install
* If the Refresh screen shows a "Contacting the snap store..." animation, let it timeout.
* Then press back when on the Storage screen
* Let the Refresh screen timeout again
* Then press back ... This time it looks like nothing happened, because the refresh screen moved us forward again.

This is affecting https://github.com/canonical/subiquity/pull/1968

[Olivier Gayot (ogayot)]

Presumably, this is the code that automatically moves forward when status.availability = RefreshCheckState.UNKNOWN:

https://github.com/canonical/subiquity/blob/979f546f27fb8cfbfe9d0ac4f763b86b49718d41/subiquity/ui/views/refresh.py#L165

[Olivier Gayot (ogayot)]

The right way to "skip" a screen is to raise the Skip exception from make_ui. That said we have no way to make a distinction between {"availability": "UNKNOWN"} as in "We are still checking" and {"availability": "UNKNOWN"} as in "We checked and we failed to find out" when we call /refresh with wait=False.

In the latter scenario, we need a call to /refresh?wait=True, which we can't really do as part of the make_ui call.