Have a test to check that Tor Browser updater is working
We should have a test to check that the Tor Browser updater is not broken and will be able to update the browser to futur releases when they are available. This test would help when making updater changes such as #17442 (moved).
Mozilla has some marionette tests that we can use for that: https://github.com/mozilla/firefox-ui-tests/blob/mozilla-central/firefox_ui_tests/update/direct/test_direct_update.py https://github.com/mozilla/firefox-ui-tests/blob/mozilla-central/firefox_ui_tests/update/fallback/test_fallback_update.py
Those tests take as arguments a channel name, a target version (the expected version after the update is installed), a target buildid.
Mozilla QA team is posting on their wiki the config file they use when testing each new release: https://wiki.mozilla.org/QA/Desktop_Firefox/Releases/Configs/Fx42RC2
Using this config they are testing that previous releases can be updated to the new release, but I could not find tests to check if the new release has a working updater that will be able to update to a futur release.
To be able to check that the updater can update to an other release, I think we could create a "test-updater" channel, which provides a signed mar file with a large version number, so that the updater always accept this update. However the problem would be that an adversary can use this mar file to update users to an old browser. Maybe we can include a non-working browser in the mar file to avoid this, but this is still not very good.
An alternative could be to have a "test-updater" channel which provides an old version, and patch our updater to have a preference that allows downgrades. We can then set this preference when testing the updater on this channel.