Your installation updates to the latest build, not the highest number

An installation that keeps itself up to date has to answer one question on every check: of everything published, which one should I be running? Until now it answered by comparing version numbers, highest wins. That sounds obviously right, and it worked for a long time — until the day a version number was wrong.

What happened

On 5 September a version number was raised by mistake and reverted a few minutes later. Ten builds had already gone out under the raised number. They were removed from circulation two days later.

In between, two installations checked for updates and found those ten. The number was higher, so they took it — and moved themselves backwards, onto builds three days older than what they had been running, missing work that had shipped since.

Then they got stuck, and this is the part worth explaining. Once an installation is on the highest-numbered build in existence, nothing can ever be higher. Every later check found the same answer: nothing newer than what you are running. That is exactly the sentence a perfectly up-to-date installation prints, so from the outside nothing looked wrong at all. Newer builds were published throughout. None of them could win. Both installations had to be moved by hand.

Removing the mistaken builds did not help either. The next-highest number belonged to a different older build, for a different reason, and the installations went for that one instead. Sorting by the number simply found the next wrong answer.

What changed

Two things, and the second is the one that matters most.

Updates now follow publication order. Every build carries the sequence number of the run that published it, and that number only ever goes up — it is produced by the machine that did the publishing, not typed by a person. That is now what "newer" means. A build labelled with a higher version but published earlier loses, which is precisely the case that caused this. The version number is still what you see and still what an official release is named by; it is simply no longer the thing that decides which build is more recent.

An installation now notices when the version it runs no longer exists. "Nothing newer" and "the build I am running has been withdrawn" used to print the same sentence, and they are opposite situations: the first is healthy, the second means the installation cannot start a fresh instance of itself at all and can never be rescued by a future release. The update check already had the information needed to tell them apart — it just was not asking. Now it does, and:

• The Updates screen says so plainly, at the top, before anything else.
• The installation moves itself to the best build that actually exists, even if that means going back a step. A build that exists beats one that does not.
• When there is genuinely nothing to move to, it says that too — and says what an administrator has to do — instead of reporting that everything is fine.

There is a deliberate third answer as well. If the check cannot reach the list of published builds, it says the question was not answered, rather than treating a failed lookup as "your version is gone". Acting on that mistake would send every installation backwards at once.

If you were affected

If an installation reported "no newer version detected" while sitting on an old build, it was not up to date — it was stuck, and there was no way to tell from the screen. It will now correct itself on its next check, with no action from you.