Once upon a time, a long time Liferay user decided to upgrade from an old version of Liferay (Liferay 6.0 GA2) to a new version of Liferay (Liferay 6.1 GA2). They performed this upgrade by shutting down Liferay 6.0 running on an old VM, starting Liferay 6.1 on a new VM, and then doing some DNS magic and some load balancer magic to switch to using the new VM after the upgrade completed.
A day later, a routine Cron job restarted Liferay 6.0 on the old VM, which still pointed at the same database used by Liferay 6.1.
Liferay 6.0's upgrade code checked to see if it had to run any of its upgrade processes, found that the build number stored in the Release_ table (6101) was higher than any of the upgrade processes it wanted to run, and proceeded to update the Release_ table with its own version number (6006).
Naturally, because nobody was accessing that old version of the portal, it didn't matter. It didn't touch any other tables, so Liferay 6.1 continued to run fine. That is, until Liferay 6.1 had to be restarted a week later for some routine maintenance.
As Liferay restarted, Liferay 6.1's upgrade code checked to see if it had to run any of its upgrade processes, found that the build number stored in the Release_ table (6006) was lower than some of the upgrade processes it wanted to run, and so ran them it did. Of course, because the data was actually in the 6101 state, all of these upgrade processes wound up corrupting the data, rendering the whole Liferay instance unusable.
No problem, the client had followed industry best practices and regularly backed up their system. So, they restored the database from yesterday's backup, thinking that all would now be well.
Alas, it was not meant to be. The damage to the database had been done a week before, and so the bad data was already in the database. Every time the client restored the database from yesterday's backup , Liferay re-detected the version number was different and that it needed to re-upgrade from 6006 to 6101. As a result, Liferay obediently re-corrupted the database.
This raises the following question: how do we avoid the data corruption resulting from an older release running against a newer release's database?
Enter LPS-21250, which prevents Liferay from starting up if its release number is lower than what is seen in the Release_ table. This means that if this situation were to repeat with 6.1 GA2 and 6.2 M3, 6.1 GA2 would fail to start up and not update the Release_ table.
Of course, this doesn't fix the problem of earlier versions of Liferay (such as Liferay 6.0) updating the Release_ table, raising a second question: how do we handle the problem where earlier versions of Liferay (such as Liferay 6.0) might update the Release_ table?
In Liferay 6.0 and in Liferay 6.1 GA1, it is possible to prevent upgrade processes from running by setting the upgrade.processes property to blank. The Release_ table will still be updated, but at least Liferay wouldn't attempt to re-corrupt its own data. This strategy will work to prevent upgrade processes from running if you were to accidentally start a Liferay 5.x release against a Liferay 6.0 database or a Liferay 6.1 GA1 database.
In Liferay 6.1 GA2, a blank value for upgrade.processes has been given a new meaning while adding support for LPS-25637: automatically try to detect the upgrade processes that need to be run. So, this workaround no longer works.
So at this time, for Liferay 6.1 GA2 and possibly the Liferay 6.2 releases, you have two ways of handling the problem of a Liferay 5.x or a Liferay 6.0 running against your database. You can either create a no-op upgrade process, or you can prevent Liferay from starting up at all if it detects a version number change.
In the former case, there's no native UpgradeProcess in Liferay 6.1 which truly does nothing. So, you can either use an upgrade process that is very unlikely to run (such as the 4.4.2 to 5.0.0 upgrade process), or you'll have to write an EXT plugin which contains a custom do-nothing upgrade process and specify it in your portal properties, or you'll have to specify a bad value for upgrade.processes and get a stack trace on every startup.
In the latter case, LPS-25637 also introduced a new feature which skips all upgrade processes if there's a version number match (previous versions of Liferay would check each one individually). Leveraging that, you can make use of a built-in run-on-all-releases upgrade process that was originally intended to allow you to split an upgrade in chunks, and effectively does nothing more than shut down Liferay.