Pull Request for Issue #16166

Summary of Changes

In 3.x, only plugins make preflight available to extension scripts during the uninstall process and none make postflight available. This PR will make those hooks available in 4.0.

Testing Instructions

An extension's install script, when the extension is uninstalled, can act on the preflight, uninstall, and postflight hooks.

Documentation Changes Required

• For plugins, where the uninstall hooks is triggered is now actually different. In 3.x, uninstall is triggered after the SQL queries are performed (if any), inconsistent with the other adapters which are doing it before. This is a B/C concern.
• All extensions implementing preflight and postflight will now have to check the install route (given as one of the method parameters) and not assume they only apply in an install/update context.

Discussion Points

The current implementation triggers both preflight and uninstall immediately, this seems a bit illogical to trigger two hooks back-to-back when nothing else has happened in the middle. I think moving uninstall somewhere in the middle (after SQL and before files is where it is for plugins in 3.x) may make sense, but there may be other options as well.