Fully unload add-ons under a mutex when shutting down

[vinistock]

Motivation

While investigating the possible add-on duplication bug in Tapioca, I noticed two small issues with our add-on deactivation:

1. The major one is not running under a mutex. This is problematic because if an add-on performs IO operations during deactivation (like the Rails add-on shutting down its runtime server) or if it simply takes too long, threads may be switched in the middle of the shutdown request. That means that, in theory, we can receive a shutdown request, switch threads and then process an exit request before the shutdown is finished - which is incorrect and may lead to hanging child processes
2. The much minor issues is that we were simply not clearing the add-on list. In theory, this shouldn't matter, since the process fully exits, but I think it's good form to fully cleanup before quitting

Implementation

Moved the entire shutdown request under the mutex to avoid any race conditions between shutdown and exit.

Create a convenience method to unload all add-ons and started using that.

[vinistock]

• Fully unload add-ons under a mutex when shutting down #2970 👈 (View in Graphite)
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