Today I attempted to upgrade our Mattermost install from 4.8.1 to 4.9.2. Our install (Linux, not Docker) runs a custom plugin we wrote to modify client CSS. After following the docs to upgrade the site, we discovered that the plugin was no longer installed.
I first attempted to “transfer” the plugin by copying the folder from the install directory backup to the new install directory, but the plugin did not appear in the system console. I next tried stopping and starting the mattermost service (after moving the folder), but the service failed to start. Finally, I removed the folder, successfully started the service, and uploaded the plugin to the system console directly (which worked).
Is there currently an “official” supported method for handling plugins during upgrades?