Bug in multisite with custom upload directory

Hi there,

We’ve been troubleshooting for a while, and finally found the case of a bug in multisite.

The scenario: The plugin is individually activated on the primary site in the network (not network activated), and the content wp-content directory has been moved outside the WordPress root folder.

This works like it should for subsites in the network (not the primary site), where the backup directory is correctly identified as writable and available.

On the primary network site, this is not the case, and a custom filter needs to be added, we took inspiration form the Imagify Custom Site Root plugin which we found elsewhere on the internet to accomplish this, but the procedure is the same, the different is that we have chosen to not name our folder wp-content, so things got even more muddied.

The plugin works fine in single-site instances with the same moved content directory, and as mentioned also for any other sub-site in a network that is not the primary site, which leads me to believe this is likely a bug in the way paths are determined on a multisite environment.