So, I've spent a couple of hours looking around the internet to try to figure out this problem, since it seems like a dumb one that would probably be a duplicate, but apparently it isn't.
I'm hosting a WordPress site in a subdirectory on my main site (http://christopherdumas.org is the main site, http://christopherdumas.org/astraterra is the WP site) and when I try to upload files to a post in the WordPress site, it smacks me with this error:
Missing a temporary folder.
Now, this was working before I upgraded to WP 5.0, and I didn't test it after the update, assuming that it would just continue to work. It didn't. I thought I'd done everything necessary for the PHP server and WordPress setup to persist on boot, but I recently restarted my server and I had to restart/fix a couple things with WP so apparently not-- which might have something to do with this issue. Interestingly, I did the normal path to fix this the first time this happened, and I think that fixed it but I don't clearly remember, and now I'm having that problem again, even with the "fixed" configs.
Even worse, adding the usual lines to my
wp-config.php file (setting
WP_TEMP_DIR, etc) isn't helping, and neither does editing my
php.ini (in the root directory of the server). Furthermore, the
temp directory has
www-data permissions (as does the entire
wp-content directory) and I added a bit to the beginning of
index.php to make sure that it is using
www-data as the user, which it is. At this point, I can't think of anything more to do. I'm running this server on top of Lighttpd instead of Apache 2, which I did originally because of Lighttpd's ease of use, but I'm starting to regret doing that...