I upgraded my Discourse software today. It is installed on a DigitalOcean droplet with Ubuntu 14.04
The upgrade included an upgrade to Docker.
After upgrade, no-one can access the forum, they get 502 Bad Gateway nginx error.
Restarting the droplet did not fix the error.
Looking into this problem, found that nginx service had not started automatically, and after trying several methods I could not get nginx started manually.
In the /var/log/nginx/error.log this message occurred several times:
[emerg] 21412#0: bind() to 0.0.0.0:80 failed (98:Address already in use)
So looking into this further, this appears to be related to a Docker issue, opened two days ago, related to upgrading to the latest version of Docker. This Docker issue describes upgrading on AWS with Ubuntu, and after the upgrade nginx cannot start because port 80 is detected to be in use, but it actually isn’t.