Mattermost upgrade fails with Postgres error

Problem

Sometimes, a migration in Mattermost might fail; while this is obviously suboptimal, there are several safety nets in place that will prevent your environment from corrupting. Usually, after fixing the root cause of the failed migration, you can restart the Mattermost server, and it will pick back up where it failed before and complete the migrations.

Below are some common errors that might come up when upgrading on a Postgres based system.

pq: remaining connection slots are reserved for non-replication superuser connections

This error comes up when the Postgres server has exhausted the maximum amount of available connections.

Solution

Increase the max_connections setting in the Postgres server configuration and restart the server.