Mattermost "An existing user is already attached to your gitlab account" - github

Question

We use Mattermost using the 'Production Docker' setup as described in Mattermost documentation. For authentication, we federate using GitHub:Enterprise.
To setup our Mattermost team, I imported the whole Slack history. This lead to the problem that everyone who did not yet log into Mattermost via GitHub:Enterprise was not able to login. Mattermost helpfully returned the error message
"An existing user is already attached to your gitlab account"
How can I fix this issue without having to setup a new Mattermost instance and force everyone to login once before importing the Slack data?

Answer 1

Prerequisites
In order for this to work, you need
GitHub:Enterprise Administrator permissions
On the Mattermost machine, either root permissions or an account that is allowed to control docker, and, if psql is not installed, a way to install the psql command-line tool.
Steps
ssh into the Mattermost vm/machine (where the mattermost docker containers are running).
Change to an account with docker permissions (root; or the account you setup during Mattermost installation; or ... )
Use docker ps and note the hash of the container mattermostdocker_db. We will assume it starts with 5c23.
Run docker inspect 5c23 | grep IPAddress. Note the IP address of the container. We will assume it is 172.17.0.2.
Ensure that the psql commandline tool is installed on the machine where mattermost/docker is running.
On debian: apt-get install postgresql-client
Connect to the mattermost db of postgresql running inside the docker container:
psql -h 172.17.0.2 -p 5432 -d mattermost -U postgres -W
The (default?) password seems to be postgres.
Verify that a user account with the correct email exists. Assume the email of the account that has the problem is 'john#example.com`
mattermost-# select email, authdata from users where email = 'john#example.com';
Connect to GitHub:Enterprise and open the admin console. We will assume the local github enterprise instance is at https://github.example.com.
Click on the rocket symbol, or
https://github.example.com/stafftools
Click on all users and find the user that cannot login. We assume the github username is john, which would correspond to https://github.example.com/john
Visit the stafftools user security page for that user.
https://github.example.com/stafftools/users/john/security
Click on the 'Search logs' link under the 'Audit logs' header. This will open a page with a query field. On this page, you will find the internal github user number for that user. Note this number. We will assume the number is 37.
Back in the psql console, update the user entry with the correct number:
update users set authservice = 'gitlab', authdata = '37' where email = 'john#example.com' ;
Exit the psql console with \q:
mattermost-# \q
Done. The user can now log into Mattermost with GitHub:Enterprise user authentication.
Notes
Don't forget to complete each statement in psql with a ;
It's gitlab, not github, even if you use GitHub:Enterprise
Tested with Mattermost 3.0, GitHub:Enterprise 2.6.2