By accident, I forcefully pushed from my local repository to the GitLab repository having mirroring active which then deleted all the hidden and GitLab related refs (for the merge request 'refs/merge-request') in the remote.
I told my coworkers to stop interacting with the remote repository and asked OPs if I could get the daily backup for that repository. I received a repositoryname.bundle file.
Now how can I recover the remote with this bundle file?
I solved it on my own like this.
Create a temporary directory and cd there
cd $(mktemp -d)
Get the bundle from your OPs (full snapshot) of the repository there
mv /tmp/yourrepo.bundle .
Clone the broken repository from GitLab to BROKEN
git clone --mirror URL_to_yourrepo BROKEN.git
cd in BROKEN and run git bundle verify ../reponame.bundle. It should not report any errors, if so, continue
cd BROKEN.git
git bundle verify ../yourrepo.bundle
Go back and then clone a new repository from the bundle file
cd ..
cd git clone --mirror yourrepo.bundle LASTKNOWNGOOD.git
cd there and verify that all your refs are there as u expect them to be
cd LASTKNOWNGOOD.git
git show-ref
Now set the repositories remote to the local BROKEN clone from GitLab
git remote add BROKEN ../BROKEN.git
Then push the contents of here to the BROKEN remote
git push --tags --force --mirror BROKEN
By then, the BROKEN repository should be healed. cd into the BROKEN and simulate a push to verify that it will do what you expect
cd ../BROKEN.git
git push --tags --verbose --dry-run --mirror origin
If it looks like u expect it to be, run without --dry-run to heal the remote repository.
It may report about remote rejected but as long as they are from the keep-around refs group you can safely ignore them.
You also need to reopen all the automatically closed merge request in GitLab for that repository.