Obliterating history from Git

It should not happen often that this is necessary, but it is useful to know.

To obliterate history means removing it from three different sources: Gerrit, Forgejo, and GitHub.

A tool has been written, called gerrit-rewrite-branch, to rewrite Gerrit history completely, including the meta on past CLs.

To use it, build it as --release (it will stack overflow on debug mode), and find the following repos, and make backups of them:

• /var/lib/gerrit/git/lix.git
• /var/lib/forgejo/repositories/*/lix.git

To start off, stop Gerrit and find the Git repo for it. The tool requires four things: The email address to obliterate, and a replacement name + email address. It also needs a cutoff date for where to remove commits before. To find this, run git cat-file -p {commit} for a commit earlier than the oldest you want to remove, and note down the timestamp on the committer line.

Call the tool. It will churn for a while, and rewrite all previous Git commits, plus the Gerrit metadata of affected commits. As a bonus, run a git gc --prune=now.

Before turning on Gerrit, run systemd-run -p DynamicUser=yes -p StateDirectory=gerrit -t gerrit reindex -d /var/lib/gerrit. This ensures Gerrit is aware of the changes made outside of its existence.

For forgejo, no special steps are needed; just run the same tool over these repos plus all their forks, and run git gc --prune=now as well.

Once Gerrit and Forgejo are back up, run ssh gerrit.lix.systems replication start --now --url github to propagate the changes to GitHub.