Add Gitlab::Ssh::Commit class

What does this MR do and why?

Describe in detail what your merge request does and why.

In order to add Sign commits with your SSH key (#343879 - closed), this MR adds a new Gitlab::Ssh::Commit class which is responsible for creating CommitSignatures::SshSignature records in the database and returning them. This class mirrors the existing Gitlab::Gpg::Commit and Gitlab::X509::Commit classes, and will be called from Commit#signature

My testing approach differs from the existing Gitlab::Gpg::Commit and Gitlab::X509::Commit commit specs, which I find difficult to maintain due to the sheer amount of bootstrapping they do. The Gitlab::Ssh::Signature spec already thoroughly tests the correctness of signature verification, so there is no need to re-test that in these specs. Instead, we only check that we correctly handle the output of Gitlab::Ssh::Signature.

Since it is expected for signatures with the unknown_key status to not have a key_id available, this MR also removes the NOT NULL constraint from the key_id column on the ssh_signatures table.

Migrations

Up

main: == 20220728114136 MakeSshSignatureKeyNullable: migrating ======================
main: -- change_column_null(:ssh_signatures, :key_id, true)
main:    -> 0.0015s
main: == 20220728114136 MakeSshSignatureKeyNullable: migrated (0.0020s) =============

Down

main: == 20220728114136 MakeSshSignatureKeyNullable: reverting ======================
main: -- change_column_null(:ssh_signatures, :key_id, false)
main:    -> 0.0067s
main: == 20220728114136 MakeSshSignatureKeyNullable: reverted (0.0143s) =============

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Start the rails console:

bundle exec rails c

Run this Ruby code:

project = Project.find_by_path("gitlab-test")
commit = project.commit_by(oid: '7b5160f9bb23a3d58a0accdbe89da13b96b1ece9')
Gitlab::Ssh::Commit.new(commit).signature

This will show the commit signature for gitlab-test@7b5160f9, which should be pre-seeded into your GDK. If it's not there, then you can either re-seed your GDK with gdk reset-data or add it to your local gitlab-test from upstream:

git clone git@gitlab.com:gitlab-org/gitlab-test.git
cd gitlab-test
git remote add gdk ssh://git@gdk.local:2222/gitlab-org/gitlab-test.git
git checkout ssh-signed-commit
git push -u gdk ssh-signed-commit

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.