Fix remote mirrors failing if Git remotes have not been added

Remote mirrors only get created when the URL changes, However, during the GCP
migration, the remote mirror did not get created automatically. Plus, there's
no guarantee someone restoring a repository from backup would have this
remote. We now add the remote each time we attempt to fetch from the
repository.

This works because Gitaly doesn't throw up an exception or error if the
remote already exists:
https://gitlab.com/gitlab-org/gitaly/issues/1317

In the future, we should attempt to add if the remote doesn't exist:
https://gitlab.com/gitlab-org/gitaly/issues/1316

Closes #50562

app/models/remote_mirror.rb

@@ -150,6 +150,15 @@ class RemoteMirror < ActiveRecord::Base
     result.to_s
   end
 
+  def ensure_remote!
+    return unless project
+    return unless remote_name && url
+
+    # If this fails or the remote already exists, we won't know due to
+    # https://gitlab.com/gitlab-org/gitaly/issues/1317
+    project.repository.add_remote(remote_name, url)
+  end
+
   private
 
   def raw

app/services/projects/update_remote_mirror_service.rb

@@ -10,6 +10,7 @@ module Projects
       return success unless remote_mirror.enabled?
 
       begin
+        remote_mirror.ensure_remote!
         repository.fetch_remote(remote_mirror.remote_name, no_tags: true)
 
         opts = {}

changelogs/unreleased/sh-fix-issue-50562.yml

+---
+title: Fix remote mirrors failing if Git remotes have not been added
+merge_request: 21351
+author:
+type: fixed

spec/models/remote_mirror_spec.rb

@@ -220,6 +220,18 @@ describe RemoteMirror do
     end
   end
 
+  context '#ensure_remote!' do
+    let(:remote_mirror) { create(:project, :repository, :remote_mirror).remote_mirrors.first }
+
+    it 'adds a remote multiple times with no errors' do
+      expect(remote_mirror.project.repository).to receive(:add_remote).with(remote_mirror.remote_name, remote_mirror.url).twice.and_call_original
+
+      2.times do
+        remote_mirror.ensure_remote!
+      end
+    end
+  end
+
   context '#updated_since?' do
     let(:remote_mirror) { create(:project, :repository, :remote_mirror).remote_mirrors.first }
     let(:timestamp) { Time.now - 5.minutes }

spec/services/projects/update_remote_mirror_service_spec.rb

@@ -18,6 +18,7 @@ describe Projects::UpdateRemoteMirrorService do
     end
 
     it "fetches the remote repository" do
+      expect(remote_mirror).to receive(:ensure_remote!).and_call_original
       expect(repository).to receive(:fetch_remote).with(remote_mirror.remote_name, no_tags: true) do
         sync_remote(repository, remote_mirror.remote_name, local_branch_names)
       end