Add Praefect rake task to print out replica checksums

When praefect is being used, the administrator needs to be able to know
whether or not a given project's repository has the same checksum across
its replicas.

changelogs/unreleased/jc-replicas-rake.yml

+---
+title: Add Praefect rake task to print out replica checksums
+merge_request: 28369
+author:
+type: added

lib/tasks/gitlab/praefect.rake

+namespace :gitlab do
+  namespace :praefect do
+    def int?(string)
+      true if Integer(string) rescue false
+    end
+
+    def print_checksums(header, row)
+      header.each_with_index do |val, i|
+        width = [val.length, row[i].length].max
+        header[i] = header[i].ljust(width)
+        row[i] = row[i].ljust(width)
+      end
+
+      header_str = header.join(' | ')
+      puts header_str
+      puts '-' * header_str.length
+      puts row.join(' | ')
+    end
+
+    desc 'GitLab | Praefect | Check replicas'
+    task :replicas, [:project_id] => :gitlab_environment do |t, args|
+      warn_user_is_not_gitlab
+
+      unless int?(args.project_id)
+        puts 'argument must be a valid project_id'
+        next
+      end
+
+      project = Project.find_by_id(args.project_id)
+      if project.nil?
+        puts 'No project was found with that id'
+        next
+      end
+
+      begin
+        replicas_resp = project.repository.replicas
+
+        sorted_replicas = replicas_resp.replicas.sort_by { |r| r.repository.storage_name }
+
+        header = ['Project name'] << "#{replicas_resp.primary.repository.storage_name} (primary)"
+        header.concat(sorted_replicas.map { |r| r.repository.storage_name })
+
+        row = [project.name] << replicas_resp.primary.checksum
+        row.concat(sorted_replicas.map {|r| r.checksum})
+      rescue
+        puts 'Something went wrong when getting replicas.'
+        next
+      end
+
+      puts "\n"
+      print_checksums(header, row)
+    end
+  end
+end

spec/tasks/gitlab/praefect_rake_spec.rb

+# frozen_string_literal: true
+
+require 'rake_helper'
+
+describe 'gitlab:praefect:replicas' do
+  before do
+    Rake.application.rake_require 'tasks/gitlab/praefect'
+  end
+
+  let(:project) { create(:project, :repository) }
+  let(:repository) { project.repository }
+
+  describe 'replicas', :praefect do
+    context 'when a valid project id is used as the argument' do
+      let(:project_arg) { project.id }
+
+      it "calls praefect info service's replicas method" do
+        expect_any_instance_of(Gitlab::GitalyClient::PraefectInfoService).to receive(:replicas).and_call_original
+
+        run_rake_task('gitlab:praefect:replicas', project_arg)
+      end
+
+      it 'prints out the expected row' do
+        row = /#{project.name}\s+\| #{project.repository.checksum}/
+
+        expect { run_rake_task('gitlab:praefect:replicas', project_arg) }.to output(row).to_stdout
+      end
+    end
+
+    context 'when a non existent project id is used as the argument' do
+      let(:project_arg) { '2' }
+
+      it "does not call praefect info service's replicas method" do
+        expect_any_instance_of(Gitlab::GitalyClient::PraefectInfoService).not_to receive(:replicas)
+
+        run_rake_task('gitlab:praefect:replicas', project_arg)
+      end
+    end
+
+    context 'when replicas throws an exception' do
+      before do
+        allow_next_instance_of(Gitlab::GitalyClient::PraefectInfoService) do |instance|
+          expect(instance).to receive(:replicas).and_raise("error")
+        end
+      end
+
+      it 'aborts with the correct error message' do
+        expect { run_rake_task('gitlab:praefect:replicas', project.id) }.to output("Something went wrong when getting replicas.\n").to_stdout
+      end
+    end
+  end
+end