Add --list-queues option to sidekiq-cluster

The --dryrun option shows the full command sidekiq-cluster will run.
This new option just lists the matched queues (mostly useful in
conjunction with --queue-selector), one per line.

lib/gitlab/sidekiq_cluster/cli.rb

@@ -37,6 +37,7 @@ module Gitlab
         @logger.formatter = ::Gitlab::SidekiqLogging::JSONFormatter.new
         @rails_path = Dir.pwd
         @dryrun = false
+        @list_queues = false
       end
 
       def run(argv = ARGV)
@@ -47,6 +48,11 @@ module Gitlab
 
         option_parser.parse!(argv)
 
+        if @dryrun && @list_queues
+          raise CommandError,
+            'The --dryrun and --list-queues options are mutually exclusive'
+        end
+
         worker_metadatas = SidekiqConfig::CliMethods.worker_metadatas(@rails_path)
         worker_queues = SidekiqConfig::CliMethods.worker_queues(@rails_path)
 
@@ -73,6 +79,12 @@ module Gitlab
             'No queues found, you must select at least one queue'
         end
 
+        if @list_queues
+          puts queue_groups.map(&:sort) # rubocop:disable Rails/Output
+
+          return
+        end
+
         unless @dryrun
           @logger.info("Starting cluster with #{queue_groups.length} processes")
         end
@@ -202,6 +214,10 @@ module Gitlab
           opt.on('-d', '--dryrun', 'Print commands that would be run without this flag, and quit') do |int|
             @dryrun = true
           end
+
+          opt.on('--list-queues', 'List matching queues, and quit') do |int|
+            @list_queues = true
+          end
         end
       end
     end

spec/lib/gitlab/sidekiq_cluster/cli_spec.rb

@@ -81,7 +81,7 @@ RSpec.describe Gitlab::SidekiqCluster::CLI do
         end
       end
 
-      context '-timeout flag' do
+      context 'with --timeout flag' do
         it 'when given', 'starts Sidekiq workers with given timeout' do
           expect(Gitlab::SidekiqCluster).to receive(:start)
             .with([['foo']], default_options.merge(timeout: 10))
@@ -97,6 +97,27 @@ RSpec.describe Gitlab::SidekiqCluster::CLI do
         end
       end
 
+      context 'with --list-queues flag' do
+        it 'errors when given --list-queues and --dryrun' do
+          expect { cli.run(%w(foo --list-queues --dryrun)) }.to raise_error(described_class::CommandError)
+        end
+
+        it 'prints out a list of queues in alphabetical order' do
+          expected_queues = [
+            'epics:epics_update_epics_dates',
+            'epics_new_epic_issue',
+            'new_epic',
+            'todos_destroyer:todos_destroyer_confidential_epic'
+          ]
+
+          allow(Gitlab::SidekiqConfig::CliMethods).to receive(:query_queues).and_return(expected_queues.shuffle)
+
+          expect(cli).to receive(:puts).with([expected_queues])
+
+          cli.run(%w(--queue-selector feature_category=epics --list-queues))
+        end
+      end
+
       context 'queue namespace expansion' do
         it 'starts Sidekiq workers for all queues in all_queues.yml with a namespace in argv' do
           expect(Gitlab::SidekiqConfig::CliMethods).to receive(:worker_queues).and_return(['cronjob:foo', 'cronjob:bar'])