Allow daemon to receive a synchronous option that joins threads

lib/gitlab/daemon.rb

@@ -2,16 +2,16 @@
 
 module Gitlab
   class Daemon
-    def self.initialize_instance(*args)
+    def self.initialize_instance(...)
       raise "#{name} singleton instance already initialized" if @instance
 
-      @instance = new(*args)
+      @instance = new(...)
       Kernel.at_exit(&@instance.method(:stop))
       @instance
     end
 
-    def self.instance(*args)
-      @instance ||= initialize_instance(*args)
+    def self.instance(...)
+      @instance ||= initialize_instance(...)
     end
 
     attr_reader :thread
@@ -20,7 +20,8 @@ module Gitlab
       !thread.nil?
     end
 
-    def initialize
+    def initialize(**options)
+      @synchronous = options[:synchronous]
       @mutex = Mutex.new
     end
 
@@ -43,6 +44,10 @@ module Gitlab
             Thread.current.name = thread_name
             run_thread
           end
+
+          @thread.join if @synchronous
+
+          @thread
         end
       end
     end

spec/lib/gitlab/daemon_spec.rb

@@ -46,6 +46,30 @@ RSpec.describe Gitlab::Daemon do
 
           expect(subject).to have_received(:run_thread)
         end
+
+        context '@synchronous' do
+          context 'when @synchronous is set to true' do
+            subject { described_class.instance(synchronous: true) }
+
+            it 'calls join on the thread' do
+              # Thread has to be run in a block, expect_next_instance_of does not support this.
+              expect_any_instance_of(Thread).to receive(:join) # rubocop:disable RSpec/AnyInstanceOf
+
+              subject.start
+            end
+          end
+
+          context 'when @synchronous is not set to a truthy value' do
+            subject { described_class.instance }
+
+            it 'does not call join on the thread' do
+              # Thread has to be run in a block, expect_next_instance_of does not support this.
+              expect_any_instance_of(Thread).not_to receive(:join) # rubocop:disable RSpec/AnyInstanceOf
+
+              subject.start
+            end
+          end
+        end
       end
 
       describe '#stop' do