[Fix rubocop#11164] Suppress a server mode message

Fixes rubocop#11164

This PR suppresses "RuboCop server starting..." message with `--server --format json`
because JSON format does not expected output message when IDE integration with server mode.

changelog/fix_suppress_a_server_mode_message.md

@@ -0,0 +1 @@
+* [#11164](https://github.com/rubocop/rubocop/issues/11164): Suppress "RuboCop server starting..." message with `--server --format json`. ([@koic][])

lib/rubocop/server/core.rb

@@ -17,6 +17,8 @@ module Server
     # The core of server process. It starts TCP server and perform socket communication.
     # @api private
     class Core
+      JSON_FORMATS = %w[json j].freeze
+
       def self.token
         @token ||= SecureRandom.hex(4)
       end
@@ -57,6 +59,10 @@ def server_mode?
       def start_server(host, port)
         @server = TCPServer.open(host, port)
 
+        # JSON format does not expected output message when IDE integration with server mode.
+        # See: https://github.com/rubocop/rubocop/issues/11164
+        return if use_json_format?
+
         output_stream = ARGV.include?('--stderr') ? $stderr : $stdout
         output_stream.puts "RuboCop server starting on #{@server.addr[3]}:#{@server.addr[1]}."
       end
@@ -76,6 +82,15 @@ def read_socket(socket)
       ensure
         socket.close
       end
+
+      def use_json_format?
+        return true if ARGV.include?('--format=json') || ARGV.include?('--format=j')
+        return false unless (index = ARGV.index('--format'))
+
+        format = ARGV[index + 1]
+
+        JSON_FORMATS.include?(format)
+      end
     end
   end
 end

spec/rubocop/server/rubocop_server_spec.rb

@@ -67,6 +67,64 @@
       end
     end
 
+    context 'when using `--server` and json is specified as the format' do
+      context 'when `--format=json`' do
+        it 'does not display the server start message' do
+          create_file('example.rb', <<~RUBY)
+            puts 0
+          RUBY
+
+          stdout, _stderr, _status = Open3.capture3(
+            'ruby', '-I', '.',
+            rubocop, '--server', '--format=json', '--stdin', 'example.rb', stdin_data: 'puts 0'
+          )
+          expect(stdout).not_to start_with 'RuboCop server starting on '
+        end
+      end
+
+      context 'when `--format=j`' do
+        it 'does not display the server start message' do
+          create_file('example.rb', <<~RUBY)
+            puts 0
+          RUBY
+
+          stdout, _stderr, _status = Open3.capture3(
+            'ruby', '-I', '.',
+            rubocop, '--server', '--format=j', '--stdin', 'example.rb', stdin_data: 'puts 0'
+          )
+          expect(stdout).not_to start_with 'RuboCop server starting on '
+        end
+      end
+
+      context 'when `--format json`' do
+        it 'does not display the server start message' do
+          create_file('example.rb', <<~RUBY)
+            puts 0
+          RUBY
+
+          stdout, _stderr, _status = Open3.capture3(
+            'ruby', '-I', '.',
+            rubocop, '--server', '--format', 'json', '--stdin', 'example.rb', stdin_data: 'puts 0'
+          )
+          expect(stdout).not_to start_with 'RuboCop server starting on '
+        end
+      end
+
+      context 'when `--format j`' do
+        it 'does not display the server start message' do
+          create_file('example.rb', <<~RUBY)
+            puts 0
+          RUBY
+
+          stdout, _stderr, _status = Open3.capture3(
+            'ruby', '-I', '.',
+            rubocop, '--server', '--format', 'j', '--stdin', 'example.rb', stdin_data: 'puts 0'
+          )
+          expect(stdout).not_to start_with 'RuboCop server starting on '
+        end
+      end
+    end
+
     context 'when using `--server` option after running server and updating configuration' do
       it 'applies .rubocop.yml configuration changes even during server startup' do
         create_file('example.rb', <<~RUBY)