Created rake restart task.

Fixes #18876. Rake restart touches `tmp/restart.txt` to restart
application on next request. Updated tests and documentation
accordingly.

railties/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Created rake restart task. Restarts your Rails app by touching the 
+    `tmp/restart.txt`.
+
+    Fixes #18876.
+
+    *Hyonjee Joo*
+
 *   Set Rails console to use log formatter and log level as specified for the
     given environment.
 

railties/lib/rails/tasks.rb

@@ -7,6 +7,7 @@ require 'rake'
   log
   middleware
   misc
+  restart
   routes
   statistics
   tmp

railties/lib/rails/tasks/restart.rake

@@ -0,0 +1,4 @@
+desc "Restart app by touching tmp/restart.txt"
+task restart: :environment do
+  FileUtils.touch('tmp/restart.txt')
+end

railties/test/application/rake/restart_test.rb

@@ -0,0 +1,31 @@
+require "isolation/abstract_unit"
+
+module ApplicationTests
+  module RakeTests
+    class RakeRestartTest < ActiveSupport::TestCase
+      include ActiveSupport::Testing::Isolation
+
+      def setup
+        build_app
+        boot_rails
+      end
+
+      def teardown
+        teardown_app
+      end
+      
+      test 'rake restart touches tmp/restart.txt' do
+        Dir.chdir(app_path) do
+          `rake restart`
+          assert File.exist?("tmp/restart.txt")
+          
+          prev_mtime = File.mtime("tmp/restart.txt")
+          sleep(1)
+          `rake restart`
+          curr_mtime = File.mtime("tmp/restart.txt")
+          assert_not_equal prev_mtime, curr_mtime
+        end
+      end
+    end
+  end
+end