Fix `jekyll serve --detach` with jekyll-sass-converter 3.x (#9304)

Merge pull request 9304

lib/jekyll/commands/serve.rb

@@ -274,12 +274,19 @@ module Jekyll
         def boot_or_detach(server, opts)
           if opts["detach"]
             pid = Process.fork do
+              # Detach the process from controlling terminal
+              $stdin.reopen("/dev/null", "r")
+              $stdout.reopen("/dev/null", "w")
+              $stderr.reopen("/dev/null", "w")
               server.start
             end
 
             Process.detach(pid)
             Jekyll.logger.info "Server detached with pid '#{pid}'.",
                                "Run `pkill -f jekyll' or `kill -9 #{pid}' to stop the server."
+
+            # Exit without running `at_exit` inherited by the forked process.
+            Process.exit! 0
           else
             t = Thread.new { server.start }
             trap("INT") { server.shutdown }

test/test_commands_serve.rb

@@ -313,4 +313,36 @@ class TestCommandsServe < JekyllUnitTest
       @merc.execute(:serve, "watch" => false)
     end
   end
+
+  context "using --detach" do
+    setup do
+      skip_if_windows "fork is not supported on Windows"
+      skip("fork is not supported by JRuby") if jruby?
+
+      @temp_dir = Dir.mktmpdir("jekyll_serve_detach_test")
+      @destination = File.join(@temp_dir, "_site")
+      Dir.mkdir(@destination) || flunk("Could not make directory #{@destination}")
+    end
+
+    teardown do
+      FileUtils.remove_entry_secure(@temp_dir, true)
+    end
+
+    should "fork into daemon process" do
+      process, output = Jekyll::Utils::Exec.run("jekyll", "serve",
+                                                "--source", @temp_dir,
+                                                "--dest", @destination,
+                                                "--detach")
+
+      re = %r!Server detached with pid '(?<pid>\d+)'!
+
+      assert_match re, output
+      assert_equal 0, process.exitstatus
+
+      pid = re.match(output)["pid"].to_i
+      assert pid > 1
+
+      Process.kill 9, pid
+    end
+  end
 end