Delay Utils.getservername until needed.

There is no need to call Utils.getservername when the :ServerName option is
specified, so delay Utils.getservername until needed to avoid unnecessary
DNS lookups.  [ruby-core:78492] [Bug #13007]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57014 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/webrick/config.rb

@@ -20,8 +20,14 @@ module WEBrick
     LIBDIR = File::dirname(__FILE__) # :nodoc:
 
     # for GenericServer
-    General = {
-      :ServerName     => Utils::getservername,
+    General = Hash.new { |hash, key|
+      case key
+      when :ServerName
+        hash[key] = Utils.getservername
+      else
+        nil
+      end
+    }.update(
       :BindAddress    => nil,   # "0.0.0.0" or "::" or nil
       :Port           => nil,   # users MUST specify this!!
       :MaxClients     => 100,   # maximum number of the concurrent connections
@@ -36,7 +42,7 @@ module WEBrick
       :AcceptCallback => nil,
       :DoNotReverseLookup => true,
       :ShutdownSocketWithoutClose => false,
-    }
+    )
 
     # for HTTPServer, HTTPRequest, HTTPResponse ...
     HTTP = General.dup.update(

test/webrick/test_config.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: false
+require "test/unit"
+require "webrick/config"
+
+class TestWEBrickConfig < Test::Unit::TestCase
+  def test_server_name_default
+    config = WEBrick::Config::General.dup
+    assert_equal(false, config.key?(:ServerName))
+    assert_equal(WEBrick::Utils.getservername, config[:ServerName])
+    assert_equal(true, config.key?(:ServerName))
+  end
+
+  def test_server_name_set_nil
+    config = WEBrick::Config::General.dup.update(ServerName: nil)
+    assert_equal(nil, config[:ServerName])
+  end
+end