Add Utils::Internet.connected? to determine whether host machine has internet connection. (#5870)

Merge pull request 5870

lib/jekyll/utils.rb

@@ -6,6 +6,7 @@ module Jekyll
     extend self
     autoload :Ansi, "jekyll/utils/ansi"
     autoload :Exec, "jekyll/utils/exec"
+    autoload :Internet, "jekyll/utils/internet"
     autoload :Platforms, "jekyll/utils/platforms"
     autoload :Rouge, "jekyll/utils/rouge"
     autoload :WinTZ, "jekyll/utils/win_tz"

lib/jekyll/utils/internet.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+module Jekyll
+  module Utils
+    module Internet
+
+      # Public: Determine whether the present device has a connection to
+      # the Internet. This allows plugin writers which require the outside
+      # world to have a neat fallback mechanism for offline building.
+      #
+      # Example:
+      #   if Internet.connected?
+      #     Typhoeus.get("https://pages.github.com/versions.json")
+      #   else
+      #     Jekyll.logger.warn "Warning:", "Version check has been disabled."
+      #     Jekyll.logger.warn "", "Connect to the Internet to enable it."
+      #     nil
+      #   end
+      #
+      # Returns true if a DNS call can successfully be made, or false if not.
+      module_function
+      def connected?
+        !dns("example.com").nil?
+      end
+
+      private
+      module_function
+      def dns(domain)
+        require "resolv"
+        Resolv::DNS.open do |resolver|
+          resolver.getaddress(domain)
+        end
+      rescue Resolv::ResolvError, Resolv::ResolvTimeout
+        nil
+      end
+
+    end
+  end
+end

test/test_utils.rb

@@ -403,4 +403,10 @@ class TestUtils < JekyllUnitTest
       assert_equal "bom|another", merged[:encoding]
     end
   end
+
+  context "Utils::Internet.connected?" do
+    should "return true if there's internet" do
+      assert Utils::Internet.connected?
+    end
+  end
 end