Add Gem.disable_system_update_message to disable gem update --system if needed.

Merged: https://github.com/ruby/ruby/pull/3229

lib/rubygems.rb

@@ -1187,6 +1187,12 @@ An Array (#{env.inspect}) was passed in from #{caller[3]}
   # other.
   class << self
 
+    ##
+    # RubyGems distributors (like operating system package managers) can
+    # disable RubyGems update by setting this to error message printed to
+    # end-users on gem update --system instead of actual update.
+    attr_accessor :disable_system_update_message
+
     ##
     # Hash of loaded Gem::Specification keyed by name
 

lib/rubygems/commands/update_command.rb

@@ -262,6 +262,11 @@ command to remove old versions.
   # Update RubyGems software to the latest version.
 
   def update_rubygems
+    if Gem.disable_system_update_message
+      alert_error Gem.disable_system_update_message
+      return
+    end
+
     check_update_arguments
 
     version, requirement = rubygems_target_version

test/rubygems/test_gem_commands_update_command.rb

@@ -238,6 +238,23 @@ class TestGemCommandsUpdateCommand < Gem::TestCase
                  @ui.error
   end
 
+  def test_execute_system_with_disabled_update
+    old_disable_system_update_message = Gem.disable_system_update_message
+    Gem.disable_system_update_message = "Please use package manager instead."
+
+    @cmd.options[:args] = []
+    @cmd.options[:system] = true
+
+    use_ui @ui do
+      @cmd.execute
+    end
+
+    assert_empty @ui.output
+    assert_equal "ERROR:  Please use package manager instead.\n", @ui.error
+  ensure
+    Gem.disable_system_update_message = old_disable_system_update_message
+  end
+
   # before:
   #   a1 -> c1.2
   # after: