Load plugin immediately

We can install RubyGems plugin by "gem install XXX". The installed
plugin is used from the NEXT "gem ...".

For example, "gem install gem-src kaminari" doesn't use gem-src plugin
for kaminari. "gem install gem-src && gem install kaminari" uses
gem-src plugin for kaminari.

How about loading a plugin immediately when the plugin is installed?
If this proposal is implemented, "gem install gem-src kaminari" works
like "gem install gem-src && gem install kaminari".

Author: kou

lib/rubygems/installer.rb

@@ -345,6 +345,8 @@ def install
 
     run_post_install_hooks
 
+    load_plugin
+
     spec
 
   # TODO: This rescue is in the wrong place. What is raising this exception?
@@ -1006,4 +1008,16 @@ def bash_prolog_script
       ""
     end
   end
+
+  def load_plugin
+    latest = Gem::Specification.latest_spec_for(spec.name)
+    # If pre-release version is only installed, latest is nil.
+    # In this case, we use pre-release version.
+    return if latest && latest.version != spec.version
+
+    plugin_files = spec.plugins.map do |plugin|
+      File.join(@plugins_dir, "#{spec.name}_plugin#{File.extname(plugin)}")
+    end
+    Gem.load_plugin_files(plugin_files)
+  end
 end

test/rubygems/test_gem.rb

@@ -1449,6 +1449,8 @@ def test_self_vendor_dir_missing
   def test_load_plugins
     plugin_path = File.join "lib", "rubygems_plugin.rb"
 
+    foo1_plugin_path = nil
+    foo2_plugin_path = nil
     Dir.chdir @tempdir do
       FileUtils.mkdir_p "lib"
       File.open plugin_path, "w" do |fp|
@@ -1458,17 +1460,22 @@ def test_load_plugins
       foo1 = util_spec "foo", "1" do |s|
         s.files << plugin_path
       end
+      foo1_plugin_path = File.join(foo1.gem_dir, plugin_path)
 
       install_gem foo1
 
       foo2 = util_spec "foo", "2" do |s|
         s.files << plugin_path
       end
+      foo2_plugin_path = File.join(foo2.gem_dir, plugin_path)
 
       install_gem foo2
     end
 
     Gem::Specification.reset
+    PLUGINS_LOADED.clear
+    $LOADED_FEATURES.delete(foo1_plugin_path)
+    $LOADED_FEATURES.delete(foo2_plugin_path)
 
     gem "foo"
 

test/rubygems/test_gem_installer.rb

@@ -769,7 +769,7 @@ def test_generate_bin_with_dangling_symlink
   def test_generate_plugins
     installer = util_setup_installer do |spec|
       write_file File.join(@tempdir, "lib", "rubygems_plugin.rb") do |io|
-        io.write "puts __FILE__"
+        io.write "# do nothing"
       end
 
       spec.files += %w[lib/rubygems_plugin.rb]
@@ -856,11 +856,32 @@ def test_generate_plugins_with_build_root
     refute_includes File.read(build_root_path), build_root
   end
 
+  class << self
+    attr_accessor :plugin_loaded
+  end
+
+  def test_use_plugin_immediately
+    self.class.plugin_loaded = false
+    installer = util_setup_installer do |spec|
+      write_file File.join(@tempdir, "lib", "rubygems_plugin.rb") do |io|
+        io.write "#{self.class}.plugin_loaded = true"
+      end
+
+      spec.files += %w[lib/rubygems_plugin.rb]
+    end
+
+    build_rake_in do
+      installer.install
+    end
+
+    assert self.class.plugin_loaded, "plugin is not loaded"
+  end
+
   def test_keeps_plugins_up_to_date
     # NOTE: version a-2 is already installed by setup hooks
 
     write_file File.join(@tempdir, "lib", "rubygems_plugin.rb") do |io|
-      io.write "puts __FILE__"
+      io.write "# do nothing"
     end
 
     build_rake_in do