Add external plugins support to extend system hooks

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/services/system_hooks_service.rb

@@ -11,6 +11,15 @@ class SystemHooksService
     SystemHook.hooks_for(hooks_scope).find_each do |hook|
       hook.async_execute(data, 'system_hooks')
     end
+
+    # Execute external plugins
+    PLUGINS.each do |plugin|
+      begin
+        plugin.new.execute(data)
+      rescue => e
+        Rails.logger.warn("GitLab -> Plugins -> #{plugin.class.name} raised an axception during execution. #{e}")
+      end
+    end
   end
 
   private

config/application.rb

@@ -28,6 +28,7 @@ module Gitlab
     config.eager_load_paths.push(*%W[#{config.root}/lib
                                      #{config.root}/app/models/hooks
                                      #{config.root}/app/models/members
+                                     #{config.root}/plugins
                                      #{config.root}/app/models/project_services
                                      #{config.root}/app/workers/concerns
                                      #{config.root}/app/services/concerns

config/initializers/9_plugins.rb

@@ -0,0 +1,29 @@
+class PluginsSystem
+  attr_accessor :plugins, :files
+
+  def initialize
+    @files = Dir.glob(Rails.root.join('plugins', '*_plugin.rb'))
+  end
+
+  def valid_plugins
+    files.map do |file|
+      file_name = File.basename(file, '.rb')
+
+      # Just give sample data to method and expect it to not crash.
+      begin
+        klass = Object.const_get(file_name.classify)
+        klass.new.execute(Gitlab::DataBuilder::Push::SAMPLE_DATA)
+      rescue => e
+        Rails.logger.warn("GitLab -> Plugins -> #{file_name} raised an exception during boot check. #{e}")
+        next
+      else
+        Rails.logger.info "GitLab -> Plugins -> #{file_name} passed boot check"
+        klass
+      end
+    end
+  end
+end
+
+# Load external plugins from /plugins directory
+# and set into PLUGINS variable
+PLUGINS = PluginsSystem.new.valid_plugins