FIX: Remove `plugin.enabled?` checks at initialization time (#6166)

Checking `plugin.enabled?` while initializing plugins causes issues in two ways:
  - An application restart is required for changes to take effect. A load-balanced multi-server environment could behave very weirdly if containers restart at different times.
  - In a multisite environment, it takes the `enabled?` setting from the default site. Changes on that site affect all other sites in the cluster.

Instead, `plugin.enabled?` should be checked at runtime, in the context of a request. This commit removes `plugin.enabled?` from many `instance.rb` methods.

I have added a working `plugin.enabled?` implementation for methods that actually affect security/functionality:
  - `post_custom_fields_whitelist`
  - `whitelist_staff_user_custom_field`
  - `add_permitted_post_create_param`

app/controllers/posts_controller.rb

@@ -615,8 +615,8 @@ class PostsController < ApplicationController
       :visible
     ]
 
-    if Post.permitted_create_params.present?
-      permitted.concat(Post.permitted_create_params.to_a)
+    Post.plugin_permitted_create_params.each do |key, plugin|
+      permitted << key if plugin.enabled?
     end
 
     # param munging for WordPress

app/models/post.rb

@@ -19,8 +19,8 @@ class Post < ActiveRecord::Base
   include HasCustomFields
   include LimitedEdit
 
-  cattr_accessor :permitted_create_params
-  self.permitted_create_params = Set.new
+  cattr_accessor :plugin_permitted_create_params
+  self.plugin_permitted_create_params = {}
 
   # increase this number to force a system wide post rebake
   # Version 1, was the initial version

lib/plugin/instance.rb

@@ -92,26 +92,26 @@ class Plugin::Instance
     end
   end
 
+  # Applies to all sites in a multisite environment. Ignores plugin.enabled?
   def add_report(name, &block)
     reloadable_patch do |plugin|
-      if plugin.enabled?
-        Report.add_report(name, &block)
-      end
+      Report.add_report(name, &block)
     end
   end
 
+  # Applies to all sites in a multisite environment. Ignores plugin.enabled?
   def replace_flags
     settings = ::FlagSettings.new
     yield settings
 
     reloadable_patch do |plugin|
-      ::PostActionType.replace_flag_settings(settings) if plugin.enabled?
+      ::PostActionType.replace_flag_settings(settings)
     end
   end
 
   def whitelist_staff_user_custom_field(field)
     reloadable_patch do |plugin|
-      ::User.register_plugin_staff_custom_field(field, plugin) if plugin.enabled?
+      ::User.register_plugin_staff_custom_field(field, plugin) # plugin.enabled? is checked at runtime
     end
   end
 
@@ -122,9 +122,10 @@ class Plugin::Instance
     end
   end
 
+  # Applies to all sites in a multisite environment. Ignores plugin.enabled?
   def add_body_class(class_name)
     reloadable_patch do |plugin|
-      ::ApplicationHelper.extra_body_classes << class_name if plugin.enabled?
+      ::ApplicationHelper.extra_body_classes << class_name
     end
   end
 
@@ -180,27 +181,34 @@ class Plugin::Instance
     end
   end
 
+  # Add a post_custom_fields_whitelister block to the TopicView, respecting if the plugin is enabled
   def topic_view_post_custom_fields_whitelister(&block)
     reloadable_patch do |plugin|
-      ::TopicView.add_post_custom_fields_whitelister(&block) if plugin.enabled?
+      ::TopicView.add_post_custom_fields_whitelister do |user|
+        return [] unless plugin.enabled?
+        block.call(user)
+      end
     end
   end
 
+  # Applies to all sites in a multisite environment. Ignores plugin.enabled?
   def add_preloaded_group_custom_field(field)
     reloadable_patch do |plugin|
-      ::Group.preloaded_custom_field_names << field if plugin.enabled?
+      ::Group.preloaded_custom_field_names << field
     end
   end
 
+  # Applies to all sites in a multisite environment. Ignores plugin.enabled?
   def add_preloaded_topic_list_custom_field(field)
     reloadable_patch do |plugin|
-      ::TopicList.preloaded_custom_fields << field if plugin.enabled?
+      ::TopicList.preloaded_custom_fields << field
     end
   end
 
+  # Add a permitted_create_param to Post, respecting if the plugin is enabled
   def add_permitted_post_create_param(name)
     reloadable_patch do |plugin|
-      ::Post.permitted_create_params << name if plugin.enabled?
+      ::Post.plugin_permitted_create_params[name] = plugin
     end
   end
 
@@ -286,27 +294,31 @@ class Plugin::Instance
     end
   end
 
+  # Applies to all sites in a multisite environment. Ignores plugin.enabled?
   def register_category_custom_field_type(name, type)
     reloadable_patch do |plugin|
-      Category.register_custom_field_type(name, type) if plugin.enabled?
+      Category.register_custom_field_type(name, type)
     end
   end
 
+  # Applies to all sites in a multisite environment. Ignores plugin.enabled?
   def register_topic_custom_field_type(name, type)
     reloadable_patch do |plugin|
-      ::Topic.register_custom_field_type(name, type) if plugin.enabled?
+      ::Topic.register_custom_field_type(name, type)
     end
   end
 
+  # Applies to all sites in a multisite environment. Ignores plugin.enabled?
   def register_post_custom_field_type(name, type)
     reloadable_patch do |plugin|
-      ::Post.register_custom_field_type(name, type) if plugin.enabled?
+      ::Post.register_custom_field_type(name, type)
     end
   end
 
+  # Applies to all sites in a multisite environment. Ignores plugin.enabled?
   def register_group_custom_field_type(name, type)
     reloadable_patch do |plugin|
-      ::Group.register_custom_field_type(name, type) if plugin.enabled?
+      ::Group.register_custom_field_type(name, type)
     end
   end