FEATURE: when under extreme load disable search

The global setting disable_search_queue_threshold
(DISCOURSE_DISABLE_SEARCH_QUEUE_THRESHOLD) which default to 1 second was
added.

This protection ensures that when the application is unable to keep up with
requests it will simply turn off search till it is not backed up.

To disable this protection set this to 0.

app/controllers/search_controller.rb

@@ -6,6 +6,8 @@ class SearchController < ApplicationController
 
   skip_before_action :check_xhr, only: :show
 
+  before_action :cancel_overloaded_search, only: [:query]
+
   def self.valid_context_types
     %w{user topic category private_messages}
   end
@@ -44,10 +46,14 @@ class SearchController < ApplicationController
     search_args[:ip_address] = request.remote_ip
     search_args[:user_id] = current_user.id if current_user.present?
 
-    search = Search.new(@search_term, search_args)
-    result = search.execute
-
-    result.find_user_data(guardian) if result
+    if site_overloaded?
+      result = Search::GroupedSearchResults.new(search_args[:type_filter], @search_term, context, false, 0)
+      result.error = I18n.t("search.extreme_load_error")
+    else
+      search = Search.new(@search_term, search_args)
+      result = search.execute
+      result.find_user_data(guardian) if result
+    end
 
     serializer = serialize_data(result, GroupedSearchResultSerializer, result: result)
 
@@ -82,8 +88,12 @@ class SearchController < ApplicationController
     search_args[:user_id] = current_user.id if current_user.present?
     search_args[:restrict_to_archetype] = params[:restrict_to_archetype] if params[:restrict_to_archetype].present?
 
-    search = Search.new(params[:term], search_args)
-    result = search.execute
+    if site_overloaded?
+      result = GroupedSearchResults.new(search_args["type_filter"], params[:term], context, false, 0)
+    else
+      search = Search.new(params[:term], search_args)
+      result = search.execute
+    end
     render_serialized(result, GroupedSearchResultSerializer, result: result)
   end
 
@@ -118,6 +128,18 @@ class SearchController < ApplicationController
 
   protected
 
+  def site_overloaded?
+    (queue_time = request.env['REQUEST_QUEUE_SECONDS']) &&
+      (GlobalSetting.disable_search_queue_threshold > 0) &&
+      (queue_time > GlobalSetting.disable_search_queue_threshold)
+  end
+
+  def cancel_overloaded_search
+    if site_overloaded?
+      render_json_error I18n.t("search.extreme_load_error"), status: 409
+    end
+  end
+
   def lookup_search_context
 
     return if params[:skip_context] == "true"