Use etag caching for environments JSON

app/controllers/projects/environments_controller.rb

@@ -15,6 +15,8 @@ class Projects::EnvironmentsController < Projects::ApplicationController
     respond_to do |format|
       format.html
       format.json do
+        Gitlab::PollingInterval.set_header(response, interval: 15_000)
+
         render json: {
           environments: EnvironmentSerializer
             .new(project: @project, current_user: @current_user)

app/models/environment.rb

@@ -57,6 +57,10 @@ class Environment < ActiveRecord::Base
 
     state :available
     state :stopped
+
+    after_transition do |environment, _|
+      environment.expire_etag_cache
+    end
   end
 
   def predefined_variables
@@ -205,4 +209,12 @@ class Environment < ActiveRecord::Base
   def random_suffix
     (0..5).map { SUFFIX_CHARS.sample }.join
   end
+
+  def expire_etag_cache
+    Gitlab::EtagCaching::Store.new.tap do |store|
+      store.touch(
+        Gitlab::Routing.url_helpers.namespace_project_environments_path(project)
+      )
+    end
+  end
 end

lib/gitlab/etag_caching/router.rb

@@ -9,7 +9,8 @@ module Gitlab
       #   - Ending in `noteable/issue/<id>/notes` for the `issue_notes` route
       #   - Ending in `issues/id`/rendered_title` for the `issue_title` route
       USED_IN_ROUTES = %w[noteable issue notes issues rendered_title
-                          commit pipelines merge_requests new].freeze
+                          commit pipelines merge_requests new
+                          environments].freeze
       RESERVED_WORDS = DynamicPathValidator::WILDCARD_ROUTES - USED_IN_ROUTES
       RESERVED_WORDS_REGEX = Regexp.union(*RESERVED_WORDS)
       ROUTES = [
@@ -40,6 +41,10 @@ module Gitlab
         Gitlab::EtagCaching::Router::Route.new(
           %r(^(?!.*(#{RESERVED_WORDS})).*/pipelines/\d+\.json\z),
           'project_pipeline'
+        ),
+        Gitlab::EtagCaching::Router::Route.new(
+          %r(^(?!.*(#{RESERVED_WORDS})).*/environments\.json\z),
+          'environments'
         )
       ].freeze
 

spec/controllers/projects/environments_controller_spec.rb

@@ -76,6 +76,26 @@ describe Projects::EnvironmentsController do
           expect(json_response['stopped_count']).to eq 1
         end
       end
+
+      context "when using etag caching" do
+        before do
+          RequestStore.begin!
+        end
+
+        after do
+          RequestStore.end!
+          RequestStore.clear!
+        end
+
+        it "limits the queries being executed" do
+          control_count = ActiveRecord::QueryRecorder.new { get :index, environment_params }.count
+
+          expect do
+            get :index, environment_params
+            get :index, environment_params
+          end.not_to exceed_query_limit(control_count)
+        end
+      end
     end
   end