only read webpack/gulp manifests 1x/req even in dev

We already have logic in here to only read these once ever in prod
but in dev mode we were reading the files multiple times. We purposely
don’t want to cache it for the entire life of the rails process in dev
mode so that if you change anything and have gulp or webpack running
in watch mode, it picks up on your change. but the way it was, it would
read these json files multiple times per request. I didn’t care
because it was dev mode. But this should make even dev mode a little 
faster while still being able to handle picking up any changes between
requests

Change-Id: Ic0ee13df1727e998b828eec012fc2701cf5f8ab7
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/218100
Tested-by: Jenkins
Tested-by: Service Cloud Jenkins <[email protected]>
Reviewed-by: Clay Diffrient <[email protected]>
QA-Review: Steven Burnett <[email protected]>
Product-Review: Ryan Shaw <[email protected]>

lib/canvas/cdn/rev_manifest.rb

@@ -22,6 +22,12 @@ module Canvas
   module Cdn
     module RevManifest
       class << self
+        include ActiveSupport::Benchmarkable
+
+        # ActiveSupport::Benchmarkable#benchmark needs a `logger` defined
+        def logger
+          Rails.logger
+        end
 
         def include?(source)
           if webpack_request?(source)
@@ -93,27 +99,35 @@ def url_for(source)
         private
         def load_gulp_data_if_needed
           return if ActionController::Base.perform_caching && defined? @gulp_manifest
-          file = Rails.root.join('public', 'dist', 'rev-manifest.json')
-          if file.exist?
-            Rails.logger.debug "reading rev-manifest.json"
-            @gulp_manifest = JSON.parse(file.read).freeze
-          elsif Rails.env.production?
-            raise "you need to run `gulp rev` first"
-          else
-            @gulp_manifest = {}.freeze
+          RequestCache.cache("rev-manifest") do
+            benchmark("reading rev-manifest") do
+              file = Rails.root.join('public', 'dist', 'rev-manifest.json')
+              if file.exist?
+                Rails.logger.debug "reading rev-manifest.json"
+                @gulp_manifest = JSON.parse(file.read).freeze
+              elsif Rails.env.production?
+                raise "you need to run `gulp rev` first"
+              else
+                @gulp_manifest = {}.freeze
+              end
+              @gulp_revved_urls = Set.new(@gulp_manifest.values.map{|s| "/dist/#{s}" }).freeze
+            end
           end
-          @gulp_revved_urls = Set.new(@gulp_manifest.values.map{|s| "/dist/#{s}" }).freeze
         end
 
         def load_webpack_data_if_needed
           return if (ActionController::Base.perform_caching || webpack_prod?) && defined? @webpack_manifest
-          file = Rails.root.join('public', webpack_dir, 'webpack-manifest.json')
-          if file.exist?
-            Rails.logger.debug "reading #{file}"
-            @webpack_manifest = JSON.parse(file.read).freeze
-          else
-            raise "you need to run webpack" unless Rails.env.test?
-            @webpack_manifest = Hash.new(["Error: you need to run webpack"]).freeze
+          RequestCache.cache("webpack_manifest") do
+            benchmark("reading webpack_manifest") do
+              file = Rails.root.join('public', webpack_dir, 'webpack-manifest.json')
+              if file.exist?
+                Rails.logger.debug "reading #{file}"
+                @webpack_manifest = JSON.parse(file.read).freeze
+              else
+                raise "you need to run webpack" unless Rails.env.test?
+                @webpack_manifest = Hash.new(["Error: you need to run webpack"]).freeze
+              end
+            end
           end
         end