Conditionally remove no-cache from Fastly headers (#9631) [deploy]

* Rename FastlyHeaders to CachingHeaders, conditionally experiment with removing no-cache

We suspect that it is safe to remove `no-cache` entirely from the Cache-Control headers that are sent
along to Fastly, but there is no great way to confirm this hypothesis. We're testing this by experimenting
on one single article. We will conditionally default to the same headers that we send to Nginx, and send
those headers to Fastly (by removing `no-cache` and adding in `max-age`, which should be ignored since Fastly
also has Surrogate-Control headers added to it, which include a `max-age`, which Fastly should prioritize
over any other `max-age`.

Once we've confirmed that this doesn't cause issues on the Fastly side, we can remove this check and use
the same Cache-Control headers for both Fastly and Nginx.

* Add check for request.env[REQUEST_PATH].present?

app/controllers/application_controller.rb

@@ -6,7 +6,7 @@ class ApplicationController < ActionController::Base
   include SessionCurrentUser
   include ValidRequest
   include Pundit
-  include FastlyHeaders
+  include CachingHeaders
   include ImageUploads
   include VerifySetupCompleted
 

app/controllers/concerns/caching_headers.rb

@@ -1,4 +1,4 @@
-module FastlyHeaders
+module CachingHeaders
   extend ActiveSupport::Concern
 
   # The following is from the now deprecated fastly-rails gem
@@ -12,11 +12,21 @@ module FastlyHeaders
   # custom config example:
   #  {cache_control: 'public, no-cache, maxage=xyz', surrogate_control: 'max-age: 100'}
   def set_cache_control_headers(
-    max_age = 1.day.to_i, cache_control: "public, no-cache",
+    max_age = 1.day.to_i,
     surrogate_control: nil, stale_while_revalidate: nil, stale_if_error: 26_400
   )
-    request.session_options[:skip] = true # no cookies
 
+    # TODO: [@forem/delightful]: Once we've tested that removing `no-cache` from the Cache-Control headers
+    # does not adversely affect fastly, we should remove this specific check entirely.
+    article_requested = request.env["REQUEST_PATH"].present? &&
+      request.env["REQUEST_PATH"].include?("the-3-biggest-misconceptions-about-diversity-3hkm")
+    cache_control = if article_requested
+                      "public, max-age=86400"
+                    else
+                      "public, no-cache"
+                    end
+
+    request.session_options[:skip] = true # no cookies
     response.headers["Cache-Control"] = cache_control
 
     response.headers["Surrogate-Control"] = surrogate_control.presence || build_surrogate_control(