nginx is stripping ETags, just use last modified instead

app/controllers/site_customizations_controller.rb

@@ -2,13 +2,19 @@ class SiteCustomizationsController < ApplicationController
   skip_before_filter :preload_json, :check_xhr, :redirect_to_login_if_required
 
   def show
-    version = params["v"]
+    cache_time = request.env["HTTP_IF_MODIFIED_SINCE"]
+    cache_time = Time.rfc2822(cache_time) rescue nil if cache_time
+    stylesheet_time = SiteCustomization.where(key: params[:key].to_s).pluck(:created_at).first
 
-    if version && version == request.headers['If-None-Match']
+    if !stylesheet_time
+      raise Discourse::NotFound
+    end
+
+    if cache_time && stylesheet_time <= cache_time
       return render nothing: true, status: 304
     end
 
-    response.headers["ETag"] = version if version
+    response.headers["Last-Modified"] = stylesheet_time.httpdate
     expires_in 1.year, public: true
     render text: SiteCustomization.stylesheet_contents(params[:key], params[:target] == "mobile" ? :mobile : :desktop),
            content_type: "text/css"

app/controllers/stylesheets_controller.rb

@@ -5,17 +5,11 @@ class StylesheetsController < ApplicationController
 
     target,digest = params[:name].split("_")
     digest_orig = digest
-
-    if digest_orig && digest_orig == request.headers['If-None-Match']
-      return render nothing: true, status: 304
-    end
-
     digest = "_" + digest if digest
 
-    # Security note, safe due to route constraint
-    location = "#{Rails.root}/#{DiscourseStylesheets::CACHE_PATH}/#{target}#{digest}.css"
+    cache_time = request.env["HTTP_IF_MODIFIED_SINCE"]
+    cache_time = Time.rfc2822(cache_time) rescue nil if cache_time
 
-    unless File.exist?(location)
     query = StylesheetCache.where(target: target)
     if digest
       query = query.where(digest: digest_orig)
@@ -23,6 +17,19 @@ class StylesheetsController < ApplicationController
       query = query.order('id desc')
     end
 
+    stylesheet_time = query.pluck(:created_at).first
+    if !stylesheet_time
+      return render nothing: true, status: 404
+    end
+
+    if cache_time && stylesheet_time && stylesheet_time <= cache_time
+      return render nothing: true, status: 304
+    end
+
+    # Security note, safe due to route constraint
+    location = "#{Rails.root}/#{DiscourseStylesheets::CACHE_PATH}/#{target}#{digest}.css"
+
+    unless File.exist?(location)
       if current = query.first
         File.write(location, current.content)
       else
@@ -30,7 +37,7 @@ class StylesheetsController < ApplicationController
       end
     end
 
-    response.headers['ETag'] = digest_orig if digest_orig
+    response.headers['Last-Modified'] = stylesheet_time.httpdate
     expires_in 1.year, public: true unless Rails.env == "development"
     send_file(location, disposition: :inline)