Always use DiscourseSassImporter, add optional sprockets deps tracking.

app/models/site_customization.rb

@@ -32,7 +32,7 @@ class SiteCustomization < ActiveRecord::Base
       filesystem_importer: DiscourseSassImporter,
       sprockets: {
         context: context,
-        environment: env
+        environment: context.environment
       }
     }).render
 

config/initializers/sprockets.rb

@@ -0,0 +1,4 @@
+require_dependency 'discourse_sass_importer'
+
+Sprockets.send(:remove_const, :SassImporter)
+Sprockets::SassImporter = DiscourseSassImporter

lib/discourse_sass_importer.rb

@@ -4,7 +4,11 @@
 class DiscourseSassImporter < Sass::Importers::Filesystem
   GLOB = /\*|\[.+\]/
 
+  # Depending upon where this is passed we might either be passed a string as the
+  # first argument or a sprockets context. If the first argument is a sprockets
+  # context we store it and use it to mark dependencies.
   def initialize(*args)
+    @context = args.first unless args.first.is_a? String
     @root = Rails.root.join('app', 'assets', 'stylesheets').to_s
     @same_name_warnings = Set.new
   end
@@ -48,6 +52,7 @@ class DiscourseSassImporter < Sass::Importers::Filesystem
   def glob_imports(glob, base_pathname, options)
     contents = ""
     each_globbed_file(glob, base_pathname.dirname, options) do |filename|
+      depend_on(filename)
       unless File.directory?(filename)
         contents << "@import #{Pathname.new(filename).relative_path_from(base_pathname.dirname).to_s.inspect};\n"
       end
@@ -62,6 +67,21 @@ class DiscourseSassImporter < Sass::Importers::Filesystem
 
   private
 
+    def depend_on(filename)
+      if @context
+        @context.depend_on(filename)
+        @context.depend_on(globbed_file_parent(filename))
+      end
+    end
+
+    def globbed_file_parent(filename)
+      if File.directory?(filename)
+        File.expand_path('..', filename)
+      else
+        File.dirname(filename)
+      end
+    end
+
     def engine_from_path(name, dir, options)
       full_filename, syntax = Sass::Util.destructure(find_real_file(dir, name, options))
       return unless full_filename && File.readable?(full_filename)

lib/tasks/assets.rake

@@ -16,9 +16,6 @@ task 'assets:precompile:before' do
   # let's make precompile faster using redis magic
   require 'sprockets'
   require 'digest/sha1'
-  require_dependency 'discourse_sass_importer'
-
-  Sprockets::SassImporter = DiscourseSassImporter
 
   module ::Sprockets