FIX: add protection for scss removal during upgrade

In some cases plugins would remove scss files or change them, but CSS
was still calculated based off stale data in old instance cache

lib/stylesheet/manager.rb

@@ -256,7 +256,18 @@ class Stylesheet::Manager
       raise "attempting to look up theme digest for invalid field"
     end
 
-    Digest::SHA1.hexdigest(scss.to_s + color_scheme_digest.to_s + settings_digest)
+    Digest::SHA1.hexdigest(scss.to_s + color_scheme_digest.to_s + settings_digest + plugins_digest)
+  end
+
+  # this protects us from situations where new versions of a plugin removed a file
+  # old instances may still be serving CSS and not aware of the change
+  # so we could end up poisoning the cache with a bad file that can not be removed
+  def plugins_digest
+    assets = []
+    assets += DiscoursePluginRegistry.stylesheets.to_a
+    assets += DiscoursePluginRegistry.mobile_stylesheets.to_a
+    assets += DiscoursePluginRegistry.desktop_stylesheets.to_a
+    Digest::SHA1.hexdigest(assets.sort.join)
   end
 
   def settings_digest

spec/components/stylesheet/manager_spec.rb

@@ -65,6 +65,30 @@ describe Stylesheet::Manager do
     expect(new_link).to include("/stylesheets/desktop_theme_#{theme.id}_")
   end
 
+  describe 'digest' do
+    after do
+      DiscoursePluginRegistry.stylesheets.delete "fake_file"
+    end
+
+    it 'can correctly account for plugins in digest' do
+
+      theme = Theme.create!(
+        name: 'parent',
+        user_id: -1
+      )
+
+      manager = Stylesheet::Manager.new(:desktop_theme, theme.key)
+      digest1 = manager.digest
+
+      DiscoursePluginRegistry.stylesheets.add "fake_file"
+
+      manager = Stylesheet::Manager.new(:desktop_theme, theme.key)
+      digest2 = manager.digest
+
+      expect(digest1).not_to eq(digest2)
+    end
+  end
+
   describe 'color_scheme_digest' do
     it "changes with category background image" do
       theme = Theme.new(