Use Redis for CacheMarkDownField on non AR models

This allows using `CacheMarkdownField` for models that are not backed
by ActiveRecord.

When the including class inherits `ActiveRecord::Base` we include
`Gitlab::MarkdownCache::ActiveRecord::Extension`. This will cause the
markdown fields to be rendered and the generated HTML stored in a
`<field>_html` attribute on the record. We also store the version
used for generating the markdown.

All other classes that include this model will include the
`Gitlab::MarkdownCache::Redis::Extension`. This add the `<field>_html`
attributes to that model and will generate the html in them. The
generated HTML will be cached in redis under the key
`markdown_cache:<class>:<id>`. The class this included in must
therefore respond to `id`.

1 files changed, 12 insertions, 0 deletions

diff --git a/lib/gitlab/markdown_cache.rb b/lib/gitlab/markdown_cache.rb
new file mode 100644
index 00000000000..0354c710a3f
--- /dev/null
+++ b/lib/gitlab/markdown_cache.rb
@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module MarkdownCache
+    # Increment this number every time the renderer changes its output
+    CACHE_COMMONMARK_VERSION_START  = 10
+    CACHE_COMMONMARK_VERSION        = 16
+
+    BaseError = Class.new(StandardError)
+    UnsupportedClassError = Class.new(BaseError)
+  end
+end