Add markdown cache columns to the database, but don't use them yet

This commit adds a number of _html columns and, with the exception of Note,
starts updating them whenever the content of their partner fields changes.

Note has a collision with the note_html attr_accessor; that will be fixed later

A background worker for clearing these cache columns is also introduced - use
`rake cache:clear` to set it off. You can clear the database or Redis caches
separately by running `rake cache:clear:db` or `rake cache:clear:redis`,
respectively.

3 files changed, 59 insertions, 16 deletions

diff --git a/lib/banzai.rb b/lib/banzai.rb
index 9ebe379f454..35ca234c1ba 100644
--- a/lib/banzai.rb
+++ b/lib/banzai.rb
@@ -3,6 +3,10 @@ module Banzai
     Renderer.render(text, context)
   end
 
+  def self.render_field(object, field)
+    Renderer.render_field(object, field)
+  end
+
   def self.cache_collection_render(texts_and_contexts)
     Renderer.cache_collection_render(texts_and_contexts)
   end
diff --git a/lib/banzai/renderer.rb b/lib/banzai/renderer.rb
index a4ae27eefd8..6924a293da8 100644
--- a/lib/banzai/renderer.rb
+++ b/lib/banzai/renderer.rb
@@ -31,6 +31,34 @@ module Banzai
       end
     end
 
+    # Convert a Markdown-containing field on an object into an HTML-safe String
+    # of HTML. This method is analogous to calling render(object.field), but it
+    # can cache the rendered HTML in the object, rather than Redis.
+    #
+    # The context to use is learned from the passed-in object by calling
+    # #banzai_render_context(field), and cannot be changed. Use #render, passing
+    # it the field text, if a custom rendering is needed. The generated context
+    # is returned along with the HTML.
+    def render_field(object, field)
+      html_field = object.markdown_cache_field_for(field)
+
+      html = object.__send__(html_field)
+      return html if html.present?
+
+      html = cacheless_render_field(object, field)
+      object.update_column(html_field, html) unless object.new_record? || object.destroyed?
+
+      html
+    end
+
+    # Same as +render_field+, but without consulting or updating the cache field
+    def cacheless_render_field(object, field)
+      text = object.__send__(field)
+      context = object.banzai_render_context(field)
+
+      cacheless_render(text, context)
+    end
+
     # Perform multiple render from an Array of Markdown String into an
     # Array of HTML-safe String of HTML.
     #
diff --git a/lib/tasks/cache.rake b/lib/tasks/cache.rake
index 2214f855200..a95a3455a4a 100644
--- a/lib/tasks/cache.rake
+++ b/lib/tasks/cache.rake
@@ -1,22 +1,33 @@
 namespace :cache do
-  CLEAR_BATCH_SIZE = 1000 # There seems to be no speedup when pushing beyond 1,000
-  REDIS_SCAN_START_STOP = '0' # Magic value, see http://redis.io/commands/scan
+  namespace :clear do
+    REDIS_CLEAR_BATCH_SIZE = 1000 # There seems to be no speedup when pushing beyond 1,000
+    REDIS_SCAN_START_STOP = '0' # Magic value, see http://redis.io/commands/scan
 
-  desc "GitLab | Clear redis cache"
-  task :clear => :environment do
-    Gitlab::Redis.with do |redis|
-      cursor = REDIS_SCAN_START_STOP
-      loop do
-        cursor, keys = redis.scan(
-          cursor,
-          match: "#{Gitlab::Redis::CACHE_NAMESPACE}*", 
-          count: CLEAR_BATCH_SIZE
-        )
-  
-        redis.del(*keys) if keys.any?
-  
-        break if cursor == REDIS_SCAN_START_STOP
+    desc "GitLab | Clear redis cache"
+    task redis: :environment do
+      Gitlab::Redis.with do |redis|
+        cursor = REDIS_SCAN_START_STOP
+        loop do
+          cursor, keys = redis.scan(
+            cursor,
+            match: "#{Gitlab::Redis::CACHE_NAMESPACE}*",
+            count: REDIS_CLEAR_BATCH_SIZE
+          )
+
+          redis.del(*keys) if keys.any?
+
+          break if cursor == REDIS_SCAN_START_STOP
+        end
       end
     end
+
+    desc "GitLab | Clear database cache (in the background)"
+    task db: :environment do
+      ClearDatabaseCacheWorker.perform_async
+    end
+
+    task all: [:db, :redis]
   end
+
+  task clear: 'cache:clear:all'
 end