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`.

6 files changed, 222 insertions, 1 deletions

diff --git a/lib/banzai/commit_renderer.rb b/lib/banzai/commit_renderer.rb
index f346151a3c1..2acc9d13f07 100644
--- a/lib/banzai/commit_renderer.rb
+++ b/lib/banzai/commit_renderer.rb
@@ -2,7 +2,7 @@
 
 module Banzai
   module CommitRenderer
-    ATTRIBUTES = [:description, :title].freeze
+    ATTRIBUTES = [:description, :title, :full_title].freeze
 
     def self.render(commits, project, user = nil)
       obj_renderer = ObjectRenderer.new(user: user, default_project: project)
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
diff --git a/lib/gitlab/markdown_cache/active_record/extension.rb b/lib/gitlab/markdown_cache/active_record/extension.rb
new file mode 100644
index 00000000000..bcc3432bd31
--- /dev/null
+++ b/lib/gitlab/markdown_cache/active_record/extension.rb
@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module MarkdownCache
+    module ActiveRecord
+      module Extension
+        extend ActiveSupport::Concern
+
+        included do
+          # Always exclude _html fields from attributes (including serialization).
+          # They contain unredacted HTML, which would be a security issue
+          alias_method :attributes_before_markdown_cache, :attributes
+          def attributes
+            attrs = attributes_before_markdown_cache
+            html_fields = cached_markdown_fields.html_fields
+            whitelisted = cached_markdown_fields.html_fields_whitelisted
+            exclude_fields = html_fields - whitelisted
+
+            exclude_fields.each do |field|
+              attrs.delete(field)
+            end
+
+            if whitelisted.empty?
+              attrs.delete('cached_markdown_version')
+            end
+
+            attrs
+          end
+
+          # Using before_update here conflicts with elasticsearch-model somehow
+          before_create :refresh_markdown_cache, if: :invalidated_markdown_cache?
+          before_update :refresh_markdown_cache, if: :invalidated_markdown_cache?
+        end
+
+        def changed_markdown_fields
+          changed_attributes.keys.map(&:to_s) & cached_markdown_fields.markdown_fields.map(&:to_s)
+        end
+
+        def write_markdown_field(field_name, value)
+          write_attribute(field_name, value)
+        end
+
+        def markdown_field_changed?(field_name)
+          attribute_changed?(field_name)
+        end
+
+        def save_markdown(updates)
+          return unless persisted? && Gitlab::Database.read_write?
+
+          update_columns(updates)
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/markdown_cache/field_data.rb b/lib/gitlab/markdown_cache/field_data.rb
new file mode 100644
index 00000000000..14622c0f186
--- /dev/null
+++ b/lib/gitlab/markdown_cache/field_data.rb
@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module MarkdownCache
+    # Knows about the relationship between markdown and html field names, and
+    # stores the rendering contexts for the latter
+    class FieldData
+      def initialize
+        @data = {}
+      end
+
+      delegate :[], :[]=, to: :@data
+
+      def markdown_fields
+        @data.keys
+      end
+
+      def html_field(markdown_field)
+        "#{markdown_field}_html"
+      end
+
+      def html_fields
+        @html_fields ||= markdown_fields.map { |field| html_field(field) }
+      end
+
+      def html_fields_whitelisted
+        markdown_fields.each_with_object([]) do |field, fields|
+          if @data[field].fetch(:whitelisted, false)
+            fields << html_field(field)
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/markdown_cache/redis/extension.rb b/lib/gitlab/markdown_cache/redis/extension.rb
new file mode 100644
index 00000000000..97fc23343b4
--- /dev/null
+++ b/lib/gitlab/markdown_cache/redis/extension.rb
@@ -0,0 +1,63 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module MarkdownCache
+    module Redis
+      module Extension
+        extend ActiveSupport::Concern
+
+        attr_reader :cached_markdown_version
+
+        class_methods do
+          def cache_markdown_field(markdown_field, context = {})
+            super
+
+            # define the `[field]_html` accessor
+            html_field = cached_markdown_fields.html_field(markdown_field)
+            define_method(html_field) do
+              load_cached_markdown unless markdown_data_loaded?
+
+              instance_variable_get("@#{html_field}")
+            end
+          end
+        end
+
+        private
+
+        def save_markdown(updates)
+          markdown_store.save(updates)
+        end
+
+        def write_markdown_field(field_name, value)
+          instance_variable_set("@#{field_name}", value)
+        end
+
+        def markdown_field_changed?(field_name)
+          false
+        end
+
+        def changed_markdown_fields
+          []
+        end
+
+        def cached_markdown
+          @cached_data ||= markdown_store.read
+        end
+
+        def load_cached_markdown
+          cached_markdown.each do |field_name, value|
+            write_markdown_field(field_name, value)
+          end
+        end
+
+        def markdown_data_loaded?
+          cached_markdown_version.present? || markdown_store.loaded?
+        end
+
+        def markdown_store
+          @store ||= Gitlab::MarkdownCache::Redis::Store.new(self)
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/markdown_cache/redis/store.rb b/lib/gitlab/markdown_cache/redis/store.rb
new file mode 100644
index 00000000000..a35eebc6a1b
--- /dev/null
+++ b/lib/gitlab/markdown_cache/redis/store.rb
@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module MarkdownCache
+    module Redis
+      class Store
+        EXPIRES_IN = 1.day
+
+        def initialize(subject)
+          @subject = subject
+          @loaded = false
+        end
+
+        def save(updates)
+          @loaded = false
+
+          Gitlab::Redis::Cache.with do |r|
+            r.mapped_hmset(markdown_cache_key, updates)
+            r.expire(markdown_cache_key, EXPIRES_IN)
+          end
+        end
+
+        def read
+          @loaded = true
+
+          results = Gitlab::Redis::Cache.with do |r|
+            r.mapped_hmget(markdown_cache_key, *fields)
+          end
+          # The value read from redis is a string, so we're converting it back
+          # to an int.
+          results[:cached_markdown_version] = results[:cached_markdown_version].to_i
+          results
+        end
+
+        def loaded?
+          @loaded
+        end
+
+        private
+
+        def fields
+          @fields ||= @subject.cached_markdown_fields.html_fields + [:cached_markdown_version]
+        end
+
+        def markdown_cache_key
+          unless @subject.respond_to?(:id)
+            raise Gitlab::MarkdownCache::UnsupportedClassError,
+                  "This class has no id to use for caching"
+          end
+
+          "markdown_cache:#{@subject.class}:#{@subject.id}"
+        end
+      end
+    end
+  end
+end