1 files changed, 131 insertions, 0 deletions
diff --git a/app/models/concerns/cache_markdown_field.rb b/app/models/concerns/cache_markdown_field.rb
new file mode 100644
index 00000000000..90bd6490a02
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+++ b/app/models/concerns/cache_markdown_field.rb
@@ -0,0 +1,131 @@
+# This module takes care of updating cache columns for Markdown-containing
+# fields. Use like this in the body of your class:
+#
+#     include CacheMarkdownField
+#     cache_markdown_field :foo
+#     cache_markdown_field :bar
+#     cache_markdown_field :baz, pipeline: :single_line
+#
+# Corresponding foo_html, bar_html and baz_html fields should exist.
+module CacheMarkdownField
+  # Knows about the relationship between markdown and html field names, and
+  # stores the rendering contexts for the latter
+  class FieldData
+    extend Forwardable
+
+    def initialize
+      @data = {}
+    end
+
+    def_delegators :@data, :[], :[]=
+    def_delegator :@data, :keys, :markdown_fields
+
+    def html_field(markdown_field)
+      "#{markdown_field}_html"
+    end
+
+    def html_fields
+      markdown_fields.map {|field| html_field(field) }
+    end
+  end
+
+  # Dynamic registries don't really work in Rails as it's not guaranteed that
+  # every class will be loaded, so hardcode the list.
+  CACHING_CLASSES = %w[
+    AbuseReport
+    Appearance
+    ApplicationSetting
+    BroadcastMessage
+    Issue
+    Label
+    MergeRequest
+    Milestone
+    Namespace
+    Note
+    Project
+    Release
+    Snippet
+  ]
+
+  def self.caching_classes
+    CACHING_CLASSES.map(&:constantize)
+  end
+
+  extend ActiveSupport::Concern
+
+  included do
+    cattr_reader :cached_markdown_fields do
+      FieldData.new
+    end
+
+    # Returns the default Banzai render context for the cached markdown field.
+    def banzai_render_context(field)
+      raise ArgumentError.new("Unknown field: #{field.inspect}") unless
+        cached_markdown_fields.markdown_fields.include?(field)
+
+      # Always include a project key, or Banzai complains
+      project = self.project if self.respond_to?(:project)
+      context = cached_markdown_fields[field].merge(project: project)
+
+      # Banzai is less strict about authors, so don't always have an author key
+      context[:author] = self.author if self.respond_to?(:author)
+
+      context
+    end
+
+    # Allow callers to look up the cache field name, rather than hardcoding it
+    def markdown_cache_field_for(field)
+      raise ArgumentError.new("Unknown field: #{field}") unless
+        cached_markdown_fields.markdown_fields.include?(field)
+
+      cached_markdown_fields.html_field(field)
+    end
+
+    # 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
+
+      cached_markdown_fields.html_fields.each do |field|
+        attrs.delete(field)
+      end
+
+      attrs
+    end
+  end
+
+  class_methods do
+    private
+
+    # Specify that a field is markdown. Its rendered output will be cached in
+    # a corresponding _html field. Any custom rendering options may be provided
+    # as a context.
+    def cache_markdown_field(markdown_field, context = {})
+      raise "Add #{self} to CacheMarkdownField::CACHING_CLASSES" unless
+        CacheMarkdownField::CACHING_CLASSES.include?(self.to_s)
+
+      cached_markdown_fields[markdown_field] = context
+
+      html_field = cached_markdown_fields.html_field(markdown_field)
+      cache_method = "#{markdown_field}_cache_refresh".to_sym
+      invalidation_method = "#{html_field}_invalidated?".to_sym
+
+      define_method(cache_method) do
+        html = Banzai::Renderer.cacheless_render_field(self, markdown_field)
+        __send__("#{html_field}=", html)
+        true
+      end
+
+      # The HTML becomes invalid if any dependent fields change. For now, assume
+      # author and project invalidate the cache in all circumstances.
+      define_method(invalidation_method) do
+        changed_fields = changed_attributes.keys
+        invalidations = changed_fields & [markdown_field.to_s, "author", "project"]
+        !invalidations.empty?
+      end
+
+      before_save cache_method, if: invalidation_method
+    end
+  end
+end