Cache default_issues_tracker? in Banzai

Every object processed by ExternalIssueReferenceFilter can return a
different Project instance when calling "project". For example, every
note processed will have it's own associated Project. If we were to
cache Project#default_issues_tracker? on Project level this would have
no impact on Markdown rendering timings as the cache would have to be
built for every Project instance without it ever being re-used.

To work around this we cache Project#default_issues_tracker? in
Banzai::Filter::ExternalIssueReferenceFilter using the project's _id_
instead of the whole object. This setup allows re-using of the cached
data even when the Project instances used are different, as long as the
actual project IDs are the same.

lib/banzai/filter/external_issue_reference_filter.rb

@@ -35,7 +35,7 @@ module Banzai
 
       def call
         # Early return if the project isn't using an external tracker
-        return doc if project.nil? || project.default_issues_tracker?
+        return doc if project.nil? || default_issues_tracker?
 
         ref_pattern = ExternalIssue.reference_pattern
         ref_start_pattern = /\A#{ref_pattern}\z/
@@ -90,6 +90,21 @@ module Banzai
       def url_for_issue(*args)
         IssuesHelper.url_for_issue(*args)
       end
+
+      def default_issues_tracker?
+        if RequestStore.active?
+          default_issues_tracker_cache[project.id] ||=
+            project.default_issues_tracker?
+        else
+          project.default_issues_tracker?
+        end
+      end
+
+      private
+
+      def default_issues_tracker_cache
+        RequestStore[:banzai_default_issues_tracker_cache] ||= {}
+      end
     end
   end
 end