Refactor TrendingProjectsFinder to support caching

== Public Projects

This finder class now _only_ returns public projects. Previously this
finder would also return private and internal projects. Including these
projects makes caching data much harder and less efficient. Meanwhile
including this data isn't very useful as very few users would be
interested in seeing projects they have access to as trending. That is,
the feature is more useful when you want to see what _other_ popular
projects there are.

== Caching

The data returned by TrendingProjectsFinder is now cached for a day
based on the number of months the data should be restricted to. The
cache is not flushed explicitly, instead it's rebuilt whenever it
expires.

== Timings

To measure the impact I changed the finder code to use the last 24
months instead of the last month. I then executed and measured 10
requests to the explore page. On the current "master" branch (commit
88fa5916ffa0aea491cd339272ed7437c3f52dc7) this would take an average of
2.43 seconds. Using the changes of this commit this was reduced to
around 1.7 seconds.

Fixes gitlab-org/gitlab-ce#22164

1 files changed, 9 insertions, 4 deletions

diff --git a/app/finders/trending_projects_finder.rb b/app/finders/trending_projects_finder.rb
index 81a12403801..c1e434d9926 100644
--- a/app/finders/trending_projects_finder.rb
+++ b/app/finders/trending_projects_finder.rb
@@ -1,11 +1,16 @@
+# Finder for retrieving public trending projects in a given time range.
 class TrendingProjectsFinder
-  def execute(current_user, start_date = 1.month.ago)
-    projects_for(current_user).trending(start_date)
+  # current_user - The currently logged in User, if any.
+  # last_months - The number of months to limit the trending data to.
+  def execute(months_limit = 1)
+    Rails.cache.fetch(cache_key_for(months_limit), expires_in: 1.day) do
+      Project.public_only.trending(months_limit.months.ago)
+    end
   end
 
   private
 
-  def projects_for(current_user)
-    ProjectsFinder.new.execute(current_user)
+  def cache_key_for(months)
+    "trending_projects/#{months}"
   end
 end