Refactor Participable

There are several changes to this module:

1. The use of an explicit stack in Participable#participants
2. Proc behaviour has been changed
3. Batch permissions checking

== Explicit Stack

Participable#participants no longer uses recursion to process "self" and
all child objects, instead it uses an Array and processes objects in
breadth-first order. This allows us to for example create a single
Gitlab::ReferenceExtractor instance and pass this to any Procs. Re-using
a ReferenceExtractor removes the need for running potentially many SQL
queries every time a Proc is called on a new object.

== Proc Behaviour Changed

Previously a Proc in Participable was expected to return an Array of
User instances. This has been changed and instead it's now expected that
a Proc modifies the Gitlab::ReferenceExtractor passed to it. The return
value of the Proc is ignored.

== Permissions Checking

The method Participable#participants uses
Ability.users_that_can_read_project to check if the returned users have
access to the project of "self" _without_ running multiple SQL queries
for every user.

1 files changed, 13 insertions, 10 deletions

diff --git a/app/models/issue.rb b/app/models/issue.rb
index 2d4a9b9f19a..bd0fbc96d18 100644
--- a/app/models/issue.rb
+++ b/app/models/issue.rb
@@ -95,14 +95,13 @@ class Issue < ActiveRecord::Base
   end
 
   def referenced_merge_requests(current_user = nil)
-    @referenced_merge_requests ||= {}
-    @referenced_merge_requests[current_user] ||= begin
-      Gitlab::ReferenceExtractor.lazily do
-        [self, *notes].flat_map do |note|
-          note.all_references(current_user).merge_requests
-        end
-      end.sort_by(&:iid).uniq
+    ext = all_references(current_user)
+
+    notes_with_associations.each do |object|
+      object.all_references(current_user, extractor: ext)
     end
+
+    ext.merge_requests.sort_by(&:iid)
   end
 
   # All branches containing the current issue's ID, except for
@@ -139,9 +138,13 @@ class Issue < ActiveRecord::Base
   def closed_by_merge_requests(current_user = nil)
     return [] unless open?
 
-    notes.system.flat_map do |note|
-      note.all_references(current_user).merge_requests
-    end.uniq.select { |mr| mr.open? && mr.closes_issue?(self) }
+    ext = all_references(current_user)
+
+    notes.system.each do |note|
+      note.all_references(current_user, extractor: ext)
+    end
+
+    ext.merge_requests.select { |mr| mr.open? && mr.closes_issue?(self) }
   end
 
   def moved?