Link issues from comments and automatically close them

Any mention of Issues, MergeRequests, or Commits via GitLab-flavored markdown
references in descriptions, titles, or attached Notes creates a back-reference
Note that links to the original referencer. Furthermore, pushing commits with
commit messages that match a (configurable) regexp to a project's default
branch will close any issues mentioned by GFM in the matched closing phrase.
If accepting a merge request would close any Issues in this way, a banner is
appended to the merge request's main panel to indicate this.

1 files changed, 34 insertions, 2 deletions

diff --git a/app/models/note.rb b/app/models/note.rb
index 7598978ad4d..e819a5516b5 100644
--- a/app/models/note.rb
+++ b/app/models/note.rb
@@ -24,6 +24,7 @@ class Note < ActiveRecord::Base
 
   attr_accessible :note, :noteable, :noteable_id, :noteable_type, :project_id,
                   :attachment, :line_code, :commit_id
+  attr_mentionable :note
 
   belongs_to :project
   belongs_to :noteable, polymorphic: true
@@ -54,15 +55,36 @@ class Note < ActiveRecord::Base
   serialize :st_diff
   before_create :set_diff, if: ->(n) { n.line_code.present? }
 
-  def self.create_status_change_note(noteable, project, author, status)
+  def self.create_status_change_note(noteable, project, author, status, source)
+    body = "_Status changed to #{status}#{' by ' + source.gfm_reference if source}_"
+
+    create({
+      noteable: noteable,
+      project: project,
+      author: author,
+      note: body,
+      system: true
+    }, without_protection: true)
+  end
+
+  # +noteable+ was referenced from +mentioner+, by including GFM in either +mentioner+'s description or an associated Note.
+  # Create a system Note associated with +noteable+ with a GFM back-reference to +mentioner+.
+  def self.create_cross_reference_note(noteable, mentioner, author, project)
     create({
       noteable: noteable,
+      commit_id: (noteable.sha if noteable.respond_to? :sha),
       project: project,
       author: author,
-      note: "_Status changed to #{status}_"
+      note: "_mentioned in #{mentioner.gfm_reference}_",
+      system: true
     }, without_protection: true)
   end
 
+  # Determine whether or not a cross-reference note already exists.
+  def self.cross_reference_exists?(noteable, mentioner)
+    where(noteable_id: noteable.id, system: true, note: "_mentioned in #{mentioner.gfm_reference}_").any?
+  end
+
   def commit_author
     @commit_author ||=
       project.users.find_by_email(noteable.author_email) ||
@@ -191,6 +213,16 @@ class Note < ActiveRecord::Base
     for_issue? || (for_merge_request? && !for_diff_line?)
   end
 
+  # Mentionable override.
+  def gfm_reference
+    noteable.gfm_reference
+  end
+
+  # Mentionable override.
+  def local_reference
+    noteable
+  end
+
   def noteable_type_name
     if noteable_type.present?
       noteable_type.downcase