1 files changed, 53 insertions, 41 deletions

diff --git a/app/models/concerns/participable.rb b/app/models/concerns/participable.rb
index fc6f83b918b..9056722f45e 100644
--- a/app/models/concerns/participable.rb
+++ b/app/models/concerns/participable.rb
@@ -3,8 +3,6 @@
 # Contains functionality related to objects that can have participants, such as
 # an author, an assignee and people mentioned in its description or comments.
 #
-# Used by Issue, Note, MergeRequest, Snippet and Commit.
-#
 # Usage:
 #
 #     class Issue < ActiveRecord::Base
@@ -12,22 +10,36 @@
 #
 #       # ...
 #
-#       participant :author, :assignee, :notes, ->(current_user) { mentioned_users(current_user) }
+#       participant :author
+#       participant :assignee
+#       participant :notes
+#
+#       participant -> (current_user, ext) do
+#         ext.analyze('...')
+#       end
 #     end
 #
 #     issue = Issue.last
 #     users = issue.participants
-#     # `users` will contain the issue's author, its assignee,
-#     # all users returned by its #mentioned_users method,
-#     # as well as all participants to all of the issue's notes,
-#     # since Note implements Participable as well.
-#
 module Participable
   extend ActiveSupport::Concern
 
   module ClassMethods
-    def participant(*attrs)
-      participant_attrs.concat(attrs)
+    # Adds a list of participant attributes. Attributes can either be symbols or
+    # Procs.
+    #
+    # When using a Proc instead of a Symbol the Proc will be given two
+    # arguments:
+    #
+    # 1. The current user (as an instance of User)
+    # 2. An instance of `Gitlab::ReferenceExtractor`
+    #
+    # It is expected that a Proc populates the given reference extractor
+    # instance with data. The return value of the Proc is ignored.
+    #
+    # attr - The name of the attribute or a Proc
+    def participant(attr)
+      participant_attrs << attr
     end
 
     def participant_attrs
@@ -35,42 +47,42 @@ module Participable
     end
   end
 
-  # Be aware that this method makes a lot of sql queries.
-  # Save result into variable if you are going to reuse it inside same request
-  def participants(current_user = self.author)
-    participants =
-      Gitlab::ReferenceExtractor.lazily do
-        self.class.participant_attrs.flat_map do |attr|
-          value =
-            if attr.respond_to?(:call)
-              instance_exec(current_user, &attr)
-            else
-              send(attr)
-            end
+  # Returns the users participating in a discussion.
+  #
+  # This method processes attributes of objects in breadth-first order.
+  #
+  # Returns an Array of User instances.
+  def participants(current_user = nil)
+    current_user ||= author
+    ext = Gitlab::ReferenceExtractor.new(project, current_user)
+    participants = Set.new
+    process = [self]
 
-          participants_for(value, current_user)
-        end.compact.uniq
-      end
+    until process.empty?
+      source = process.pop
 
-    unless Gitlab::ReferenceExtractor.lazy?
-      participants.select! do |user|
-        user.can?(:read_project, project)
+      case source
+      when User
+        participants << source
+      when Participable
+        source.class.participant_attrs.each do |attr|
+          if attr.respond_to?(:call)
+            source.instance_exec(current_user, ext, &attr)
+          else
+            process << source.__send__(attr)
+          end
+        end
+      when Enumerable, ActiveRecord::Relation
+        # This uses reverse_each so we can use "pop" to get the next value to
+        # process (in order). Using unshift instead of pop would require
+        # moving all Array values one index to the left (which can be
+        # expensive).
+        source.reverse_each { |obj| process << obj }
       end
     end
 
-    participants
-  end
-
-  private
+    participants.merge(ext.users)
 
-  def participants_for(value, current_user = nil)
-    case value
-    when User, Banzai::LazyReference
-      [value]
-    when Enumerable, ActiveRecord::Relation
-      value.flat_map { |v| participants_for(v, current_user) }
-    when Participable
-      value.participants(current_user)
-    end
+    Ability.users_that_can_read_project(participants.to_a, project)
   end
 end