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, 38 insertions, 0 deletions

diff --git a/spec/models/issue_spec.rb b/spec/models/issue_spec.rb
index 6540d77fbc0..87b3d8d650a 100644
--- a/spec/models/issue_spec.rb
+++ b/spec/models/issue_spec.rb
@@ -231,4 +231,42 @@ describe Issue, models: true do
       expect(issue.to_branch_name).to match /confidential-issue\z/
     end
   end
+
+  describe '#participants' do
+    context 'using a public project' do
+      let(:project) { create(:project, :public) }
+      let(:issue) { create(:issue, project: project) }
+
+      let!(:note1) do
+        create(:note_on_issue, noteable: issue, project: project, note: 'a')
+      end
+
+      let!(:note2) do
+        create(:note_on_issue, noteable: issue, project: project, note: 'b')
+      end
+
+      it 'includes the issue author' do
+        expect(issue.participants).to include(issue.author)
+      end
+
+      it 'includes the authors of the notes' do
+        expect(issue.participants).to include(note1.author, note2.author)
+      end
+    end
+
+    context 'using a private project' do
+      it 'does not include mentioned users that do not have access to the project' do
+        project = create(:project)
+        user = create(:user)
+        issue = create(:issue, project: project)
+
+        create(:note_on_issue,
+               noteable: issue,
+               project: project,
+               note: user.to_reference)
+
+        expect(issue.participants).not_to include(user)
+      end
+    end
+  end
 end