Handle external issues in IssueReferenceFilter

In the past this class would use Project#get_issue to retrieve an issue
by its ID. This method would automatically determine whether to return
an Issue or ExternalIssue.

This commit changes IssueReferenceFilter to handle external issues again
and in a somewhat more explicit manner than before.

Fixes gitlab-org/gitlab-ce#18827

lib/banzai/filter/issue_reference_filter.rb

@@ -31,10 +31,14 @@ module Banzai
           projects_per_reference.each do |path, project|
             issue_ids = references_per_project[path]
 
-            next unless project.default_issues_tracker?
+            if project.default_issues_tracker?
+              issues = project.issues.where(iid: issue_ids.to_a)
+            else
+              issues = issue_ids.map { |id| ExternalIssue.new(id, project) }
+            end
 
-            project.issues.where(iid: issue_ids.to_a).each do |issue|
-              hash[project][issue.iid] = issue
+            issues.each do |issue|
+              hash[project][issue.iid.to_i] = issue
             end
           end
 

spec/lib/banzai/filter/issue_reference_filter_spec.rb

@@ -198,4 +198,40 @@ describe Banzai::Filter::IssueReferenceFilter, lib: true do
       expect(doc.to_html).to match(/\(<a.+>Reference<\/a>\.\)/)
     end
   end
+
+  describe '#issues_per_Project' do
+    context 'using an internal issue tracker' do
+      it 'returns a Hash containing the issues per project' do
+        doc = Nokogiri::HTML.fragment('')
+        filter = described_class.new(doc, project: project)
+
+        expect(filter).to receive(:projects_per_reference).
+          and_return({ project.path_with_namespace => project })
+
+        expect(filter).to receive(:references_per_project).
+          and_return({ project.path_with_namespace => Set.new([issue.iid]) })
+
+        expect(filter.issues_per_project).
+          to eq({ project => { issue.iid => issue } })
+      end
+    end
+
+    context 'using an external issue tracker' do
+      it 'returns a Hash containing the issues per project' do
+        doc = Nokogiri::HTML.fragment('')
+        filter = described_class.new(doc, project: project)
+
+        expect(project).to receive(:default_issues_tracker?).and_return(false)
+
+        expect(filter).to receive(:projects_per_reference).
+          and_return({ project.path_with_namespace => project })
+
+        expect(filter).to receive(:references_per_project).
+          and_return({ project.path_with_namespace => Set.new([1]) })
+
+        expect(filter.issues_per_project[project][1]).
+          to be_an_instance_of(ExternalIssue)
+      end
+    end
+  end
 end