Merge branch 'feature/cross-project-labels' into 'master'

Add support for cross project references for labels

## Summary

Support for cross project references for labels.

## Rationale

1.   Cross project label references are currently not supported in GitLab
1.   `to_reference` method signature in `Label` model breaks the abstraction introduced in `Referable`.

      `concerns/referable.rb:  def to_reference(_from_project = nil)`

      Signatures:

      ```
      label.rb:           def to_reference(format = :id)

      commit_range.rb:    def to_reference(from_project = nil)
      commit.rb:          def to_reference(from_project = nil)
      external_issue.rb:  def to_reference(_from_project = nil)
      group.rb:           def to_reference(_from_project = nil)
      issue.rb:           def to_reference(from_project = nil)
      merge_request.rb:   def to_reference(from_project = nil)
      milestone.rb:       def to_reference(from_project = nil)
      project.rb:         def to_reference(_from_project = nil)
      snippet.rb:         def to_reference(from_project = nil)
      user.rb:            def to_reference(_from_project = nil)
      ```

     This MR suggests using `def to_reference(from_project = nil, format: :id)` which makes use of keyword arguments and preserves abstract interface.

1.   We need support for cross project label references when we want to move issue to another project

     It may happen that issue description, system notes or comments contain reference to label and this reference will be invalid after moving issue to another project and will not be displayed correctly unless we have support for cross project references.

     Merge request that needs this feature: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2831


I think that cross project label references may be useful, (example: `Hey, see our issues for CI in GitLab CE! - gitab-org/gitlab-ce~"CI"`).

cc @JobV @DouweM @rspeicher 

See merge request !2966

3 files changed, 54 insertions, 5 deletions

diff --git a/spec/fixtures/markdown.md.erb b/spec/fixtures/markdown.md.erb
index fe6d42acee2..1772cc3f6a4 100644
--- a/spec/fixtures/markdown.md.erb
+++ b/spec/fixtures/markdown.md.erb
@@ -209,7 +209,7 @@ References should be parseable even inside _<%= merge_request.to_reference %>_ e
 
 - Label by ID: <%= simple_label.to_reference %>
 - Label by name: <%= Label.reference_prefix %><%= simple_label.name %>
-- Label by name in quotes: <%= label.to_reference(:name) %>
+- Label by name in quotes: <%= label.to_reference(format: :name) %>
 - Ignored in code: `<%= simple_label.to_reference %>`
 - Ignored in links: [Link to <%= simple_label.to_reference %>](#label-link)
 - Link to label by reference: [Label](<%= label.to_reference %>)
diff --git a/spec/lib/banzai/filter/label_reference_filter_spec.rb b/spec/lib/banzai/filter/label_reference_filter_spec.rb
index b46ccc47605..e2d21f53b7e 100644
--- a/spec/lib/banzai/filter/label_reference_filter_spec.rb
+++ b/spec/lib/banzai/filter/label_reference_filter_spec.rb
@@ -111,7 +111,7 @@ describe Banzai::Filter::LabelReferenceFilter, lib: true do
 
   context 'String-based multi-word references in quotes' do
     let(:label)     { create(:label, name: 'gfm references', project: project) }
-    let(:reference) { label.to_reference(:name) }
+    let(:reference) { label.to_reference(format: :name) }
 
     it 'links to a valid reference' do
       doc = reference_filter("See #{reference}")
@@ -176,4 +176,29 @@ describe Banzai::Filter::LabelReferenceFilter, lib: true do
       expect(result[:references][:label]).to eq [label]
     end
   end
+
+  describe 'cross project label references' do
+    let(:another_project)  { create(:empty_project, :public) }
+    let(:project_name) { another_project.name_with_namespace }
+    let(:label) { create(:label, project: another_project, color: '#00ff00') }
+    let(:reference) { label.to_reference(project) }
+
+    let!(:result) { reference_filter("See #{reference}") }
+
+    it 'points to referenced project issues page' do
+      expect(result.css('a').first.attr('href'))
+        .to eq urls.namespace_project_issues_url(another_project.namespace,
+                                                 another_project,
+                                                 label_name: label.name)
+    end
+
+    it 'has valid color' do
+      expect(result.css('a span').first.attr('style'))
+        .to match /background-color: #00ff00/
+    end
+
+    it 'contains cross project content' do
+      expect(result.css('a').first.text).to eq "#{label.name} in #{project_name}"
+    end
+  end
 end
diff --git a/spec/models/label_spec.rb b/spec/models/label_spec.rb
index 696fbf7e0aa..0614ca1e7c9 100644
--- a/spec/models/label_spec.rb
+++ b/spec/models/label_spec.rb
@@ -59,18 +59,42 @@ describe Label, models: true do
     context 'using id' do
       it 'returns a String reference to the object' do
         expect(label.to_reference).to eq "~#{label.id}"
-        expect(label.to_reference(double('project'))).to eq "~#{label.id}"
       end
     end
 
     context 'using name' do
       it 'returns a String reference to the object' do
-        expect(label.to_reference(:name)).to eq %(~"#{label.name}")
+        expect(label.to_reference(format: :name)).to eq %(~"#{label.name}")
       end
 
       it 'uses id when name contains double quote' do
         label = create(:label, name: %q{"irony"})
-        expect(label.to_reference(:name)).to eq "~#{label.id}"
+        expect(label.to_reference(format: :name)).to eq "~#{label.id}"
+      end
+    end
+
+    context 'using invalid format' do
+      it 'raises error' do
+        expect { label.to_reference(format: :invalid) }
+          .to raise_error StandardError, /Unknown format/
+      end
+    end
+
+    context 'cross project reference' do
+      let(:project) { create(:project) }
+
+      context 'using name' do
+        it 'returns cross reference with label name' do
+          expect(label.to_reference(project, format: :name))
+            .to eq %Q(#{label.project.to_reference}~"#{label.name}")
+        end
+      end
+
+      context 'using id' do
+        it 'returns cross reference with label id' do
+          expect(label.to_reference(project, format: :id))
+            .to eq %Q(#{label.project.to_reference}~#{label.id})
+        end
       end
     end
   end