Allow public access to some Tag API endpoints

Signed-off-by: Rémy Coutable <remy@rymai.me>

changelogs/unreleased/public-tags-api.yml

+---
+title: Allow public access to some Tag API endpoints
+merge_request: 
+author: 

doc/api/tags.md

@@ -2,7 +2,9 @@
 
 ## List project repository tags
 
-Get a list of repository tags from a project, sorted by name in reverse alphabetical order.
+Get a list of repository tags from a project, sorted by name in reverse
+alphabetical order. This endpoint can be accessed without authentication if the
+repository is publicly accessible.
 
 ```
 GET /projects/:id/repository/tags
@@ -40,7 +42,8 @@ Parameters:
 
 ## Get a single repository tag
 
-Get a specific repository tag determined by its name.
+Get a specific repository tag determined by its name. This endpoint can be
+accessed without authentication if the repository is publicly accessible.
 
 ```
 GET /projects/:id/repository/tags/:tag_name

lib/api/tags.rb

 module API
   # Git Tags API
   class Tags < Grape::API
-    before { authenticate! }
     before { authorize! :download_code, user_project }
 
     params do

spec/requests/api/tags_spec.rb

@@ -15,6 +15,31 @@ describe API::Tags, api: true  do
     let(:tag_name) { project.repository.tag_names.sort.reverse.first }
     let(:description) { 'Awesome release!' }
 
+    shared_examples_for 'repository tags' do
+      it 'returns the repository tags' do
+        get api("/projects/#{project.id}/repository/tags", current_user)
+
+        expect(response).to have_http_status(200)
+
+        first_tag = json_response.first
+
+        expect(first_tag['name']).to eq(tag_name)
+      end
+    end
+
+    context 'when unauthenticated' do
+      it_behaves_like 'repository tags' do
+        let(:project) { create(:project, :public) }
+        let(:current_user) { nil }
+      end
+    end
+
+    context 'when authenticated' do
+      it_behaves_like 'repository tags' do
+        let(:current_user) { user }
+      end
+    end
+
     context 'without releases' do
       it "returns an array of project tags" do
         get api("/projects/#{project.id}/repository/tags", user)
@@ -45,20 +70,36 @@ describe API::Tags, api: true  do
   describe 'GET /projects/:id/repository/tags/:tag_name' do
     let(:tag_name) { project.repository.tag_names.sort.reverse.first }
 
-    it 'returns a specific tag' do
-      get api("/projects/#{project.id}/repository/tags/#{tag_name}", user)
+    shared_examples_for 'repository tag' do
+      it 'returns the repository tag' do
+        get api("/projects/#{project.id}/repository/tags/#{tag_name}", current_user)
 
         expect(response).to have_http_status(200)
+
         expect(json_response['name']).to eq(tag_name)
       end
 
       it 'returns 404 for an invalid tag name' do
-      get api("/projects/#{project.id}/repository/tags/foobar", user)
+        get api("/projects/#{project.id}/repository/tags/foobar", current_user)
 
         expect(response).to have_http_status(404)
       end
     end
 
+    context 'when unauthenticated' do
+      it_behaves_like 'repository tag' do
+        let(:project) { create(:project, :public) }
+        let(:current_user) { nil }
+      end
+    end
+
+    context 'when authenticated' do
+      it_behaves_like 'repository tag' do
+        let(:current_user) { user }
+      end
+    end
+  end
+
   describe 'POST /projects/:id/repository/tags' do
     context 'lightweight tags' do
       it 'creates a new tag' do