Improve commit builds API endpoint RESTful behavior

1. Return 404 if commit is not found (RESTful resource not found)
2. Return an empty array if pipeline is not found (resource present, no
   associated builds found)
3. Return an empty array if pipeline found but no builds there (resource
   present, no associated builds)

lib/api/builds.rb

@@ -33,8 +33,10 @@ module API
       get ':id/repository/commits/:sha/builds' do
         authorize_read_builds!
 
+        return not_found! unless user_project.commit(params[:sha])
+
         pipelines = user_project.pipelines.where(sha: params[:sha])
-        return not_found! if pipelines.empty?
+        return [] if pipelines.empty?
 
         builds = user_project.builds.where(pipeline: pipelines).order('id DESC')
         builds = filter_builds(builds, params[:scope])

spec/requests/api/builds_spec.rb

@@ -63,6 +63,19 @@ describe API::API, api: true  do
   end
 
   describe 'GET /projects/:id/repository/commits/:sha/builds' do
+    context 'when commit does not exist in repository' do
+      before do
+        get api("/projects/#{project.id}/repository/commits/1a271fd1/builds", api_user)
+      end
+
+      it 'responds with 404' do
+        expect(response).to have_http_status(404)
+      end
+    end
+
+    context 'when commit exists in repository' do
+      context 'when user is authorized' do
+        context 'when pipeline has builds' do
           before do
             create(:ci_pipeline, project: project, sha: project.commit.id)
             create(:ci_build, pipeline: pipeline)
@@ -71,7 +84,6 @@ describe API::API, api: true  do
             get api("/projects/#{project.id}/repository/commits/#{project.commit.id}/builds", api_user)
           end
 
-    context 'authorized user' do
           it 'should return project builds for specific commit' do
             expect(response).to have_http_status(200)
             expect(json_response).to be_an Array
@@ -79,11 +91,32 @@ describe API::API, api: true  do
           end
         end
 
-    context 'unauthorized user' do
-      let(:api_user) { nil }
+        context 'when pipeline has no builds' do
+          before do
+            branch_head = project.commit('feature').id
+            get api("/projects/#{project.id}/repository/commits/#{branch_head}/builds", api_user)
+          end
+
+          it 'returns an empty array' do
+            expect(response).to have_http_status(200)
+            expect(json_response).to be_an Array
+            expect(json_response).to be_empty
+          end
+        end
+      end
+
+      context 'when user is not authorized' do
+        before do
+          create(:ci_pipeline, project: project, sha: project.commit.id)
+          create(:ci_build, pipeline: pipeline)
+
+          get api("/projects/#{project.id}/repository/commits/#{project.commit.id}/builds", nil)
+        end
 
         it 'should not return project builds' do
           expect(response).to have_http_status(401)
+          expect(json_response.except('message')).to be_empty
+        end
       end
     end
   end