Optimize Projects::JobsController#trace endpoint

Optimizes Projects::JobsController#trace endpoint to:

- Mark the job as being watched only if it's running
- Serialize the job trace only if there is a trace to serialize

app/controllers/projects/jobs_controller.rb

@@ -49,25 +49,25 @@ class Projects::JobsController < Projects::ApplicationController
   # rubocop: enable CodeReuse/ActiveRecord
 
   def trace
+    @build.trace.being_watched! if @build.running?
+
+    if @build.has_trace?
       @build.trace.read do |stream|
         respond_to do |format|
           format.json do
-          @build.trace.being_watched!
-
             build_trace = Ci::BuildTrace.new(
               build: @build,
               stream: stream,
               state: params[:state])
 
-          if @build.has_trace?
             render json: BuildTraceSerializer
               .new(project: @project, current_user: @current_user)
               .represent(build_trace)
-          else
-            head :no_content
           end
         end
       end
+    else
+      head :no_content
     end
   end
 

spec/controllers/projects/jobs_controller_spec.rb

@@ -700,6 +700,9 @@ RSpec.describe Projects::JobsController, :clean_gitlab_redis_shared_state do
         expect(json_response['lines']).to eq [{ 'content' => [{ 'text' => 'BUILD TRACE' }], 'offset' => 0 }]
       end
 
+      context 'when job is running' do
+        let(:job) { create(:ci_build, :trace_live, :running, pipeline: pipeline) }
+
         it 'sets being-watched flag for the job' do
           expect(response).to have_gitlab_http_status(:ok)
 
@@ -707,6 +710,15 @@ RSpec.describe Projects::JobsController, :clean_gitlab_redis_shared_state do
         end
       end
 
+      context 'when job is not running' do
+        it 'does not set being-watched flag for the job' do
+          expect(response).to have_gitlab_http_status(:ok)
+
+          expect(job.trace.being_watched?).to be(false)
+        end
+      end
+    end
+
     context 'when job has no traces' do
       let(:job) { create(:ci_build, pipeline: pipeline) }