Remove legacy Runners support in /api/v4/jobs/request

In Runner v1.3.0 we've started to send User-Agent header with Runner's
version data. Since GitLab v8.12.0 we've started to use this header to check
if used Runner's version supports 204 status code instead of 404 as a
response when there is no jobs to execute by a Runner.

In APIv4 (introduced in GitLab 9.0.0) will require Runner v9.0.0. And
writing more accurately: GitLab Runner v9.0.0 will require GitLab at
least 9.0.0. Because of such breaking change we are able to switch
entirely to 204 response code and there is no need to do check of
User-Agent.

This commit removes useless code and complexity.

lib/api/helpers/runner.rb

@@ -41,14 +41,6 @@ module API
           (Time.now - current_runner.contacted_at) >= contacted_at_max_age
       end
 
-      def job_not_found!
-        if headers['User-Agent'].to_s =~ /gitlab(-ci-multi)?-runner \d+\.\d+\.\d+(~beta\.\d+\.g[0-9a-f]+)? /
-          no_content!
-        else
-          not_found!
-        end
-      end
-
       def validate_job!(job)
         not_found! unless job
 

lib/api/runner.rb

@@ -63,6 +63,9 @@ module API
     resource :jobs do
       desc 'Request a job' do
         success Entities::JobRequest::Response
+        http_codes [[201, 'Job was scheduled'],
+                    [204, 'No job for Runner'],
+                    [403, 'Forbidden']]
       end
       params do
         requires :token, type: String, desc: %q(Runner's authentication token)
@@ -71,13 +74,13 @@ module API
       end
       post '/request' do
         authenticate_runner!
-        not_found! unless current_runner.active?
+        no_content! unless current_runner.active?
         update_runner_info
 
         if current_runner.is_runner_queue_value_latest?(params[:last_update])
           header 'X-GitLab-Last-Update', params[:last_update]
           Gitlab::Metrics.add_event(:build_not_found_cached)
-          return job_not_found!
+          return no_content!
         end
 
         new_update = current_runner.ensure_runner_queue_value
@@ -91,7 +94,7 @@ module API
           else
             Gitlab::Metrics.add_event(:build_not_found)
             header 'X-GitLab-Last-Update', new_update
-            job_not_found!
+            no_content!
           end
         else
           # We received build that is invalid due to concurrency conflict

spec/requests/api/runner_spec.rb

@@ -248,18 +248,6 @@ describe API::Runner do
             it { expect(response).to have_http_status(204) }
           end
         end
-
-        context "when runner doesn't send version in User-Agent" do
-          let(:user_agent) { 'Go-http-client/1.1' }
-
-          it { expect(response).to have_http_status(404) }
-        end
-
-        context "when runner doesn't have a User-Agent" do
-          let(:user_agent) { nil }
-
-          it { expect(response).to have_http_status(404) }
-        end
       end
 
       context 'when no token is provided' do
@@ -282,10 +270,10 @@ describe API::Runner do
         context 'when Runner is not active' do
           let(:runner) { create(:ci_runner, :inactive) }
 
-          it 'returns 404 error' do
+          it 'returns 204 error' do
             request_job
 
-            expect(response).to have_http_status 404
+            expect(response).to have_http_status 204
           end
         end