Add incremental build trace update API

app/models/ci/build.rb

@@ -231,11 +231,23 @@ module Ci
     end
 
     def trace=(trace)
+      recreate_trace_dir
+      File.write(path_to_trace, trace)
+    end
+
+    def recreate_trace_dir
       unless Dir.exists?(dir_to_trace)
         FileUtils.mkdir_p(dir_to_trace)
       end
+    end
+    private :recreate_trace_dir
 
-      File.write(path_to_trace, trace)
+    def append_trace(trace_part)
+      recreate_trace_dir
+
+      File.open(path_to_trace, 'a') do |f|
+        f.write(trace_part)
+      end
     end
 
     def dir_to_trace

lib/ci/api/builds.rb

@@ -50,6 +50,15 @@ module Ci
           end
         end
 
+        patch ":id/trace.txt" do
+          authenticate_runner!
+          update_runner_last_contact
+          build = Ci::Build.where(runner_id: current_runner.id).running.find(params[:id])
+          forbidden!('Build has been erased!') if build.erased?
+
+          build.append_trace(params[:trace_part])
+        end
+
         # Authorize artifacts uploading for build - Runners only
         #
         # Parameters:

spec/requests/ci/api/builds_spec.rb

@@ -156,6 +156,28 @@ describe Ci::API::API do
       end
     end
 
+    describe 'PATCH /builds/:id/trace.txt' do
+      let(:build) { create(:ci_build, :trace, runner_id: runner.id) }
+
+      before do
+        build.run!
+        patch ci_api("/builds/#{build.id}/trace.txt"), trace_part: ' appended', token: runner.token
+      end
+
+      it 'should append trace part to the trace' do
+        expect(response.status).to eq 200
+        expect(build.reload.trace).to eq 'BUILD TRACE appended'
+      end
+
+      context 'when build has been erased' do
+        let(:build) { create(:ci_build, runner_id: runner.id, erased_at: Time.now) }
+
+        it 'should respond with forbidden' do
+          expect(response.status).to eq 403
+        end
+      end
+    end
+
     context "Artifacts" do
       let(:file_upload) { fixture_file_upload(Rails.root + 'spec/fixtures/banana_sample.gif', 'image/gif') }
       let(:file_upload2) { fixture_file_upload(Rails.root + 'spec/fixtures/dk.png', 'image/gif') }