Improve comments

app/services/ci/archive_trace_service.rb

@@ -7,19 +7,29 @@ module Ci
     rescue ::Gitlab::Ci::Trace::AlreadyArchivedError
       # It's already archived, thus we can safely ignore this exception.
     rescue => e
+      # Tracks this error with application logs, Sentry, and Prometheus.
+      # If `archive!` keeps failing for over a week, that could incur data loss.
+      # (See more https://docs.gitlab.com/ee/administration/job_traces.html#new-live-trace-architecture)
+      # In order to avoid interrupting the system, we do not raise an exception here.
       archive_error(e, job)
     end
 
     private
 
     def failed_archive_counter
-      @failed_archive_counter ||= Gitlab::Metrics.counter(:job_trace_archive_failed_total, "Counter of failed attempts of trace archiving")
+      @failed_archive_counter ||=
+        Gitlab::Metrics.counter(:job_trace_archive_failed_total,
+                                "Counter of failed attempts of trace archiving")
     end
 
     def archive_error(error, job)
       failed_archive_counter.increment
-      Gitlab::Sentry.track_exception(error, issue_url: 'https://gitlab.com/gitlab-org/gitlab-ce/issues/51502', extra: { job_id: job.id })
       Rails.logger.error "Failed to archive trace. id: #{job.id} message: #{error.message}"
+
+      Gitlab::Sentry
+        .track_exception(error,
+                         issue_url: 'https://gitlab.com/gitlab-org/gitlab-ce/issues/51502',
+                         extra: { job_id: job.id })
     end
   end
 end

spec/workers/ci/archive_traces_cron_worker_spec.rb

@@ -48,6 +48,21 @@ describe Ci::ArchiveTracesCronWorker do
         expect(build2.job_artifacts_trace).to be_exist
       end
     end
+
+    context 'when an unexpected exception happened during archiving' do
+      let!(:build) { create(:ci_build, :success, :trace_live) }
+
+      before do
+        allow(Gitlab::Sentry).to receive(:track_exception)
+        allow_any_instance_of(Gitlab::Ci::Trace).to receive(:archive!).and_raise('Unexpected error')
+      end
+
+      it 'puts a log' do
+        expect(Rails.logger).to receive(:error).with("Failed to archive trace. id: #{build.id} message: Unexpected error")
+
+        subject
+      end
+    end
   end
 
   context 'when a job was cancelled' do