Move default_path to legacy_default_path. Switch to the new path for live-trace

lib/gitlab/ci/trace.rb

+##
+# Current status of paths
+# Era 1: Live/Full traces in database (ci_builds.trace)
+# Era 2: Live/Full traces in `setting_root/YYYY_MM/project_ci_id/job_id.log`
+# Era 3: Live/Full traces in `setting_root/YYYY_MM/project_id/job_id.log`
+# Era 4: Live traces in `setting_root/live_trace/job_id.log`. Full traces in JobArtifactUploader#legacy_default_path.
+#
+# The legacy paths are to be migrated to the latest era.
 module Gitlab
   module Ci
     class Trace
@@ -99,12 +107,12 @@ module Gitlab
         return current_path if current_path
 
         ensure_directory
-        default_path
+        live_trace_default_path
       end
 
       def ensure_directory
-        unless Dir.exist?(default_directory)
-          FileUtils.mkdir_p(default_directory)
+        unless Dir.exist?(live_trace_default_directory)
+          FileUtils.mkdir_p(live_trace_default_directory)
         end
       end
 
@@ -115,24 +123,34 @@ module Gitlab
       end
 
       ##
-      # This doesn't include the latest path JobArtifactUploader#default_path.
+      # This method doesn't include the latest path, which is JobArtifactUploader#default_path,
+      # Because, in EE, traces can be moved to ObjectStorage, so checking paths in Filestorage doesn't make sense.
+      # All legacy paths (`legacy_default_path` and `deprecated_path`) are to be migrated to JobArtifactUploader#default_path
       def paths
         [
-          default_path,
+          live_trace_default_path,
+          legacy_default_path,
           deprecated_path
         ].compact
       end
 
-      def default_directory
+      def live_trace_default_directory
         File.join(
           Settings.gitlab_ci.builds_path,
-          job.created_at.utc.strftime("%Y_%m"),
-          job.project_id.to_s
+          'live_trace'
         )
       end
 
-      def default_path
-        File.join(default_directory, "#{job.id}.log")
+      def live_trace_default_path
+        File.join(live_trace_default_directory, "#{job.id}.log")
+      end
+
+      def legacy_default_path
+        File.join(
+          Settings.gitlab_ci.builds_path,
+          job.created_at.utc.strftime("%Y_%m"),
+          job.project_id.to_s,
+          "#{job.id}.log")
       end
 
       def deprecated_path